Optimize (x<y)&1

Add an optimization to simplifyExpressionsPre to replace (x<y)&1 with x<y if possible. This comes up frequently in C++ with bool variables.
emscripten-core · Jul 2, 2013 · 0a2260a · 0a2260a
1 parent 01708b7
commit 0a2260a
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Showing 3 changed files with 37 additions and 3 deletions.
diff --git a/tools/js-optimizer.js b/tools/js-optimizer.js
@@ -405,13 +405,16 @@ function removeUnneededLabelSettings(ast) {
 // Various expression simplifications. Pre run before closure (where we still have metadata), Post run after.
 
 var USEFUL_BINARY_OPS = set('<<', '>>', '|', '&', '^');
+var COMPARE_OPS = set('<', '<=', '>', '>=', '==', '===', '!=');
 
 function simplifyExpressionsPre(ast) {
-  // Look for (x&A)<<B>>B and replace it with X&A if possible.
-  function simplifySignExtends(ast) {
+  // Simplify common expressions used to perform integer conversion operations
+  // in cases where no conversion is needed.
+  function simplifyIntegerConversions(ast) {
     traverse(ast, function(node, type) {
       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]) {
+        // Transform (x&A)<<B>>B to X&A.
         var innerNode = node[2][2];
         var shifts = node[3][1];
         if (innerNode[0] === 'binary' && innerNode[1] === '&' && innerNode[3][0] === 'num') {
@@ -420,6 +423,15 @@ function simplifyExpressionsPre(ast) {
             return innerNode;
           }
         }
+      } else if (type === 'binary' && node[1] === '&' && node[3][0] === 'num') {
+        // Rewrite (X < Y) & 1 to (X < Y)|0. (Subsequent passes will eliminate
+        // the |0 if possible.)
+        var input = node[2];
+        var amount = node[3][1];
+        if (input[0] === 'binary' && (input[1] in COMPARE_OPS) && amount == 1) {
+          node[1] = '|';
+          node[3][1] = 0;
+        }
       }
     });
   }
@@ -763,7 +775,7 @@ function simplifyExpressionsPre(ast) {
   }
 
   traverseGeneratedFunctions(ast, function(func) {
-    simplifySignExtends(func);
+    simplifyIntegerConversions(func);
     simplifyBitops(func);
     joinAdditions(func);
     // simplifyZeroComp(func); TODO: investigate performance

diff --git a/tools/test-js-optimizer-asm-pre-output.js b/tools/test-js-optimizer-asm-pre-output.js
@@ -119,6 +119,16 @@ function sign_extension_simplification() {
   print(5);
  }
 }
+function compare_result_simplification() {
+ HEAP32[$4] = HEAP32[$5] < HEAP32[$6];
+ HEAP32[$4] = HEAP32[$5] > HEAP32[$6];
+ HEAP32[$4] = HEAP32[$5] <= HEAP32[$6];
+ HEAP32[$4] = HEAP32[$5] <= HEAP32[$6];
+ HEAP32[$4] = HEAP32[$5] == HEAP32[$6];
+ HEAP32[$4] = HEAP32[$5] === HEAP32[$6];
+ HEAP32[$4] = HEAP32[$5] != HEAP32[$6];
+ var x = HEAP32[$5] != HEAP32[$6] | 0;
+}
 function tempDoublePtr($45, $14, $28, $42) {
  $45 = $45 | 0;
  $14 = $14 | 0;

diff --git a/tools/test-js-optimizer-asm-pre.js b/tools/test-js-optimizer-asm-pre.js
@@ -122,6 +122,18 @@ function sign_extension_simplification() {
   print(5);
  }
 }
+function compare_result_simplification() {
+ // Eliminate these '&1's.
+ HEAP32[$4] = (HEAP32[$5] < HEAP32[$6]) & 1;
+ HEAP32[$4] = (HEAP32[$5] > HEAP32[$6]) & 1;
+ HEAP32[$4] = (HEAP32[$5] <= HEAP32[$6]) & 1;
+ HEAP32[$4] = (HEAP32[$5] <= HEAP32[$6]) & 1;
+ HEAP32[$4] = (HEAP32[$5] == HEAP32[$6]) & 1;
+ HEAP32[$4] = (HEAP32[$5] === HEAP32[$6]) & 1;
+ HEAP32[$4] = (HEAP32[$5] != HEAP32[$6]) & 1;
+ // Convert the &1 to |0 here, since we don't get an implicit coersion.
+ var x = (HEAP32[$5] != HEAP32[$6]) & 1;
+}
 function tempDoublePtr($45, $14, $28, $42) {
  $45 = $45 | 0;
  $14 = $14 | 0;