Optimize (x&A)<<B>>B.

Add an optimization to simplifyExpressionsPre to replace (x&A)<<B>>B with X&A
if possible. This comes up frequently in C++ with bool variables.

3 files changed, 49 insertions, 3 deletions

diff --git a/tools/js-optimizer.js b/tools/js-optimizer.js
index 32ed1cce..5e7a2448 100644
--- a/tools/js-optimizer.js
+++ b/tools/js-optimizer.js
@@ -403,6 +403,23 @@ function removeUnneededLabelSettings(ast) {
 // Various expression simplifications. Pre run before closure (where we still have metadata), Post run after.
 
 function simplifyExpressionsPre(ast) {
+  // Look for (x&A)<<B>>B and replace it with X&A if possible.
+  function simplifySignExtends(ast) {
+    traverseGenerated(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]) {
+        var innerNode = node[2][2];
+        var shifts = node[3][1];
+        if (innerNode[0] == 'binary' && innerNode[1] == '&' && innerNode[3][0] == 'num') {
+          var mask = innerNode[3][1];
+          if (mask << shifts >> shifts == mask) {
+            return innerNode;
+          }
+        }
+      }
+    });
+  }
+
   // 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?
@@ -592,6 +609,7 @@ function simplifyExpressionsPre(ast) {
     });
   }
 
+  simplifySignExtends(ast);
   simplifyBitops(ast);
   joinAdditions(ast);
   // simplifyZeroComp(ast); TODO: investigate performance
diff --git a/tools/test-js-optimizer-asm-pre-output.js b/tools/test-js-optimizer-asm-pre-output.js
index 72608aa8..ab953e5d 100644
--- a/tools/test-js-optimizer-asm-pre-output.js
+++ b/tools/test-js-optimizer-asm-pre-output.js
@@ -18,7 +18,7 @@ function b($this, $__n) {
   }
   $4 = $this;
   $5 = HEAP8[$4 & 16777215] | 0;
-  if (($5 & 1) << 24 >> 24 == 0) {
+  if (($5 & 1) == 0) {
     $14 = 10;
     $13 = $5;
   } else {
@@ -38,14 +38,14 @@ function b($this, $__n) {
   } else {
     $30 = $13;
   }
-  if (($30 & 1) << 24 >> 24 == 0) {
+  if (($30 & 1) == 0) {
     $38 = $this + 1 | 0;
   } else {
     $38 = HEAP32[($this + 8 & 16777215) >> 2] | 0;
   }
   _memset($38 + $23 | 0, 0, $__n | 0, 1, 1213141516);
   $40 = $23 + $__n | 0;
-  if ((HEAP8[$4 & 16777215] & 1) << 24 >> 24 == 0) {
+  if ((HEAP8[$4 & 16777215] & 1) == 0) {
     HEAP8[$4 & 16777215] = $40 << 1 & 255;
   } else {
     HEAP32[($this + 4 & 16777215) >> 2] = $40;
@@ -93,4 +93,18 @@ function i32_8() {
     print(5);
   }
 }
+function sign_extension_simplification() {
+  if ((HEAP8[$4 & 16777215] & 127) == 0) {
+    print(5);
+  }
+  if ((HEAP8[$4 & 16777215] & 128) << 24 >> 24 == 0) {
+    print(5);
+  }
+  if ((HEAP32[$5 & 16777215] & 32767) == 0) {
+    print(5);
+  }
+  if ((HEAP32[$5 & 16777215] & 32768) << 16 >> 16 == 0) {
+    print(5);
+  }
+}
 
diff --git a/tools/test-js-optimizer-asm-pre.js b/tools/test-js-optimizer-asm-pre.js
index f2ffaef4..264587d2 100644
--- a/tools/test-js-optimizer-asm-pre.js
+++ b/tools/test-js-optimizer-asm-pre.js
@@ -95,4 +95,18 @@ function i32_8() {
     print(5);
   }
 }
+function sign_extension_simplification() {
+  if ((HEAP8[$4 & 16777215] & 127) << 24 >> 24 == 0) {
+    print(5);
+  }
+  if ((HEAP8[$4 & 16777215] & 128) << 24 >> 24 == 0) {
+    print(5);
+  }
+  if ((HEAP32[$5 & 16777215] & 32767) << 16 >> 16 == 0) {
+    print(5);
+  }
+  if ((HEAP32[$5 & 16777215] & 32768) << 16 >> 16 == 0) {
+    print(5);
+  }
+}
 // EMSCRIPTEN_GENERATED_FUNCTIONS: ["a", "b", "rett", "ret2t", "retf", "i32_8"]