path: root/tools/js-optimizer.js

//==============================================================================
// Optimizer tool. This is meant to be run after the emscripten compiler has
// finished generating code. These optimizations are done on the generated
// code to further improve it. Some of the modifications also work in
// conjunction with closure compiler.
//
// TODO: Optimize traverse to modify a node we want to replace, in-place,
//       instead of returning it to the previous call frame where we check?
// TODO: Share EMPTY_NODE instead of emptyNode that constructs?
//==============================================================================

// *** Environment setup code ***
var arguments_ = [];

var ENVIRONMENT_IS_NODE = typeof process === 'object';
var ENVIRONMENT_IS_WEB = typeof window === 'object';
var ENVIRONMENT_IS_WORKER = typeof importScripts === 'function';
var ENVIRONMENT_IS_SHELL = !ENVIRONMENT_IS_WEB && !ENVIRONMENT_IS_NODE && !ENVIRONMENT_IS_WORKER;

if (ENVIRONMENT_IS_NODE) {
  // Expose functionality in the same simple way that the shells work
  // Note that we pollute the global namespace here, otherwise we break in node
  print = function(x) {
    process['stdout'].write(x + '\n');
  };
  printErr = function(x) {
    process['stderr'].write(x + '\n');
  };

  var nodeFS = require('fs');
  var nodePath = require('path');

  if (!nodeFS.existsSync) {
    nodeFS.existsSync = function(path) {
      try {
        return !!nodeFS.readFileSync(path);
      } catch(e) {
        return false;
      }
    }
  }

  function find(filename) {
    var prefixes = [nodePath.join(__dirname, '..', 'src'), process.cwd()];
    for (var i = 0; i < prefixes.length; ++i) {
      var combined = nodePath.join(prefixes[i], filename);
      if (nodeFS.existsSync(combined)) {
        return combined;
      }
    }
    return filename;
  }

  read = function(filename) {
    var absolute = find(filename);
    return nodeFS['readFileSync'](absolute).toString();
  };

  load = function(f) {
    globalEval(read(f));
  };

  arguments_ = process['argv'].slice(2);

} else if (ENVIRONMENT_IS_SHELL) {
  // Polyfill over SpiderMonkey/V8 differences
  if (!this['read']) {
    this['read'] = function(f) { snarf(f) };
  }

  if (typeof scriptArgs != 'undefined') {
    arguments_ = scriptArgs;
  } else if (typeof arguments != 'undefined') {
    arguments_ = arguments;
  }

} else if (ENVIRONMENT_IS_WEB) {
  this['print'] = printErr = function(x) {
    console.log(x);
  };

  this['read'] = function(url) {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', url, false);
    xhr.send(null);
    return xhr.responseText;
  };

  if (this['arguments']) {
    arguments_ = arguments;
  }
} else if (ENVIRONMENT_IS_WORKER) {
  // We can do very little here...

  this['load'] = importScripts;

} else {
  throw 'Unknown runtime environment. Where are we?';
}

function globalEval(x) {
  eval.call(null, x);
}

if (typeof load == 'undefined' && typeof read != 'undefined') {
  this['load'] = function(f) {
    globalEval(read(f));
  };
}

if (typeof printErr === 'undefined') {
  this['printErr'] = function(){};
}

if (typeof print === 'undefined') {
  this['print'] = printErr;
}
// *** Environment setup code ***

var uglify = require('../tools/eliminator/node_modules/uglify-js');
var fs = require('fs');
var path = require('path');

// Load some modules

load('utility.js');

// Utilities

var FUNCTION = set('defun', 'function');
var LOOP = set('do', 'while', 'for');
var LOOP_FLOW = set('break', 'continue');
var ASSIGN_OR_ALTER = set('assign', 'unary-postfix', 'unary-prefix');
var CONTROL_FLOW = set('do', 'while', 'for', 'if', 'switch');
var NAME_OR_NUM = set('name', 'num');
var ASSOCIATIVE_BINARIES = set('+', '*', '|', '&', '^');

var NULL_NODE = ['name', 'null'];
var UNDEFINED_NODE = ['unary-prefix', 'void', ['num', 0]];
var TRUE_NODE = ['unary-prefix', '!', ['num', 0]];
var FALSE_NODE = ['unary-prefix', '!', ['num', 1]];

var GENERATED_FUNCTIONS_MARKER = '// EMSCRIPTEN_GENERATED_FUNCTIONS:';
var generatedFunctions = null;
function setGeneratedFunctions(metadata) {
  var start = metadata.indexOf(GENERATED_FUNCTIONS_MARKER);
  generatedFunctions = set(eval(metadata.substr(start + GENERATED_FUNCTIONS_MARKER.length)));
}
function isGenerated(ident) {
  return ident in generatedFunctions;
}

function srcToAst(src) {
  return uglify.parser.parse(src);
}

function astToSrc(ast, compress) {
    return uglify.uglify.gen_code(ast, {
    ascii_only: true,
    beautify: !compress,
    indent_level: 2
  });
}

// Traverses the children of a node. If the traverse function returns an object,
// replaces the child. If it returns true, stop the traversal and return true.
function traverseChildren(node, traverse, pre, post, stack) {
  for (var i = 0; i < node.length; i++) {
    var subnode = node[i];
    if (typeof subnode == 'object' && subnode && subnode.length) {
      var subresult = traverse(subnode, pre, post, stack);
      if (subresult == true) return true;
      if (subresult !== null && typeof subresult == 'object') node[i] = subresult;
    }
  }
}

// Traverses a JavaScript syntax tree rooted at the given node calling the given
// callback for each node.
//   @arg node: The root of the AST.
//   @arg pre: The pre to call for each node. This will be called with
//     the node as the first argument and its type as the second. If true is
//     returned, the traversal is stopped. If an object is returned,
//     it replaces the passed node in the tree. If null is returned, we stop
//     traversing the subelements (but continue otherwise).
//   @arg post: A callback to call after traversing all children.
//   @arg stack: If true, a stack will be implemented: If pre does not push on
//               the stack, we push a 0. We pop when we leave the node. The
//               stack is passed as a third parameter to the callbacks.
//   @returns: If the root node was replaced, the new root node. If the traversal
//     was stopped, true. Otherwise undefined.
function traverse(node, pre, post, stack) {
  var type = node[0], result, len;
  var relevant = typeof type == 'string';
  if (relevant) {
    if (stack) len = stack.length;
    var result = pre(node, type, stack);
    if (result == true) return true;
    if (result !== null && typeof result == 'object') node = result; // Continue processing on this node
    if (stack && len == stack.length) stack.push(0);
  }
  if (result !== null) {
    if (traverseChildren(node, traverse, pre, post, stack) == true) return true;
  }
  if (relevant) {
    if (post) {
      var postResult = post(node, type, stack);
      result = result || postResult;
    }
    if (stack) stack.pop();
  }
  return result;
}

// Only walk through the generated functions
function traverseGenerated(ast, pre, post, stack) {
  traverse(ast, function(node) {
    if (node[0] == 'defun' && isGenerated(node[1])) {
      traverse(node, pre, post, stack);
      return null;
    }
  });
}

function traverseGeneratedFunctions(ast, callback) {
  traverse(ast, function(node) {
    if (node[0] == 'defun' && isGenerated(node[1])) {
      callback(node);
      return null;
    }
  });
}

// Walk the ast in a simple way, with an understanding of which JS variables are defined)
function traverseWithVariables(ast, callback) {
  traverse(ast, function(node, type, stack) {
    if (type in FUNCTION) {
      stack.push({ type: 'function', vars: node[2] });
    } else if (type == 'var') {
      // Find our function, add our vars
      var func = stack[stack.length-1];
      if (func) {
        func.vars = func.vars.concat(node[1].map(function(varItem) { return varItem[0] }));
      }
    }
  }, function(node, type, stack) {
    if (type == 'toplevel' || type in FUNCTION) {
      // We know all of the variables that are seen here, proceed to do relevant replacements
      var allVars = stack.map(function(item) { return item ? item.vars : [] }).reduce(concatenator, []); // FIXME dictionary for speed?
      traverse(node, function(node2, type2, stack2) {
        // Be careful not to look into our inner functions. They have already been processed.
        if (sum(stack2) > 1 || (type == 'toplevel' && sum(stack2) == 1)) return;
        if (type2 in FUNCTION) stack2.push(1);
        return callback(node2, type2, allVars);
      }, null, []);
    }
  }, []);
}

function emptyNode() { // XXX do we need to create new nodes here? can't we reuse?
  return ['toplevel', []]
}

