MIPS: Ensure UseRegisterAtStart not used with fixed temp/return register

src/mips/lithium-mips.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 853 matching lines @@
 }
 
 
 void LChunkBuilder::VisitInstruction(HInstruction* current) {
   HInstruction* old_current = current_instruction_;
   current_instruction_ = current;
   if (current->has_position()) position_ = current->position();
   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);
     }
     if (FLAG_stress_environments && !instr->HasEnvironment()) {
       instr = AssignEnvironment(instr);
     }
     instr->set_hydrogen_value(current);
     chunk_->AddInstruction(instr, current_block_);
   }
   current_instruction_ = old_current;
@@ skipping 225 matching lines @@
     LMathExp* result = new(zone()) LMathExp(input, double_temp, temp1, temp2);
     return DefineAsRegister(result);
   } else if (op == kMathPowHalf) {
     // Input cannot be the same as the result.
     // See lithium-codegen-mips.cc::DoMathPowHalf.
     LOperand* input = UseFixedDouble(instr->value(), f8);
     LOperand* temp = FixedTemp(f6);
     LUnaryMathOperation* result = new(zone()) LUnaryMathOperation(input, temp);
     return DefineFixedDouble(result, f4);
   } else {
-    LOperand* input = UseRegisterAtStart(instr->value());
+    LOperand* input = UseRegister(instr->value());
 
     LOperand* temp = (op == kMathRound) ? FixedTemp(f6) :
         (op == kMathFloor) ? TempRegister() : NULL;
     LUnaryMathOperation* result = new(zone()) LUnaryMathOperation(input, temp);
     switch (op) {
       case kMathAbs:
         return AssignEnvironment(AssignPointerMap(DefineAsRegister(result)));
       case kMathFloor:
         return AssignEnvironment(AssignPointerMap(DefineAsRegister(result)));
       case kMathSqrt:
@@ skipping 554 matching lines @@
   Representation from = instr->from();
   Representation to = instr->to();
   if (from.IsTagged()) {
     if (to.IsDouble()) {
       info()->MarkAsDeferredCalling();
       LOperand* value = UseRegister(instr->value());
       LNumberUntagD* res = new(zone()) LNumberUntagD(value);
       return AssignEnvironment(DefineAsRegister(res));
     } else {
       ASSERT(to.IsInteger32());
-      LOperand* value = UseRegisterAtStart(instr->value());
+      LOperand* value = NULL;
       LInstruction* res = NULL;
       if (instr->value()->type().IsSmi()) {
+        value = UseRegisterAtStart(instr->value());
         res = DefineAsRegister(new(zone()) LSmiUntag(value, false));
       } else {
+        value = UseRegister(instr->value());
         LOperand* temp1 = TempRegister();
         LOperand* temp2 = instr->CanTruncateToInt32() ? TempRegister()
                                                       : NULL;
         LOperand* temp3 = FixedTemp(f22);
         res = DefineSameAsFirst(new(zone()) LTaggedToI(value,
                                                        temp1,
                                                        temp2,
                                                        temp3));
         res = AssignEnvironment(res);
       }
@@ skipping 701 matching lines @@
 
 
 LInstruction* LChunkBuilder::DoLoadFieldByIndex(HLoadFieldByIndex* instr) {
   LOperand* object = UseRegister(instr->object());
   LOperand* index = UseRegister(instr->index());
   return DefineAsRegister(new(zone()) LLoadFieldByIndex(object, index));
 }
 
 
 } }  // namespace v8::internal