use Math.imul for precise 32-bit mul

2 files changed, 13 insertions, 4 deletions

diff --git a/src/parseTools.js b/src/parseTools.js
index 514d74d0..e6138fc1 100644
--- a/src/parseTools.js
+++ b/src/parseTools.js
@@ -2129,9 +2129,8 @@ function processMathop(item) {
     case 'sub': return handleOverflow(getFastValue(idents[0], '-', idents[1], item.type), bits);
     case 'sdiv': case 'udiv': return makeRounding(getFastValue(idents[0], '/', idents[1], item.type), bits, op[0] === 's');
     case 'mul': {
-      if (bits == 32 && PRECISE_I32_MUL) {
-        Types.preciseI64MathUsed = true;
-        return '(i64Math' + (ASM_JS ? '_' : '.') + 'multiply(' + asmCoercion(idents[0], 'i32') + ',0,' + asmCoercion(idents[1], 'i32') + ',0),' + makeGetValue('tempDoublePtr', 0, 'i32') + ')';
+      if (bits == 32) {
+        return 'Math.imul(' + idents[0] + ',' + idents[1] + ')';
       } else {
         return '((' +getFastValue(idents[0], '*', idents[1], item.type) + ')&-1)'; // force a non-eliminatable coercion here, to prevent a double result from leaking
       }
diff --git a/src/preamble.js b/src/preamble.js
index b355f130..05269c6e 100644
--- a/src/preamble.js
+++ b/src/preamble.js
@@ -742,7 +742,17 @@ Module['writeArrayToMemory'] = writeArrayToMemory;
 {{{ unSign }}}
 {{{ reSign }}}
 
-if (!Math.imul) Math.imul = function(x, y) { return (x*y)|0 }; // # not a real polyfill since semantics not identical, but close and fast
+if (!Math.imul) Math.imul = function(a, b) {
+#if PRECISE_I32_MUL
+  var ah  = a >>> 16;
+  var al = a & 0xffff;
+  var bh  = b >>> 16;
+  var bl = b & 0xffff;
+  return (al*bl + ((ah*bl + al*bh) << 16))|0;
+#else
+  return (a*b)|0; // fast but imprecise
+#endif
+};
 
 // A counter of dependencies for calling run(). If we need to
 // do asynchronous work before running, increment this and