// Passes

// Dump the AST. Useful for debugging. For example,
//  node tools/js-optimizer.js ABSOLUTE_PATH_TO_FILE dumpAst
function dumpAst(ast) {
  printErr(JSON.stringify(ast, null, '  '));
}

function dumpSrc(ast) {
  printErr(astToSrc(ast));
}

// Undos closure's creation of global variables with values true, false,
// undefined, null. These cut down on size, but do not affect gzip size
// and make JS engine's lives slightly harder (?)
function unGlobalize(ast) {

  throw 'this is deprecated!'; // and does not work with parallel compilation

  assert(ast[0] == 'toplevel');
  var values = {};
  // Find global renamings of the relevant values
  ast[1].forEach(function(node, i) {
    if (node[0] != 'var') return;
    node[1] = node[1].filter(function(varItem, j) {
      var ident = varItem[0];
      var value = varItem[1];
      if (!value) return true;
      var possible = false;
      if (jsonCompare(value, NULL_NODE) ||
          jsonCompare(value, UNDEFINED_NODE) ||
          jsonCompare(value, TRUE_NODE) ||
          jsonCompare(value, FALSE_NODE)) {
        possible = true;
      }
      if (!possible) return true;
      // Make sure there are no assignments to this variable. (This isn't fast, we traverse many times..)
      ast[1][i][1][j] = emptyNode();
      var assigned = false;
      traverseWithVariables(ast, function(node, type, allVars) {
        if (type == 'assign' && node[2][0] == 'name' && node[2][1] == ident) assigned = true;
      });
      ast[1][i][1][j] = [ident, value];
      if (!assigned) {
        values[ident] = value;
        return false;
      }
      return true;
    });

    if (node[1].length == 0) {
      ast[1][i] = emptyNode();
    }
  });
  traverseWithVariables(ast, function(node, type, allVars) {
    if (type == 'name') {
      var ident = node[1];
      if (ident in values && allVars.indexOf(ident) < 0) {
        return copy(values[ident]);
      }
    }
  });
}

// Closure compiler, when inlining, will insert assignments to
// undefined for the shared variables. However, in compiled code
// - and in library/shell code too! - we should never rely on
// undefined being assigned. So we can simply remove those assignments.
//
// Note: An inlined function that kept a large value referenced, may
//       keep that references when inlined, if we remove the setting to
//       undefined. This is not dangerous in compiled code, but might be
//       in supporting code (for example, holding on to the HEAP when copying).
//
// This pass assumes that unGlobalize has been run, so undefined
// is now explicit.
function removeAssignsToUndefined(ast) {
  traverse(ast, function(node, type) {
    if (type == 'assign' && jsonCompare(node[3], ['unary-prefix', 'void', ['num', 0]])) {
      return emptyNode();
    } else if (type == 'var') {
      node[1] = node[1].map(function(varItem, j) {
        var ident = varItem[0];
        var value = varItem[1];
        if (jsonCompare(value, UNDEFINED_NODE)) return [ident];
        return [ident, value];
      });
    }
  });
  // cleanup (|x = y = void 0| leaves |x = ;| right now)
  var modified = true;
  while (modified) {
    modified = false;
    traverse(ast, function(node, type) {
      if (type == 'assign' && jsonCompare(node[3], emptyNode())) {
        modified = true;
        return emptyNode();
      } else if (type == 'var') {
        node[1] = node[1].map(function(varItem, j) {
          var ident = varItem[0];
          var value = varItem[1];
          if (value && jsonCompare(value, emptyNode())) return [ident];
          return [ident, value];
        });
      }
    });
  }
}

// XXX This is an invalid optimization
// We sometimes leave some settings to __label__ that are not needed, if later in
// the relooper we realize that we have a single entry, so no checks on __label__
// are actually necessary. It's easy to clean those up now.
function removeUnneededLabelSettings(ast) {
  traverse(ast, function(node, type) {
    if (type == 'defun') { // all of our compiled code is in defun nodes
      // Find all checks
      var checked = {};
      traverse(node, function(node, type) {
        if (type == 'binary' && node[1] == '==' && node[2][0] == 'name' && node[2][1] == '__label__') {
          assert(node[3][0] == 'num');
          checked[node[3][1]] = 1;
        }
      });
      // Remove unneeded sets
      traverse(node, function(node, type) {
        if (type == 'assign' && node[2][0] == 'name' && node[2][1] == '__label__') {
          assert(node[3][0] == 'num');
          if (!(node[3][1] in checked)) return emptyNode();
        }
      });
    }
  });
}

// Various expression simplifications. Pre run before closure (where we still have metadata), Post run after.

function simplifyExpressionsPre(ast) {
  // When there is a bunch of math like (((8+5)|0)+12)|0, only the external |0 is needed, one correction is enough.
  // At each node, ((X|0)+Y)|0 can be transformed into (X+Y): The inner corrections are not needed
  // TODO: Is the same is true for 0xff, 0xffff?
  // Likewise, if we have |0 inside a block that will be >>'d, then the |0 is unnecessary because some
  // 'useful' mathops already |0 anyhow.

  function simplifyBitops(ast) {
    var USEFUL_BINARY_OPS = set('<<', '>>', '|', '&', '^');
    var SAFE_BINARY_OPS = set('+', '-', '*', '%'); // division is unsafe as it creates non-ints in JS
    var ZERO = ['num', 0];
    var rerun = true;
    while (rerun) {
      rerun = false;
      traverseGenerated(ast, function(node, type, stack) {
        if (type == 'binary' && node[1] == '|' && (jsonCompare(node[2], ZERO) || jsonCompare(node[3], ZERO))) {
          stack.push(1); // From here on up, no need for this kind of correction, it's done at the top

          // We might be able to remove this correction
          for (var i = stack.length-2; i >= 0; i--) {
            if (stack[i] == 1) {
              // Great, we can eliminate
              rerun = true;
              return jsonCompare(node[2], ZERO) ? node[3] : node[2];
            } else if (stack[i] == -1) {
              break; // Too bad, we can't
            }
          }
        } else if (type == 'binary' && node[1] in USEFUL_BINARY_OPS) {
          stack.push(1);
        } else if ((type == 'binary' && node[1] in SAFE_BINARY_OPS) || type == 'num' || type == 'name') {
          stack.push(0); // This node is safe in that it does not interfere with this optimization
        } else {
          stack.push(-1); // This node is dangerous! Give up if you see this before you see '1'
        }
      }, null, []);
    }
  }

  // The most common mathop is addition, e.g. in getelementptr done repeatedly. We can join all of those,
  // by doing (num+num) ==> newnum, and (name+num)+num = name+newnum
  function joinAdditions(ast) {
    var rerun = true;
    while (rerun) {
      rerun = false;
      traverseGenerated(ast, function(node, type) {
        if (type == 'binary' && node[1] == '+') {
          if (node[2][0] == 'num' && node[3][0] == 'num') {
            rerun = true;
            return ['num', node[2][1] + node[3][1]];
          }
          for (var i = 2; i <= 3; i++) {
            var ii = 5-i;
            for (var j = 2; j <= 3; j++) {
              if (node[i][0] == 'num' && node[ii][0] == 'binary' && node[ii][1] == '+' && node[ii][j][0] == 'num') {
                rerun = true;
                node[ii][j][1] += node[i][1];
                return node[ii];
              }
            }
          }
        }
      });
    }
  }

  // if (x == 0) can be if (!x), etc.
  function simplifyZeroComp(ast) {
    traverseGenerated(ast, function(node, type) {
      var binary;
      if (type == 'if' && (binary = node[1])[0] == 'binary') {
        if ((binary[1] == '!=' || binary[1] == '!==') && binary[3][0] == 'num' && binary[3][1] == 0) {
          node[1] = binary[2];
          return node;
        } else if ((binary[1] == '==' || binary[1] == '===') && binary[3][0] == 'num' && binary[3][1] == 0) {
          node[1] = ['unary-prefix', '!', binary[2]];
          return node;
        }
      }
    });
  }

  simplifyBitops(ast);
  joinAdditions(ast);
  // simplifyZeroComp(ast); TODO: investigate performance
}

