simplifyIfs js optimizer pass

5 files changed, 160 insertions, 2 deletions

diff --git a/emcc b/emcc
index 1c57d991..3845beb1 100755
--- a/emcc
+++ b/emcc
@@ -1689,6 +1689,8 @@ try:
 
       js_optimizer_queue += ['simplifyExpressions']
 
+      if debug_level == 0 or profiling: js_optimizer_queue += ['simplifyIfs']
+
       if opt_level >= 3 and shared.Settings.PRECISE_F32: js_optimizer_queue += ['optimizeFrounds']
 
   if closure and not shared.Settings.ASM_JS:
diff --git a/tests/test_other.py b/tests/test_other.py
index 7200b9fd..a9880cf3 100644
--- a/tests/test_other.py
+++ b/tests/test_other.py
@@ -1773,6 +1773,8 @@ This pointer might make sense in another type signature: i: 0
        ['simplifyExpressions', 'optimizeShiftsConservative']),
       (path_from_root('tools', 'test-js-optimizer-t2.js'), open(path_from_root('tools', 'test-js-optimizer-t2-output.js')).read(),
        ['simplifyExpressions', 'optimizeShiftsAggressive']),
+      (path_from_root('tools', 'test-js-optimizer-si.js'), open(path_from_root('tools', 'test-js-optimizer-si-output.js')).read(),
+       ['simplifyIfs']),
       # Make sure that optimizeShifts handles functions with shift statements.
       (path_from_root('tools', 'test-js-optimizer-t3.js'), open(path_from_root('tools', 'test-js-optimizer-t3-output.js')).read(),
        ['optimizeShiftsAggressive']),
diff --git a/tools/js-optimizer.js b/tools/js-optimizer.js
index 81bf8824..7900044a 100644
--- a/tools/js-optimizer.js
+++ b/tools/js-optimizer.js
@@ -436,7 +436,7 @@ function removeUnneededLabelSettings(ast) {
   });
 }
 
-// Various expression simplifications. Pre run before closure (where we still have metadata), Post run after.
+// Various expression simplifications. Happens after elimination, which opens up many of these simplification opportunities.
 
 var USEFUL_BINARY_OPS = set('<<', '>>', '|', '&', '^');
 var COMPARE_OPS = set('<', '<=', '>', '>=', '==', '===', '!=', '!==');
@@ -819,11 +819,32 @@ function simplifyExpressions(ast) {
     simplifyIntegerConversions(func);
     simplifyBitops(func);
     joinAdditions(func);
-    // simplifyZeroComp(func); TODO: investigate performance
     simplifyNotComps(func);
+    // simplifyZeroComp(func); TODO: investigate performance
   });
 }
 
+
+function simplifyIfs(ast) {
+  traverse(ast, function(node, type) {
+    // simplify   if (x) { if (y) { .. } }   to   if (x ? y : 0) { .. }
+    if (type === 'if' && !node[3]) {
+      var body = node[2];
+      // recurse to handle chains
+      while (body[0] === 'block' && body[1].length === 1) {
+        var singleton = body[1][0];
+        if (singleton[0] === 'if' && !singleton[3]) {
+          node[1] = ['conditional', node[1], singleton[1], ['num', 0]];
+          body = node[2] = singleton[2];
+        } else {
+          break;
+        }
+      }
+    }
+  });
+}
+
+
 // 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,
@@ -5262,6 +5283,7 @@ var passes = {
   removeAssignsToUndefined: removeAssignsToUndefined,
   //removeUnneededLabelSettings: removeUnneededLabelSettings,
   simplifyExpressions: simplifyExpressions,
+  simplifyIfs: simplifyIfs,
   optimizeShiftsConservative: optimizeShiftsConservative,
   optimizeShiftsAggressive: optimizeShiftsAggressive,
   hoistMultiples: hoistMultiples,
diff --git a/tools/test-js-optimizer-si-output.js b/tools/test-js-optimizer-si-output.js
new file mode 100644
index 00000000..9a55281d
--- /dev/null
+++ b/tools/test-js-optimizer-si-output.js
@@ -0,0 +1,59 @@
+function a() {
+ if (x ? y : 0) {
+  g();
+ }
+ if (x) {
+  if (y) {
+   g();
+  } else {
+   h();
+  }
+ }
+ if (x) {
+  if (y) {
+   g();
+  }
+  h();
+ }
+ if (x) {
+  if (y) {
+   g();
+  }
+ } else {
+  h();
+ }
+ if (x) {
+  f();
+  if (y) {
+   g();
+  }
+ }
+ if ((x ? y : 0) ? z : 0) {
+  g();
+ }
+ if (x) {
+  f();
+  if (y ? z : 0) {
+   g();
+  }
+ }
+ if (x ? y : 0) {
+  f();
+  if (z) {
+   g();
+  }
+ }
+ if (x ? y : 0) {
+  if (z) {
+   g();
+  }
+  f();
+ }
+ if (x) {
+  if (y ? z : 0) {
+   g();
+  }
+  f();
+ }
+}
+
diff --git a/tools/test-js-optimizer-si.js b/tools/test-js-optimizer-si.js
new file mode 100644
index 00000000..1d214386
--- /dev/null
+++ b/tools/test-js-optimizer-si.js
@@ -0,0 +1,73 @@
+function a() {
+  if (x) {
+    if (y) {
+      g();
+    }
+  }
+  if (x) {
+    if (y) {
+      g();
+    } else {
+      h();
+    }
+  }
+  if (x) {
+    if (y) {
+      g();
+    }
+    h();
+  }
+  if (x) {
+    if (y) {
+      g();
+    }
+  } else {
+    h();
+  }
+  if (x) {
+    f();
+    if (y) {
+      g();
+    }
+  }
+  if (x) {
+    if (y) {
+      if (z) {
+        g();
+      }
+    }
+  }
+  if (x) {
+    f();
+    if (y) {
+      if (z) {
+        g();
+      }
+    }
+  }
+  if (x) {
+    if (y) {
+      f();
+      if (z) {
+        g();
+      }
+    }
+  }
+  if (x) {
+    if (y) {
+      if (z) {
+        g();
+      }
+      f();
+    }
+  }
+  if (x) {
+    if (y) {
+      if (z) {
+        g();
+      }
+    }
+    f();
+  }
+}
+