[crankshaft] Fix DoDeferredMathAbsTaggedHeapNumber overwriting the context with some temporary valu…

src/crankshaft/ia32/lithium-codegen-ia32.cc

Index: src/crankshaft/ia32/lithium-codegen-ia32.cc
diff --git a/src/crankshaft/ia32/lithium-codegen-ia32.cc b/src/crankshaft/ia32/lithium-codegen-ia32.cc
index 00761c4857a6467208403bd16cb59253dc4fec50..f806e462f0edd04858c9515ec0cd9949cea01122 100644
--- a/src/crankshaft/ia32/lithium-codegen-ia32.cc
+++ b/src/crankshaft/ia32/lithium-codegen-ia32.cc
@@ -3075,6 +3075,17 @@ void LCodeGen::DoDeferredMathAbsTaggedHeapNumber(LMathAbs* instr) {
   Register tmp = input_reg.is(eax) ? ecx : eax;
   Register tmp2 = tmp.is(ecx) ? edx : input_reg.is(ecx) ? edx : ecx;
 
+  // Make sure that the context isn't overwritten in the AllocateHeapNumber
+  // macro below.
+  if (instr->context()->IsRegister()) {
+    Register context_reg = ToRegister(instr->context());
+    if (tmp.is(context_reg)) {
+      tmp = ebx;
+    } else if (tmp2.is(context_reg)) {
+      tmp2 = ebx;
+    }
+  }

[Jarin, 2016/06/03 13:54:56]

It is not obvious that tmp/tmp2 cannot alias the input_reg after this exercise.

Could we DCHECK that here (and the other ports) for documentation?

[epertoso, 2016/06/03 15:39:06]

Changed it a bit. Feel free to Commit if you still like it.

+
   // Preserve the value of all registers.
   PushSafepointRegistersScope scope(this);