// In typed arrays mode 2, we can have
//  HEAP[x >> 2]
// very often. We can in some cases do the shift on the variable itself when it is set,
// to greatly reduce the number of shift operations.
// TODO: when shifting a variable, if there are other uses, keep an unshifted version too, to prevent slowdowns?
function optimizeShiftsInternal(ast, conservative) {
  var MAX_SHIFTS = 3;
  traverseGeneratedFunctions(ast, function(fun) {
    var funMore = true;
    var funFinished = {};
    while (funMore) {
      funMore = false;
      // Recognize variables and parameters
      var vars = {};
      function newVar(name, param, addUse) {
        if (!vars[name]) {
          vars[name] = {
            param: param,
            defs: addUse ? 1 : 0,
            uses: 0,
            timesShifted: [0, 0, 0, 0], // zero shifts of size 0, 1, 2, 3
            benefit: 0,
            primaryShift: -1
          };
        }
      }
      // params
      if (fun[2]) {
        fun[2].forEach(function(arg) {
          newVar(arg, true, true);
        });
      }
      // vars
      // XXX if var has >>=, ignore it here? That means a previous pass already optimized it
      var hasSwitch = traverse(fun, function(node, type) {
        if (type == 'var') {
          node[1].forEach(function(arg) {
            newVar(arg[0], false, arg[1]);
          });
        } else if (type == 'switch') {
          // The relooper can't always optimize functions, and we currently don't work with
          // switch statements when optimizing shifts. Bail.
          return true;
        }
      });
      if (hasSwitch) {
        break;
      }
      // uses and defs TODO: weight uses by being inside a loop (powers). without that, we
      // optimize for code size, not speed.
      traverse(fun, function(node, type, stack) {
        stack.push(node);
        if (type == 'name' && vars[node[1]] && stack[stack.length-2][0] != 'assign') {
          vars[node[1]].uses++;
        } else if (type == 'assign' && node[2][0] == 'name' && vars[node[2][1]]) {
          vars[node[2][1]].defs++;
        }
      }, null, []);
      // First, break up elements inside a shift. This lets us see clearly what to do next.
      traverse(fun, function(node, type) {
        if (type == 'binary' && node[1] == '>>' && node[3][0] == 'num') {
          var shifts = node[3][1];
          if (shifts <= MAX_SHIFTS) {
            // Push the >> inside the value elements
            function addShift(subNode) {
              if (subNode[0] == 'binary' && subNode[1] == '+') {
                subNode[2] = addShift(subNode[2]);
                subNode[3] = addShift(subNode[3]);
                return subNode;
              }
              if (subNode[0] == 'name' && !subNode[2]) { // names are returned with a shift, but we also note their being shifted
                var name = subNode[1];
                if (vars[name]) {
                  vars[name].timesShifted[shifts]++;
                  subNode[2] = true;
                }
              }
              return ['binary', '>>', subNode, ['num', shifts]];
            }
            return addShift(node[2]);
          }
        }
      });
      traverse(fun, function(node, type) {
        if (node[0] == 'name' && node[2]) {
          return node.slice(0, 2); // clean up our notes
        }
      });
      // At this point, shifted expressions are split up, and we know who the vars are and their info, so we can decide
      // TODO: vars that depend on other vars
      for (var name in vars) {
        var data = vars[name];
        var totalTimesShifted = sum(data.timesShifted);
        if (totalTimesShifted == 0) {
          continue;
        }
        if (totalTimesShifted != Math.max.apply(null, data.timesShifted)) {
          // TODO: Handle multiple different shifts
          continue;
        }
        if (funFinished[name]) continue;
        // We have one shift size (and possible unshifted uses). Consider replacing this variable with a shifted clone. If
        // the estimated benefit is >0, we will do it
        if (data.defs == 1) {
          data.benefit = totalTimesShifted - 2*(data.defs + (data.param ? 1 : 0));
        }
        if (conservative) data.benefit = 0;
        if (data.benefit > 0) {
          funMore = true; // We will reprocess this function
          for (var i = 0; i < 4; i++) {
            if (data.timesShifted[i]) {
              data.primaryShift = i;
            }
          }
        }
      }
      //printErr(JSON.stringify(vars));
      function cleanNotes() { // We need to mark 'name' nodes as 'processed' in some passes here; this cleans the notes up
        traverse(fun, function(node, type) {
          if (node[0] == 'name' && node[2]) {
            return node.slice(0, 2);
          }
        });
      }
      cleanNotes();
      // Apply changes
      function needsShift(name) {
        return vars[name] && vars[name].primaryShift >= 0;
      }
      for (var name in vars) { // add shifts for params and var's for all new variables
        var data = vars[name];
        if (needsShift(name)) {
          if (data.param) {
            fun[3].unshift(['var', [[name + '$s' + data.primaryShift, ['binary', '>>', ['name', name], ['num', data.primaryShift]]]]]);
          } else {
            fun[3].unshift(['var', [[name + '$s' + data.primaryShift]]]);
          }
        }
      }
      traverse(fun, function(node, type, stack) { // add shift to assignments
        stack.push(node);
        if (node[0] == 'assign' && node[1] === true && node[2][0] == 'name' && needsShift(node[2][1]) && !node[2][2]) {
          var name = node[2][1];
          var data = vars[name];
          var parent = stack[stack.length-3];
          var statements = getStatements(parent);
          assert(statements, 'Invalid parent for assign-shift: ' + dump(parent));
          var i = statements.indexOf(stack[stack.length-2]);
          statements.splice(i+1, 0, ['stat', ['assign', true, ['name', name + '$s' + data.primaryShift], ['binary', '>>', ['name', name, true], ['num', data.primaryShift]]]]);
        } else if (node[0] == 'var') {
          var args = node[1];
          for (var i = 0; i < args.length; i++) {
            var arg = args[i];
            var name = arg[0];
            var data = vars[name];
            if (arg[1] && needsShift(name)) {
              args.splice(i+1, 0, [name + '$s' + data.primaryShift, ['binary', '>>', ['name', name, true], ['num', data.primaryShift]]]);
            }
          }
          return node;
        }
      }, null, []);
      cleanNotes();
      traverse(fun, function(node, type, stack) { // replace shifted name with new variable
        stack.push(node);
        if (node[0] == 'binary' && node[1] == '>>' && node[2][0] == 'name' && needsShift(node[2][1]) && node[3][0] == 'num') {
          var name = node[2][1];
          var data = vars[name];
          var parent = stack[stack.length-2];
          // Don't modify in |x$sN = x >> 2|, in normal assigns and in var assigns
          if (parent[0] == 'assign' && parent[2][0] == 'name' && parent[2][1] == name + '$s' + data.primaryShift) return;
          if (parent[0] == name + '$s' + data.primaryShift) return;
          if (node[3][1] == data.primaryShift) {
            return ['name', name + '$s' + data.primaryShift];
          }
        }
      }, null, []);
      cleanNotes();
      var SIMPLE_SHIFTS = set('<<', '>>');
      var more = true;
      while (more) { // combine shifts in the same direction as an optimization
        more = false;
        traverse(fun, function(node, type) {
          if (node[0] == 'binary' && node[1] in SIMPLE_SHIFTS && node[2][0] == 'binary' && node[2][1] == node[1] &&
              node[3][0] == 'num' && node[2][3][0] == 'num') { // do not turn a << b << c into a << b + c; while logically identical, it is slower
            more = true;
            return ['binary', node[1], node[2][2], ['num', node[3][1] + node[2][3][1]]];
          }
        });
      }
      // Before recombining, do some additional optimizations
      traverse(fun, function(node, type) {
        // Apply constant shifts onto constants
        if (type == 'binary' && node[1] == '>>' && node[2][0] == 'num' && node[3][0] == 'num' && node[3][1] <= MAX_SHIFTS) {
          var subNode = node[2];
          var shifts = node[3][1];
          var result = subNode[1] / Math.pow(2, shifts);
          if (result % 1 == 0) {
            subNode[1] = result;
            return subNode;
          }
        }
        // Optimize the case of ($a*80)>>2 into ($a*20)|0
        if (type == 'binary' && node[1] in SIMPLE_SHIFTS &&
            node[2][0] == 'binary' && node[2][1] == '*') {
          var mulNode = node[2];
          if (mulNode[2][0] == 'num') {
            var temp = mulNode[2];
            mulNode[2] = mulNode[3];
            mulNode[3] = temp;
          }
          if (mulNode[3][0] == 'num') {
            if (node[1] == '<<') {
              mulNode[3][1] *= Math.pow(2, node[3][1]);
              node[1] = '|';
              node[3][1] = 0;
              return node;
            } else {
              if (mulNode[3][1] % Math.pow(2, node[3][1]) == 0) {
                mulNode[3][1] /= Math.pow(2, node[3][1]);
                node[1] = '|';
                node[3][1] = 0;
                return node;
              }
            }
          }
        }
        /* XXX - theoretically useful optimization(s), but commented out as not helpful in practice
        // Transform (x << 2) >> 2 into x & mask or something even simpler
        if (type == 'binary'       && node[1]    == '>>' && node[3][0] == 'num' &&
            node[2][0] == 'binary' && node[2][1] == '<<' && node[2][3][0] == 'num' && node[3][1] == node[2][3][1]) {
          var subNode = node[2];
          var shifts = node[3][1];
          var mask = ((0xffffffff << shifts) >>> shifts) | 0;
          return ['binary', '&', subNode[2], ['num', mask]];
          //return ['binary', '|', subNode[2], ['num', 0]];
          //return subNode[2];
        }
        */
      });
      // Re-combine remaining shifts, to undo the breaking up we did before. may require reordering inside +'s
      traverse(fun, function(node, type, stack) {
        stack.push(node);
        if (type == 'binary' && node[1] == '+' && (stack[stack.length-2][0] != 'binary' || stack[stack.length-2][1] != '+')) {
          // 'Flatten' added items
          var addedItems = [];
          function flatten(node) {
            if (node[0] == 'binary' && node[1] == '+') {
              flatten(node[2]);
              flatten(node[3]);
            } else {
              addedItems.push(node);
            }
          }
          flatten(node);
          var originalOrder = addedItems.slice();
          function key(node) { // a unique value for all relevant shifts for recombining, non-unique for stuff we don't need to bother with
            function originalOrderKey(item) {
              return -originalOrder.indexOf(item);
            }
            if (node[0] == 'binary' && node[1] in SIMPLE_SHIFTS) {
              if (node[3][0] == 'num' && node[3][1] <= MAX_SHIFTS) return 2*node[3][1] + (node[1] == '>>' ? 100 : 0); // 0-106
              return (node[1] == '>>' ? 20000 : 10000) + originalOrderKey(node);
            }
            if (node[0] == 'num') return -20000 + node[1];
            return -10000 + originalOrderKey(node); // Don't modify the original order if we don't modify anything
          }
          for (var i = 0; i < addedItems.length; i++) {
            if (addedItems[i][0] == 'string') return; // this node is not relevant for us
          }
          addedItems.sort(function(node1, node2) {
            return key(node1) - key(node2);
          });
          // Regenerate items, now sorted
          var i = 0;
          while (i < addedItems.length-1) { // re-combine inside addedItems
            var k = key(addedItems[i]), k1 = key(addedItems[i+1]);
            if (k == k1 && k >= 0 && k1 <= 106) {
              addedItems[i] = ['binary', addedItems[i][1], ['binary', '+', addedItems[i][2], addedItems[i+1][2]], addedItems[i][3]];
              addedItems.splice(i+1, 1);
            } else {
              i++;
            }
          }
          var num = 0;
          for (i = 0; i < addedItems.length; i++) { // combine all numbers into one
            if (addedItems[i][0] == 'num') {
              num += addedItems[i][1];
              addedItems.splice(i, 1);
              i--;
            }
          }
          if (num != 0) { // add the numbers into an existing shift, we 
                          // prefer (x+5)>>7 over (x>>7)+5 , since >>'s result is known to be 32-bit and is more easily optimized.
                          // Also, in the former we can avoid the parentheses, which saves a little space (the number will be bigger,
                          // so it might take more space, but normally at most one more digit).
            var added = false;
            for (i = 0; i < addedItems.length; i++) {
              if (addedItems[i][0] == 'binary' && addedItems[i][1] == '>>' && addedItems[i][3][0] == 'num' && addedItems[i][3][1] <= MAX_SHIFTS) {
                addedItems[i] = ['binary', '>>', ['binary', '+', addedItems[i][2], ['num', num << addedItems[i][3][1]]], addedItems[i][3]];
                added = true;
              }
            }
            if (!added) {
              addedItems.unshift(['num', num]);
            }
          }
          var ret = addedItems.pop();
          while (addedItems.length > 0) { // re-create AST from addedItems
            ret = ['binary', '+', ret, addedItems.pop()];
          }
          return ret;
        }
      }, null, []);
      // Note finished variables
      for (var name in vars) {
        funFinished[name] = true;
      }
    }
  });
}

