Ensure UseRegisterAtStart not used with fixed temp/return register

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
diff --git a/src/ia32/lithium-ia32.cc b/src/ia32/lithium-ia32.cc
index f2aec99770342caa126c6e5f3c33a9e95dafdf57..0ec5d71acc3df8d19980a4ee47c4f01abc025360 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -923,6 +923,35 @@ void LChunkBuilder::VisitInstruction(HInstruction* current) {
   LInstruction* instr = current->CompileToLithium(this);
 
   if (instr != NULL) {
+#if DEBUG
+    // Make sure that the lithium instruction has either no fixed register
+    // constraints in temps or the result OR no uses that are only used at
+    // start. If this invariant doesn't hold, the register allocator can decide
+    // to insert a split of a range immediately before the instruction due to an
+    // already allocated register needing to be used for the instruction's fixed
+    // register constraint. In this case, The register allocator won't see an
+    // interference between the split child and the use-at-start (it would if
+    // the it was just a plain use), so it is free to move the split child into
+    // the same register that is used for the use-at-start.
+    // See https://code.google.com/p/chromium/issues/detail?id=201590
+    if (!(instr->ClobbersRegisters() && instr->ClobbersDoubleRegisters())) {
+      int fixed = 0;
+      int used_at_start = 0;
+      for (UseIterator it(instr); !it.Done(); it.Advance()) {
+        LUnallocated* operand = LUnallocated::cast(it.Current());
+        if (operand->IsUsedAtStart()) ++used_at_start;
+      }
+      if (instr->Output() != NULL) {
+        if (LUnallocated::cast(instr->Output())->HasFixedPolicy()) ++fixed;
+      }
+      for (TempIterator it(instr); !it.Done(); it.Advance()) {
+        LUnallocated* operand = LUnallocated::cast(it.Current());
+        if (operand->HasFixedPolicy()) ++fixed;
+      }
+      ASSERT(fixed == 0 || used_at_start == 0);
+    }
+#endif
+
     if (FLAG_stress_pointer_maps && !instr->HasPointerMap()) {
       instr = AssignPointerMap(instr);
     }
@@ -1182,16 +1211,20 @@ LInstruction* LChunkBuilder::DoUnaryMathOperation(HUnaryMathOperation* instr) {
                                                                   input);
     return MarkAsCall(DefineFixedDouble(result, xmm1), instr);
   } else {
-    LOperand* input = UseRegisterAtStart(instr->value());
     LOperand* context = UseAny(instr->context());  // Deferred use by MathAbs.
+    LOperand* input = NULL;
     if (op == kMathPowHalf) {
+      input = UseRegisterAtStart(instr->value());
       LOperand* temp = TempRegister();
       LMathPowHalf* result = new(zone()) LMathPowHalf(context, input, temp);
       return DefineSameAsFirst(result);
     } else if (op == kMathRound) {
+      input = UseRegister(instr->value());
       LOperand* temp = FixedTemp(xmm4);
       LMathRound* result = new(zone()) LMathRound(context, input, temp);
       return AssignEnvironment(DefineAsRegister(result));
+    } else {
+      input = UseRegisterAtStart(instr->value());
     }
     LUnaryMathOperation* result = new(zone()) LUnaryMathOperation(context,
                                                                   input);