Xor optimizations.

Optimize x^-1 to ~x; this comes up because LLVM does not have a bitwise
negate operator. Optimize x&1^1 to !x; this comes up because of how LLVM
lowers C++ bool variables.

Also, add an optimization to simplifyExpressionsPre to eliminate |0 from
'~' expressions in more cases.

3 files changed, 17 insertions, 0 deletions

diff --git a/tools/js-optimizer.js b/tools/js-optimizer.js
index 0d1e8801..f69278b0 100644
--- a/tools/js-optimizer.js
+++ b/tools/js-optimizer.js
@@ -504,6 +504,8 @@ function simplifyExpressionsPre(ast) {
             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 if (type === 'unary-prefix' && node[1] === '~') {
+            stack.push(1);
           } else {
             stack.push(-1); // This node is dangerous! Give up if you see this before you see '1'
           }
@@ -554,6 +556,12 @@ function simplifyExpressionsPre(ast) {
             }
           }
         }
+      } else if (type === 'binary' && node[1] === '^') {
+        // LLVM represents bitwise not as xor with -1. Translate it back to an actual bitwise not.
+        if (node[3][0] === 'unary-prefix' && node[3][1] === '-' && node[3][2][0] === 'num' &&
+            node[3][2][1] === 1) {
+            return ['unary-prefix', '~', node[2]];
+        }
       } else if (type       === 'binary' && node[1]    === '>>' && node[3][0]    === 'num' &&
                  node[2][0] === 'binary' && node[2][1] === '<<' && node[2][3][0] === 'num' &&
                  node[2][2][0] === 'sub' && node[2][2][1][0] === 'name') {
diff --git a/tools/test-js-optimizer-asm-pre-output.js b/tools/test-js-optimizer-asm-pre-output.js
index 59a42010..d66c28a4 100644
--- a/tools/test-js-optimizer-asm-pre-output.js
+++ b/tools/test-js-optimizer-asm-pre-output.js
@@ -63,6 +63,9 @@ function b($this, $__n) {
   HEAP32[($this + 4 & 16777215) >> 2] = $40;
  }
  HEAP8[$38 + $40 & 16777215] = 0;
+ HEAP32[$4] = ~HEAP32[$5];
+ HEAP32[$4] = ~HEAP32[$5];
+ HEAP32[$4] = ~HEAP32[$5];
  return;
 }
 function rett() {
diff --git a/tools/test-js-optimizer-asm-pre.js b/tools/test-js-optimizer-asm-pre.js
index 7ca941fa..5c09043e 100644
--- a/tools/test-js-optimizer-asm-pre.js
+++ b/tools/test-js-optimizer-asm-pre.js
@@ -64,6 +64,12 @@ function b($this, $__n) {
   HEAP32[(($this + 4 | 0) & 16777215) >> 2] = $40;
  }
  HEAP8[($38 + $40 | 0) & 16777215] = 0;
+ // Eliminate the |0.
+ HEAP32[$4] = ((~(HEAP32[$5]|0))|0);
+ // Rewrite to ~.
+ HEAP32[$4] = HEAP32[$5]^-1;
+ // Rewrite to ~ and eliminate the |0.
+ HEAP32[$4] = ((HEAP32[$5]|0)^-1)|0;
  return;
 }
 function rett() {