function optimizeShiftsConservative(ast) {
  optimizeShiftsInternal(ast, true);
}

function optimizeShiftsAggressive(ast) {
  optimizeShiftsInternal(ast, false);
}

// We often have branchings that are simplified so one end vanishes, and
// we then get
//   if (!(x < 5))
// or such. Simplifying these saves space and time.
function simplifyNotComps(ast) {
  traverse(ast, function(node, type) {
    if (type == 'unary-prefix' && node[1] == '!' && node[2][0] == 'binary') {
      if (node[2][1] == '<') {
        return ['binary', '>=', node[2][2], node[2][3]];
      } else if (node[2][1] == '>') {
        return ['binary', '<=', node[2][2], node[2][3]];
      } else if (node[2][1] == '==') {
        return ['binary', '!=', node[2][2], node[2][3]];
      } else if (node[2][1] == '!=') {
        return ['binary', '==', node[2][2], node[2][3]];
      } else if (node[2][1] == '===') {
        return ['binary', '!==', node[2][2], node[2][3]];
      } else if (node[2][1] == '!==') {
        return ['binary', '===', node[2][2], node[2][3]];
      }
    }
  });
}

function simplifyExpressionsPost(ast) {
  simplifyNotComps(ast);
}

var NO_SIDE_EFFECTS = set('num', 'name');

function hasSideEffects(node) { // this is 99% incomplete!
  if (node[0] in NO_SIDE_EFFECTS) return false;
  if (node[0] == 'unary-prefix' && node[1] == '!') return hasSideEffects(node[2]);
  if (node[0] == 'binary') return hasSideEffects(node[2]) || hasSideEffects(node[3]);
  return true;
}

// Clear out empty ifs and blocks, and redundant blocks/stats and so forth
// Operates on generated functions only
function vacuum(ast) {
  function isEmpty(node) {
    if (!node) return true;
    if (node[0] == 'toplevel' && (!node[1] || node[1].length == 0)) return true;
    if (node[0] == 'block' && (!node[1] || (typeof node[1] != 'object') || node[1].length == 0 || (node[1].length == 1 && isEmpty(node[1])))) return true;
    return false;
  }
  function simplifyList(node, si) {
    var changed = false;
    // Merge block items into this list, thus removing unneeded |{ .. }|'s
    var statements = node[si];
    var i = 0;
    while (i < statements.length) {
      var subNode = statements[i];
      if (subNode[0] == 'block') {
        statements.splice.apply(statements, [i, 1].concat(subNode[1] || []));
        changed = true;
      } else {
        i++;
      }
    }
    // Remove empty items
    var pre = node[si].length;
    node[si] = node[si].filter(function(node) { return !isEmpty(node) });
    if (node[si].length < pre) changed = true;
    if (changed) {
      return node;
    }
  }
  function vacuumInternal(node) {
    traverseChildren(node, vacuumInternal);
    var ret;
    switch(node[0]) {
      case 'block': {
        if (node[1] && node[1].length == 1 && node[1][0][0] == 'block') {
          return node[1][0];
        } else if (typeof node[1] == 'object') {
          ret = simplifyList(node, 1);
          if (ret) return ret;
        }
      } break;
      case 'stat': {
        if (node[1][0] == 'block') {
          return node[1];
        }
      } break;
      case 'defun': {
        if (node[3].length == 1 && node[3][0][0] == 'block') {
          node[3] = node[3][0][1];
          return node;
        } else {
          ret = simplifyList(node, 3);
          if (ret) return ret;
        }
      } break;
      case 'do': {
        if (node[1][0] == 'num' && node[2][0] == 'toplevel' && (!node[2][1] || node[2][1].length == 0)) {
          return emptyNode();
        } else if (isEmpty(node[2]) && !hasSideEffects(node[1])) {
          return emptyNode();
        }
      } break;
      case 'label': {
        if (node[2][0] == 'toplevel' && (!node[2][1] || node[2][1].length == 0)) {
          return emptyNode();
        }
      } break;
      case 'if': {
        var empty2 = isEmpty(node[2]), empty3 = isEmpty(node[3]), has3 = node.length == 4;
        if (!empty2 && empty3 && has3) { // empty else clauses
          return node.slice(0, 3);
        } else if (empty2 && !empty3) { // empty if blocks
          return ['if', ['unary-prefix', '!', node[1]], node[3]];
        } else if (empty2 && empty3) {
          if (hasSideEffects(node[1])) {
            return ['stat', node[1]];
          } else {
            return emptyNode();
          }
        }
      } break;
    }
  }
  traverseGeneratedFunctions(ast, function(node) {
    vacuumInternal(node);
    simplifyNotComps(node);
  });
}

function getStatements(node) {
  if (node[0] == 'defun') {
    return node[3];
  } else if (node[0] == 'block') {
    return node[1];
  } else {
    return null;
  }
}

