Ensure UseRegisterAtStart not used with fixed temp/return register

src/x64/lithium-x64.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 861 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 1565 matching lines @@
 LInstruction* LChunkBuilder::DoLoadFieldByIndex(HLoadFieldByIndex* instr) {
   LOperand* object = UseRegister(instr->object());
   LOperand* index = UseTempRegister(instr->index());
   return DefineSameAsFirst(new(zone()) LLoadFieldByIndex(object, index));
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64