[turbofan] Fix phi-hinting problem with 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 2166 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;
+    // Select the hint from the first predecessor block that preceeds this block
+    // in the rpo ordering. Prefer non-deferred blocks. The enforcement of
+    // hinting in rpo order is required because hint resolution that happens
+    // later in the compiler pipeline visits instructions in reverse rpo,
+    // relying on the fact that phis are encountered before their hints.
+    const Instruction* instr = nullptr;
     const InstructionBlock::Predecessors& predecessors = block->predecessors();
-    const InstructionBlock* predecessor_block =
-        code()->InstructionBlockAt(predecessors[0]);
-    const Instruction* instr = GetLastInstruction(code(), predecessor_block);
-    if (predecessor_block->IsDeferred()) {
-      // "Prefer the hint from the first non-deferred predecessor, if any.
-      for (size_t i = 1; i < predecessors.size(); ++i) {
-        predecessor_block = code()->InstructionBlockAt(predecessors[i]);
-        if (!predecessor_block->IsDeferred()) {
-          instr = GetLastInstruction(code(), predecessor_block);
-          break;
-        }
+    for (size_t i = 0; i < predecessors.size(); ++i) {
+      const InstructionBlock* predecessor_block =
+          code()->InstructionBlockAt(predecessors[i]);
+      if (predecessor_block->rpo_number() < block->rpo_number()) {
+        instr = GetLastInstruction(code(), predecessor_block);
+        if (!predecessor_block->IsDeferred()) break;
       }
     }
     DCHECK_NOT_NULL(instr);
 
+    InstructionOperand* hint = nullptr;
     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 1443 matching lines @@
         }
       }
     }
   }
 }
 
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8