// Multiple blocks from the relooper are, in general, implemented by
//   if (__label__ == x) { } else if ..
// and branching into them by
//   if (condition) { __label__ == x } else ..
// We can hoist the multiple block into the condition, thus removing code and one 'if' check
function hoistMultiples(ast) {
  traverseGeneratedFunctions(ast, function(node) {
    traverse(node, function(node, type) {
      var statements = getStatements(node);
      if (!statements) return;
      var modified = false;
      for (var i = 0; i < statements.length-1; i++) {
        var modifiedI = false;
        var pre = statements[i];
        if (pre[0] != 'if') continue;
        var post = statements[i+1];
        // Look into some block types. shell() will then recreate the shell that we looked into
        var postInner = post;
        var shellLabel = false, shellDo = false;
        while (true) {
          if (postInner[0] == 'label') {
            shellLabel = postInner[1];
            postInner = postInner[2];
          } else if (postInner[0] == 'do') {
            shellDo = postInner[1];
            postInner = postInner[2][1][0];
          } else {
            break; // give up
          }
        }
        if (postInner[0] != 'if') continue;
        // Look into this if, and its elseifs
        while (postInner && postInner[0] == 'if') {
          var cond = postInner[1];
          if (cond[0] == 'binary' && cond[1] == '==' && cond[2][0] == 'name' && cond[2][1] == '__label__') {
            assert(cond[3][0] == 'num');
            // We have a valid Multiple check here. Try to hoist it, look for the source in |pre| and its else's
            var labelNum = cond[3][1];
            var labelBlock = postInner[2];
            assert(labelBlock[0] == 'block');
            var found = false;
            traverse(pre, function(preNode, preType) {
              if (!found && preType == 'assign' && preNode[2][0] == 'name' && preNode[2][1] == '__label__') {
                assert(preNode[3][0] == 'num');
                if (preNode[3][1] == labelNum) {
                  // That's it! Hoist away. We can also throw away the __label__ setting as its goal has already been achieved
                  found = true;
                  modifiedI = true;
                  postInner[2] = ['block', []];
                  return labelBlock;
                }
              }
            });
          }
          postInner = postInner[3]; // Proceed to look in the else clause
        }
        if (modifiedI) {
          if (shellDo) {
            statements[i] = ['do', shellDo, ['block', [statements[i]]]];
          }
          if (shellLabel) {
            statements[i] = ['label', shellLabel, statements[i]];
          }
        }
      }
      if (modified) return node;
    });

    // After hoisting in this function, it is safe to remove { __label__ = x; } blocks, because
    // if they were leading to the next code right after them, they would be hoisted, and if they
    // are going to some other place entirely, they would break or continue. The only risky
    // situation is if the code after us is a multiple, in which case we might be checking for
    // this label inside it (or in a later multiple, even)
    function tryEliminate(node) {
      if (node[0] == 'if') {
        var replaced;
        if (replaced = tryEliminate(node[2])) node[2] = replaced;
        if (node[3] && (replaced = tryEliminate(node[3]))) node[3] = replaced;
      } else {
        if (node[0] == 'block' && node[1] && node[1].length > 0) {
          var subNode = node[1][node[1].length-1];
          if (subNode[0] == 'stat' && subNode[1][0] == 'assign' && subNode[1][2][0] == 'name' &&
              subNode[1][2][1] == '__label__' && subNode[1][3][0] == 'num') {
            if (node[1].length == 1) {
              return emptyNode();
            } else {
              node[1].splice(node[1].length-1, 1);
              return node;
            }
          }
        }
      }
      return false;
    }
    function getActualStatement(node) { // find the actual active statement, ignoring a label and one-time do loop
      if (node[0] == 'label') node = node[2];
      if (node[0] == 'do') node = node[2];
      if (node[0] == 'block' && node[1].length == 1) node = node[1][0];
      return node;
    }
    vacuum(node);
    traverse(node, function(node, type) {
      var statements = getStatements(node);
      if (!statements) return;
      for (var i = 0; i < statements.length-1; i++) {
        var curr = getActualStatement(statements[i]);
        var next = statements[i+1];
        if (curr[0] == 'if' && next[0] != 'if' && next[0] != 'label' && next[0] != 'do' && next[0] != 'while') {
          tryEliminate(curr);
        }
      }
    });
  });

  vacuum(ast);

  // Afterpass: Reduce
  //    if (..) { .. break|continue } else { .. }
  // to
  //    if (..) { .. break|continue } ..
  traverseGenerated(ast, function(container, type) {
    var statements = getStatements(container);
    if (!statements) return;
    for (var i = 0; i < statements.length; i++) {
      var node = statements[i];
      if (node[0] == 'if' && node[2][0] == 'block' && node[3] && node[3][0] == 'block') {
        var stat1 = node[2][1], stat2 = node[3][1];
        // If break|continue in the latter and not the former, reverse them
        if (!(stat1[stat1.length-1][0] in LOOP_FLOW) && (stat2[stat2.length-1][0] in LOOP_FLOW)) {
          var temp = node[3];
          node[3] = node[2];
          node[2] = temp;
          node[1] = ['unary-prefix', '!', node[1]];
          simplifyNotComps(node[1]); // bake the ! into the expression
          stat1 = node[2][1];
          stat2 = node[3][1];
        }
        if (stat1[stat1.length-1][0] in LOOP_FLOW) {
          statements.splice.apply(statements, [i+1, 0].concat(stat2));
          node[3] = null;
        }
      }
    }
  });
}

// Simplifies loops
// WARNING: This assumes all loops and breaks/continues are labelled
function loopOptimizer(ast) {
  // Remove unneeded labels and one-time (do while(0)) loops. It is convenient to do these both at once.
  function passTwo(ast) {
    var neededDos = [];
    // Find unneeded labels
    traverseGenerated(ast, function(node, type, stack) {
      if (type == 'label' && node[2][0] in LOOP) {
        // this is a labelled loop. we don't know if it's needed yet. Mark its label for removal for now now.
        stack.push(node);
        node[1] = '+' + node[1];
      } else if (type in LOOP) {
        stack.push(node);
      } else if (type in LOOP_FLOW) {
        // Find topmost loop, and its label if there is one
        var lastLabel = null, lastLoop = null, i = stack.length-1;
        while (i >= 0 && !lastLoop) {
          if (stack[i][0] in LOOP) lastLoop = stack[i];
          i--;
        }
        assert(lastLoop, 'Cannot break/continue without a Label');
        while (i >= 0 && !lastLabel) {
          if (stack[i][0] in LOOP) break; // another loop in the middle - no label for lastLoop
          if (stack[i][0] == 'label') lastLabel = stack[i];
          i--;
        }
        var ident = node[1]; // there may not be a label ident if this is a simple break; or continue;
        var plus = '+' + ident;
        if (lastLabel && ident && (ident == lastLabel[1] || plus == lastLabel[1])) {
          // If this is a 'do' loop, this break means we actually need it.
          neededDos.push(lastLoop);
          // We don't need the control flow command to have a label - it's referring to the current loop
          return [node[0]];
        } else {
          if (!ident) {
            // No label on the break/continue, so keep the last loop alive (no need for its label though)
            neededDos.push(lastLoop);
          } else {
            // Find the label node that needs to stay alive
            stack.forEach(function(label) {
              if (!label) return;
              if (label[1] == plus) label[1] = label[1].substr(1); // Remove '+', marking it as needed
            });
          }
        }
      }
    }, null, []);
    // We return whether another pass is necessary
    var more = false;
    // Remove unneeded labels
    traverseGenerated(ast, function(node, type) {
      if (type == 'label' && node[1][0] == '+') {
        more = true;
        var ident = node[1].substr(1);
        // Remove label from loop flow commands
        traverse(node[2], function(node2, type) {
          if (type in LOOP_FLOW && node2[1] == ident) {
            return [node2[0]];
          }
        });
        return node[2]; // Remove the label itself on the loop
      }
    });
    // Remove unneeded one-time loops. We need such loops if (1) they have a label, or (2) they have a direct break so they are in neededDos.
    // First, add all labeled loops of this nature to neededDos
    traverseGenerated(ast, function(node, type) {
      if (type == 'label' && node[2][0] == 'do') {
        neededDos.push(node[2]);
      }
    });
    // Remove unneeded dos, we know who they are now
    traverseGenerated(ast, function(node, type) {
      if (type == 'do' && neededDos.indexOf(node) < 0) {
        assert(jsonCompare(node[1], ['num', 0]), 'Trying to remove a one-time do loop that is not one of our generated ones.;');
        more = true;
        return node[2];
      }
    });
    return more;
  }

  // Go

  // TODO: pass 1: Removal of unneeded continues, breaks if they get us to where we are already going. That will
  //               help the next pass.

