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

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

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_IA32
 
 #include "src/crankshaft/ia32/lithium-codegen-ia32.h"
 
 #include "src/base/bits.h"
 #include "src/code-factory.h"
@@ skipping 3057 matching lines @@
 void LCodeGen::DoDeferredMathAbsTaggedHeapNumber(LMathAbs* instr) {
   Register input_reg = ToRegister(instr->value());
   __ cmp(FieldOperand(input_reg, HeapObject::kMapOffset),
          factory()->heap_number_map());
   DeoptimizeIf(not_equal, instr, Deoptimizer::kNotAHeapNumber);
 
   Label slow, allocated, done;
   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);
 
   __ mov(tmp, FieldOperand(input_reg, HeapNumber::kExponentOffset));
   // Check the sign of the argument. If the argument is positive, just
   // return it. We do not need to patch the stack since |input| and
   // |result| are the same register and |input| will be restored
   // unchanged by popping safepoint registers.
   __ test(tmp, Immediate(HeapNumber::kSignMask));
   __ j(zero, &done, Label::kNear);
@@ skipping 2221 matching lines @@
   __ bind(deferred->exit());
   __ bind(&done);
 }
 
 #undef __
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_IA32