[turbofan] don't take hints from deferred blocks

src/compiler/register-allocator.cc

 // Copyright 2014 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.
 
 #include "src/base/adapters.h"
 #include "src/compiler/linkage.h"
 #include "src/compiler/register-allocator.h"
 #include "src/string-stream.h"
 
 namespace v8 {
@@ skipping 2179 matching lines @@
 
 
 void LiveRangeBuilder::ProcessPhis(const InstructionBlock* block,
                                    BitVector* live) {
   for (PhiInstruction* phi : block->phis()) {
     // The live range interval already ends at the first instruction of the
     // block.
     int phi_vreg = phi->virtual_register();
     live->Remove(phi_vreg);
     InstructionOperand* hint = nullptr;
-    Instruction* instr = GetLastInstruction(
-        code(), code()->InstructionBlockAt(block->predecessors()[0]));
+    const InstructionBlock::Predecessors& predecessors = block->predecessors();

[Jarin, 2016/02/25 11:13:42]

Could you add a comment with an explanation here? ("Take the hint from some
non-deferred predecessor." or some such)

[Mircea Trofin, 2016/02/25 15:22:52]

Done.

+    const InstructionBlock* predecessor_block =
+        code()->InstructionBlockAt(predecessors[0]);
+    const Instruction* instr = GetLastInstruction(code(), predecessor_block);
+    if (predecessor_block->IsDeferred()) {
+      for (size_t i = 1; i < predecessors.size(); ++i) {

[titzer, 2016/02/25 12:14:42]

Could also simplify to

for (size_t i = 0; i < predecessors.size(); ++i) {
  if (!code()->InstructionBlockAt(predecessors[i])->IsDeferred()) { instr = ...
; break; }
}

[Mircea Trofin, 2016/02/25 15:22:51]

Done.

+        predecessor_block = code()->InstructionBlockAt(predecessors[i]);
+        if (predecessor_block->IsDeferred()) continue;
+
+        instr = GetLastInstruction(code(), predecessor_block);
+        break;
+      }
+    }
+    DCHECK_NOT_NULL(instr);
+
     for (MoveOperands* move : *instr->GetParallelMove(Instruction::END)) {
       InstructionOperand& to = move->destination();
       if (to.IsUnallocated() &&
           UnallocatedOperand::cast(to).virtual_register() == phi_vreg) {
         hint = &move->source();
         break;
       }
     }
     DCHECK(hint != nullptr);
     LifetimePosition block_start = LifetimePosition::GapFromInstructionIndex(
@@ skipping 1368 matching lines @@
                            spill_operand);
       }
     }
   }
 }
 
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8