  // Multiple pass two runs may be needed, as we remove one-time loops and so forth
  do {
    var more = passTwo(ast);
    vacuum(ast);
  } while (more);

  vacuum(ast);
}

// Very simple 'registerization', coalescing of variables into a smaller number.
// We do not optimize when there are switches, so this pass only makes sense with
// relooping.
// TODO: Consider how this fits in with the rest of the optimization toolchain. Do
//       we still need the eliminator? Closure? And in what order? Perhaps just
//       closure simple?
function registerize(ast) {
  traverseGeneratedFunctions(ast, function(fun) {
    // Replace all var definitions with assignments; we will add var definitions at the top after we registerize
    // We also mark local variables - i.e., having a var definition
    var localVars = {};
    var hasSwitch = false; // we cannot optimize variables if there is a switch
    traverse(fun, function(node, type) {
      if (type == 'var') {
        node[1].forEach(function(defined) { localVars[defined[0]] = 1 });
        var vars = node[1].filter(function(varr) { return varr[1] });
        if (vars.length > 1) {
          var ret = ['stat', []];
          var curr = ret[1];
          for (var i = 0; i < vars.length-1; i++) {
            curr[0] = 'seq';
            curr[1] = ['assign', true, ['name', vars[i][0]], vars[i][1]];
            if (i != vars.length-2) curr = curr[2] = [];
          }
          curr[2] = ['assign', true, ['name', vars[vars.length-1][0]], vars[vars.length-1][1]];
          return ret;
        } else if (vars.length == 1) {
          return ['stat', ['assign', true, ['name', vars[0][0]], vars[0][1]]];
        } else {
          return emptyNode();
        }
      } else if (type == 'switch') {
        hasSwitch = true;
      }
    });
    vacuum(fun);
    // Find the # of uses of each variable.
    // While doing so, check if all a variable's uses are dominated in a simple
    // way by a simple assign, if so, then we can assign its register to it
    // just for its definition to its last use, and not to the entire toplevel loop,
    // we call such variables "optimizable"
    var varUses = {};
    var level = 1;
    var levelDominations = {};
    var varLevels = {};
    var possibles = {};
    var unoptimizables = {};
    traverse(fun, function(node, type) {
      if (type == 'name') {
        var name = node[1];
        if (localVars[name]) {
          if (!varUses[name]) varUses[name] = 0;
          varUses[name]++;
          if (possibles[name] && !varLevels[name]) unoptimizables[name] = 1; // used outside of simple domination
        }
      } else if (type == 'assign' && typeof node[1] != 'string') {
        if (node[2] && node[2][0] == 'name') {
          var name = node[2][1];
          // if local and not yet used, this might be optimizable if we dominate
          // all other uses
          if (localVars[name] && !varUses[name] && !varLevels[name]) {
            possibles[name] = 1;
            varLevels[name] = level;
            if (!levelDominations[level]) levelDominations[level] = {};
            levelDominations[level][name] = 1;
          }
        }
      } else if (type in CONTROL_FLOW) {
        level++;
      }
    }, function(node, type) {
      if (type in CONTROL_FLOW) {
        // Invalidate all dominating on this level, further users make it unoptimizable
        for (var name in levelDominations[level]) {
          varLevels[name] = 0;
        }
        levelDominations[level] = null;
        level--;
      }
    });
    var optimizables = {};
    if (!hasSwitch) {
      for (var possible in possibles) {
        if (!unoptimizables[possible]) optimizables[possible] = 1;
      }
    }
    // Go through the function's code, assigning 'registers'.
    // The only tricky bit is to keep variables locked on a register through loops,
    // since they can potentially be returned to. Optimizable variables lock onto
    // loops that they enter, unoptimizable variables lock in a conservative way
    // into the topmost loop.
    // Note that we cannot lock onto a variable in a loop if it was used and free'd
    // before! (then they could overwrite us in the early part of the loop). For now
    // we just use a fresh register to make sure we avoid this, but it could be
    // optimized to check for safe registers (free, and not used in this loop level).
    var varRegs = {}; // maps variables to the register they will use all their life
    var freeRegs = [];
    var nextReg = 1;
    var fullNames = {};
    var loopRegs = {}; // for each loop nesting level, the list of bound variables
    var loops = 0; // 0 is toplevel, 1 is first loop, etc
    var saved = 0;
    var activeOptimizables = {};
    var optimizableLoops = {};
    function decUse(name) {
      if (!varUses[name]) return false; // no uses left, or not a relevant variable
      if (optimizables[name]) activeOptimizables[name] = 1;
      var reg = varRegs[name];
      if (!reg) {
        // acquire register
        if (optimizables[name] && freeRegs.length > 0) {
          reg = freeRegs.pop();
          saved++;
        } else {
          reg = nextReg++;
          fullNames[reg] = 'r' + reg; // TODO: even smaller names
        }
        varRegs[name] = reg;
      }
      varUses[name]--;
      assert(varUses[name] >= 0);
      if (varUses[name] == 0) {
        if (optimizables[name]) delete activeOptimizables[name];
        // If we are not in a loop, or we are optimizable and not bound to a loop
        // (we might have been in one but left it), we can free the register now.
        if (loops == 0 || (optimizables[name] && !optimizableLoops[name])) {
          // free register
          freeRegs.push(reg);
        } else {
          // when the relevant loop is exited, we will free the register
          var releventLoop = optimizables[name] ? (optimizableLoops[name] || 1) : 1;
          if (!loopRegs[releventLoop]) loopRegs[releventLoop] = [];
          loopRegs[releventLoop].push(reg);
        }
      }
      return true;
    }
    traverse(fun, function(node, type) { // XXX we rely on traversal order being the same as execution order here
      if (type == 'name') {
        var name = node[1];
        if (decUse(name)) {
          node[1] = fullNames[varRegs[name]];
        }
      } else if (type in LOOP) {
        loops++;
        // Active optimizables lock onto this loop, if not locked onto one that encloses this one
        for (var name in activeOptimizables) {
          if (!optimizableLoops[name]) {
            optimizableLoops[name] = loops;
          }
        }
      }
    }, function(node, type) {
      if (type in LOOP) {
        // Free registers that were locked to this loop
        if (loopRegs[loops]) {
          freeRegs = freeRegs.concat(loopRegs[loops]);
          loopRegs[loops] = [];
        }
        loops--;
      }
    });
    // Add vars at the beginning
    if (nextReg > 1) {
      var vars = [];
      for (var i = 1; i < nextReg; i++) {
        vars.push([fullNames[i]]);
      }
      getStatements(fun).unshift(['var', vars]);
    }
    //printErr(fun[1] + ': saved ' + saved + ' / ' + (saved + nextReg - 1) + ' vars through registerization'); // not totally accurate
  });
}

// Eliminator aka Expressionizer
//
// The goal of this pass is to eliminate unneeded variables (which represent one of the infinite registers in the LLVM
// model) and thus to generate complex expressions where possible, for example
//
//  var x = a(10);
//  var y = HEAP[20];
//  print(x+y);
//
// can be transformed into
//
//  print(a(10)+HEAP[20]);
//
// The basic principle is to scan along the code in the order of parsing/execution, and keep a list of tracked
// variables that are current contenders for elimination. We must untrack when we see something that we cannot
// cross, for example, a write to memory means we must invalidate variables that depend on reading from
// memory, since if we change the order then we do not preserve the computation.
//
// We rely on some assumptions about emscripten-generated code here, which means we can do a lot more than
// a general JS optimization can. For example, we assume that 'sub' nodes (indexing like HEAP[..]) are
// memory accesses or FUNCTION_TABLE accesses, and in both cases that the symbol cannot be replaced although
// the contents can. So we assume FUNCTION_TABLE might have its contents changed but not be pointed to
// a different object, which allows
//
//  var x = f();
//  FUNCTION_TABLE[x]();
//
// to be optimized (f could replace FUNCTION_TABLE, so in general JS eliminating x is not valid).
//
// In memSafe mode, we are more careful and assume functions can replace HEAP and FUNCTION_TABLE, which
// can happen in ALLOW_MEMORY_GROWTH mode

var ELIMINATION_SAFE_NODES = set('var', 'assign', 'call', 'if', 'toplevel', 'do', 'return'); // do is checked carefully, however
var NODES_WITHOUT_ELIMINATION_SIDE_EFFECTS = set('name', 'num', 'string', 'binary', 'sub', 'unary-prefix');
var IGNORABLE_ELIMINATOR_SCAN_NODES = set('num', 'toplevel', 'string', 'break', 'continue', 'dot'); // dot can only be STRING_TABLE.*
var ABORTING_ELIMINATOR_SCAN_NODES = set('new', 'object', 'function', 'defun', 'switch', 'for', 'while', 'array', 'throw'); // we could handle some of these, TODO, but nontrivial (e.g. for while, the condition is hit multiple times after the body)

function eliminate(ast, memSafe) {
  // Find variables that have a single use, and if they can be eliminated, do so
  traverseGeneratedFunctions(ast, function(func, type) {
    //printErr('eliminate in ' + func[1]);

    // First, find the potentially eliminatable functions: that have one definition and one use
    var definitions = {};
    var uses = {};
    var values = {};
    var locals = {};
    var varsToRemove = {}; // variables being removed, that we can eliminate all 'var x;' of
    var varsToTryToRemove = {}; // variables that have 0 uses, but have side effects - when we scan we can try to remove them
    // add arguments as locals
    if (func[2]) {
      for (var i = 0; i < func[2].length; i++) {
        locals[func[2][i]] = true;
      }
    }
    // examine body and note locals
    traverse(func, function(node, type) {
      if (type === 'var') {
        var node1 = node[1];
        for (var i = 0; i < node1.length; i++) {
          var node1i = node1[i];
          var name = node1i[0];
          var value = node1i[1];
          if (value) {
            if (!definitions[name]) definitions[name] = 0;
            definitions[name]++;
            if (!values[name]) values[name] = value;
          }
          if (!uses[name]) uses[name] = 0;
          locals[name] = true;
        }
      } else if (type === 'name') {
        var name = node[1];
        if (!uses[name]) uses[name] = 0;
        uses[name]++;
      } else if (type == 'assign') {
        var target = node[2];
        if (target[0] == 'name') {
          var name = target[1];
          if (!definitions[name]) definitions[name] = 0;
          definitions[name]++;
          if (!uses[name]) uses[name] = 0;
          if (!values[name]) values[name] = node[3];
          if (node[1] === true) { // not +=, -= etc., just =
            uses[name]--; // because the name node will show up by itself in the previous case
          }
        }
      }
    });
    var potentials = {}; // local variables with 1 definition and 1 use
    for (var name in locals) {
      if (definitions[name] == 1 && uses[name] == 1) {
        potentials[name] = 1;
      } else if (uses[name] == 0) {
        var hasSideEffects = false;
        if (values[name]) {
          traverse(values[name], function(node, type) {
            if (!(type in NODES_WITHOUT_ELIMINATION_SIDE_EFFECTS)) {
              hasSideEffects = true; // cannot remove this unused variable, constructing it has side effects
              return true;
            }
          });
        }
        if (!hasSideEffects) {
          varsToRemove[name] = 1; // remove it normally
        } else {
          varsToTryToRemove[name] = 1; // try to remove it later during scanning
        }
      }
    }
    //printErr('defs: ' + JSON.stringify(definitions));
    //printErr('uses: ' + JSON.stringify(uses));
    //printErr('locals: ' + JSON.stringify(locals));
    //printErr('varsToRemove: ' + JSON.stringify(varsToRemove));
    //printErr('2varsToTryToRemove: ' + JSON.stringify(varsToTryToRemove));
    definitions = uses = values = null;
    //printErr('potentials: ' + JSON.stringify(potentials));
    // We can now proceed through the function. In each list of statements, we try to eliminate
    var tracked = {};
    var globalsInvalidated = false; // do not repeat invalidations, until we track something new
    var memoryInvalidated = false;
    var callsInvalidated = false;
    function track(name, value, defNode) { // add a potential that has just been defined to the tracked list, we hope to eliminate it
      var usesGlobals = false, usesMemory = false, deps = {}, doesCall = false;
      var ignoreName = false; // one-time ignorings of names, as first op in sub and call
      traverse(value, function(node, type) {
        if (type == 'name') {
          if (!ignoreName) {
            var name = node[1];
            if (!(name in locals)) {
              usesGlobals = true;
            }
            if (!(name in potentials)) { // deps do not matter for potentials - they are defined once, so no complexity
              deps[name] = 1;
            }
          } else {
            ignoreName = false;
          }
        } else if (type == 'sub') {
          usesMemory = true;
          ignoreName = true;
        } else if (type == 'call') {
          usesGlobals = true;
          usesMemory = true;
          doesCall = true;        
          ignoreName = true;
        } else {
          ignoreName = false;
        }
      });
      tracked[name] = {
        usesGlobals: usesGlobals,
        usesMemory: usesMemory,
        defNode: defNode,
        deps: deps,
        doesCall: doesCall
      };
      globalsInvalidated = false;
      memoryInvalidated = false;
      callsInvalidated = false;
      //printErr('track ' + [name, JSON.stringify(tracked[name])]);
    }
    var temp = [];
    // TODO: invalidate using a sequence number for each type (if you were tracked before the last invalidation, you are cancelled). remove for.in loops
    function invalidateGlobals() {
      //printErr('invalidate globals');
      temp.length = 0;
      for (var name in tracked) {
        var info = tracked[name];
        if (info.usesGlobals) {
          temp.push(name);
        }
      }
      for (var i = 0; i < temp.length; i++) {
        delete tracked[temp[i]];
      }
    }
    function invalidateMemory() {
      //printErr('invalidate memory');
      temp.length = 0;
      for (var name in tracked) {
        var info = tracked[name];
        if (info.usesMemory) {
          temp.push(name);
        }
      }
      for (var i = 0; i < temp.length; i++) {
        delete tracked[temp[i]];
      }
    }
    function invalidateByDep(dep) {
      //printErr('invalidate by dep ' + dep);
      temp.length = 0;
      for (var name in tracked) {
        var info = tracked[name];
        if (info.deps[dep]) {
          temp.push(name);
        }
      }
      for (var i = 0; i < temp.length; i++) {
        delete tracked[temp[i]];
      }
    }
    function invalidateCalls() {
      //printErr('invalidate calls');
      temp.length = 0;
      for (var name in tracked) {
        var info = tracked[name];
        if (info.doesCall) {
          temp.push(name);
        }
      }
      for (var i = 0; i < temp.length; i++) {
        delete tracked[temp[i]];
      }
    }

    // Generate the sequence of execution. This determines what is executed before what, so we know what can be reordered. Using
    // that, performs invalidations and eliminations
    function scan(node) {
      //printErr('scan: ' + JSON.stringify(node).substr(0, 50) + ' : ' + keys(tracked));
      var abort = false;
      var allowTracking = true; // false inside an if; also prevents recursing in an if
      //var nesting = 1; // printErr-related
      function traverseInOrder(node, ignoreSub, ignoreName) {
        if (abort) return;
        //nesting++; // printErr-related
        //printErr(spaces(2*(nesting+1)) + 'trav: ' + JSON.stringify(node).substr(0, 50) + ' : ' + keys(tracked) + ' : ' + [allowTracking, ignoreSub, ignoreName]);
        var type = node[0];
        if (type == 'assign') {
          var target = node[2];
          var value = node[3];
          var nameTarget = target[0] == 'name';
          traverseInOrder(target, true, nameTarget); // evaluate left
          traverseInOrder(value); // evaluate right
          // do the actual assignment
          if (nameTarget) {
            var name = target[1];
            if (!(name in potentials)) {
              if (!(name in varsToTryToRemove)) {
                // expensive check for invalidating specific tracked vars. This list is generally quite short though, because of
                // how we just eliminate in short spans and abort when control flow happens TODO: history numbers instead
                invalidateByDep(name); // can happen more than once per dep..
                if (!(name in locals) && !globalsInvalidated) {
                  invalidateGlobals();
                  globalsInvalidated = true;
                }
              } else {
                // replace it in-place
                node.length = value.length;
                for (var i = 0; i < value.length; i++) {
                  node[i] = value[i];
                }
                varsToRemove[name] = 1;
              }
            } else {
              if (allowTracking) track(name, node[3], node);
            }
          } else if (target[0] == 'sub') {
            if (!memoryInvalidated) {
              invalidateMemory();
              memoryInvalidated = true;
            }
          }
        } else if (type == 'sub') {
          traverseInOrder(node[1], false, !memSafe); // evaluate inner
          traverseInOrder(node[2]); // evaluate outer
          if (!ignoreSub) { // ignoreSub means we are a write (happening later), not a read
            // do the memory access
            if (!callsInvalidated) {
              invalidateCalls();
              callsInvalidated = true;
            }
          }
        } else if (type == 'var') {
          var vars = node[1];
          for (var i = 0; i < vars.length; i++) {
            var name = vars[i][0];
            var value = vars[i][1];
            if (value) {
              traverseInOrder(value);
              if (name in potentials && allowTracking) {
                track(name, value, node);
              } else {
                invalidateByDep(name);
              }
              if (vars.length == 1 && name in varsToTryToRemove && value) {
                // replace it in-place
                value = ['stat', value];
                node.length = value.length;
                for (var i = 0; i < value.length; i++) {
                  node[i] = value[i];
                }
                varsToRemove[name] = 1;
              }
            }
          }
        } else if (type == 'binary') {
          var flipped = false;
          if (node[1] in ASSOCIATIVE_BINARIES && !(node[2][0] in NAME_OR_NUM) && node[3][0] in NAME_OR_NUM) { // TODO recurse here?
            // associatives like + and * can be reordered in the simple case of one of the sides being a name, since we assume they are all just numbers
            var temp = node[2];
            node[2] = node[3];
            node[3] = temp;
            flipped = true;
          }
          traverseInOrder(node[2]);
          traverseInOrder(node[3]);
          if (flipped && node[2][0] in NAME_OR_NUM) { // dunno if we optimized, but safe to flip back - and keeps the code closer to the original and more readable
            var temp = node[2];
            node[2] = node[3];
            node[3] = temp;
          }
        } else if (type == 'name') {
          if (!ignoreName) { // ignoreName means we are the name of something like a call or a sub - irrelevant for us
            var name = node[1];
            if (name in tracked) {
              doEliminate(name, node);
            } else if (!(name in locals) && !callsInvalidated) {
              invalidateCalls();
              callsInvalidated = true;
            }
          }
        } else if (type == 'unary-prefix' || type == 'unary-postfix') {
          traverseInOrder(node[2]);
        } else if (type in IGNORABLE_ELIMINATOR_SCAN_NODES) {
        } else if (type == 'call') {
          traverseInOrder(node[1], false, true);
          var args = node[2];
          for (var i = 0; i < args.length; i++) {
            traverseInOrder(args[i]);
          }
          // these two invalidations will also invalidate calls
          if (!globalsInvalidated) {
            invalidateGlobals();
            globalsInvalidated = true;
          }
          if (!memoryInvalidated) {
            invalidateMemory();
            memoryInvalidated = true;
          }
        } else if (type == 'if') {
          if (allowTracking) {
            traverseInOrder(node[1]); // can eliminate into condition, but nowhere else
            allowTracking = false;
            traverseInOrder(node[2]); // 2 and 3 could be 'parallel', really..
            if (node[3]) traverseInOrder(node[3]);
            allowTracking = true;
          } else {
            tracked = {};
            abort = true;
          }
        } else if (type == 'block') {
          var stats = node[1];
          for (var i = 0; i < stats.length; i++) {
            traverseInOrder(stats[i]);
          }
        } else if (type == 'stat') {
          traverseInOrder(node[1]);
        } else if (type == 'label') {
          traverseInOrder(node[2]);
        } else if (type == 'seq') {
          traverseInOrder(node[1]);
          traverseInOrder(node[2]);
        } else if (type == 'do') {
          if (node[1][0] == 'num' && node[1][1] == 0) { // one-time loop
            traverseInOrder(node[2]);
          } else {
            tracked = {};
            abort = true;
          }
        } else if (type == 'return') {
          if (node[1]) traverseInOrder(node[1]);
        } else if (type == 'conditional') {
          traverseInOrder(node[1]);
          traverseInOrder(node[2]);
          traverseInOrder(node[3]);
        } else {
          if (!(type in ABORTING_ELIMINATOR_SCAN_NODES)) {
            printErr('unfamiliar eliminator scan node: ' + JSON.stringify(node));
          }
          tracked = {};
          abort = true;
        }
        //nesting--; // printErr-related
      }
      traverseInOrder(node);
    }
    function doEliminate(name, node) {
      //printErr('elim!!!!! ' + name);
      // yes, eliminate!
      varsToRemove[name] = 1; // both assign and var definitions can have other vars we must clean up
      var info = tracked[name];
      delete tracked[name];
      var defNode = info.defNode;
      if (defNode[0] == 'var') {
        defNode[1].forEach(function(pair) {
          if (pair[0] == name) {
            value = pair[1];
          }
        });
        assert(value);
      } else { // assign
        value = defNode[3];
        // wipe out the assign
        defNode[0] = 'toplevel';
        defNode[1] = [];
        defNode.length = 2;
      }
      // replace this node in-place
      node.length = 0;
      for (var i = 0; i < value.length; i++) {
        node[i] = value[i];
      }
    }
    traverse(func, function(block) {
      var stats = getStatements(block);
      if (!stats) return;
      tracked = {};
      //printErr('new StatBlock');
      for (var i = 0; i < stats.length; i++) {
        var node = stats[i];
        //printErr('StatBlock[' + i + '] => ' + JSON.stringify(node));
        var type = node[0];
        if (type == 'stat') {
          node = node[1];
          type = node[0];
        }
        // Check for things that affect elimination
        if (type in ELIMINATION_SAFE_NODES) {
          scan(node);
        } else {
          tracked = {}; // not a var or assign, break all potential elimination so far
        }
      }
    });

    // clean up vars
    //printErr('cleaning up ' + JSON.stringify(varsToRemove));
    traverse(func, function(node, type) {
      if (type === 'var') {
        node[1] = node[1].filter(function(pair) { return !(pair[0] in varsToRemove) });
        if (node[1].length == 0) {
          // wipe out an empty |var;|
          node[0] = 'toplevel';
          node[1] = [];
        }
      }
    });
  });

  // A class for optimizing expressions. We know that it is legitimate to collapse
  // 5+7 in the generated code, as it will always be numerical, for example. XXX do we need this? here?
  function ExpressionOptimizer(node) {
    this.node = node;

    this.run = function() {
      traverse(this.node, function(node, type) {
        if (type === 'binary' && node[1] == '+') {
          var names = [];
          var num = 0;
          var has_num = false;
          var fail = false;
          traverse(node, function(subNode, subType) {
            if (subType === 'binary') {
              if (subNode[1] != '+') {
                fail = true;
                return false;
              }
            } else if (subType === 'name') {
              names.push(subNode[1]);
              return;
            } else if (subType === 'num') {
              num += subNode[1];
              has_num = true;
              return;
            } else {
              fail = true;
              return false;
            }
          });
          if (!fail && has_num) {
            var ret = ['num', num];
            for (var i = 0; i < names.length; i++) {
              ret = ['binary', '+', ['name', names[i]], ret];
            }
            return ret;
          }
        }
      });
    };
  }
  new ExpressionOptimizer(ast).run();
}

function eliminateMemSafe(ast) {
  eliminate(ast, true);
}

// Passes table

var compress = false, printMetadata = true;

var passes = {
  dumpAst: dumpAst,
  dumpSrc: dumpSrc,
  unGlobalize: unGlobalize,
  removeAssignsToUndefined: removeAssignsToUndefined,
  //removeUnneededLabelSettings: removeUnneededLabelSettings,
  simplifyExpressionsPre: simplifyExpressionsPre,
  optimizeShiftsConservative: optimizeShiftsConservative,
  optimizeShiftsAggressive: optimizeShiftsAggressive,
  simplifyExpressionsPost: simplifyExpressionsPost,
  hoistMultiples: hoistMultiples,
  loopOptimizer: loopOptimizer,
  registerize: registerize,
  eliminate: eliminate,
  eliminateMemSafe: eliminateMemSafe,
  compress: function() { compress = true; },
  noPrintMetadata: function() { printMetadata = false; }
};

// Main

var src = read(arguments_[0]);
var ast = srcToAst(src);
//printErr(JSON.stringify(ast)); throw 1;
var metadata = src.split('\n').filter(function(line) { return line.indexOf(GENERATED_FUNCTIONS_MARKER) >= 0 })[0];
//assert(metadata, 'Must have EMSCRIPTEN_GENERATED_FUNCTIONS metadata');
if (metadata) setGeneratedFunctions(metadata);

arguments_.slice(1).forEach(function(arg) {
  passes[arg](ast);
});

var js = astToSrc(ast, compress), old;

// remove unneeded newlines+spaces, and print
do {
  old = js;
  js = js.replace(/\n *\n/g, '\n');
} while (js != old);
print(js);
if (metadata && printMetadata) print(metadata);
print('\n');