Reland "[turbofan] simplify gap ordering"

src/compiler/register-allocator.cc

Index: src/compiler/register-allocator.cc
diff --git a/src/compiler/register-allocator.cc b/src/compiler/register-allocator.cc
index 7def50e47aa069572228668614bcee6510ea9798..e0487fa8e3b1693e2ca5ea201b9f72635fb639cf 100644
--- a/src/compiler/register-allocator.cc
+++ b/src/compiler/register-allocator.cc
@@ -1071,7 +1071,7 @@ void RegisterAllocator::MeetRegisterConstraintsForLastInstructionInBlock(
       for (auto succ : block->successors()) {
         const InstructionBlock* successor = code()->InstructionBlockAt(succ);
         DCHECK(successor->PredecessorCount() == 1);
-        int gap_index = successor->first_instruction_index() + 1;
+        int gap_index = successor->first_instruction_index();
         DCHECK(code()->IsGapAt(gap_index));
 
         // Create an unconstrained operand for the same virtual register
@@ -1088,7 +1088,7 @@ void RegisterAllocator::MeetRegisterConstraintsForLastInstructionInBlock(
       for (auto succ : block->successors()) {
         const InstructionBlock* successor = code()->InstructionBlockAt(succ);
         DCHECK(successor->PredecessorCount() == 1);
-        int gap_index = successor->first_instruction_index() + 1;
+        int gap_index = successor->first_instruction_index();
         range->SpillAtDefinition(local_zone(), gap_index, output);
         range->SetSpillStartIndex(gap_index);
       }
@@ -1398,7 +1398,7 @@ ParallelMove* RegisterAllocator::GetConnectingParallelMove(
   }
   int gap_pos = pos.IsInstructionStart() ? (index - 1) : (index + 1);
   return code()->GapAt(gap_pos)->GetOrCreateParallelMove(
-      (gap_pos < index) ? GapInstruction::AFTER : GapInstruction::BEFORE,
+      (gap_pos < index) ? GapInstruction::AFTER : GapInstruction::START,
       code_zone());
 }
 
@@ -2325,7 +2325,8 @@ void RegisterAllocator::SplitAndSpillIntersecting(LiveRange* current) {
 
 bool RegisterAllocator::IsBlockBoundary(LifetimePosition pos) {
   return pos.IsInstructionStart() &&
-         InstructionAt(pos.InstructionIndex())->IsBlockStart();
+         code()->GetInstructionBlock(pos.InstructionIndex())->code_start() ==
+             pos.InstructionIndex();
 }
 
 
@@ -2420,9 +2421,13 @@ void RegisterAllocator::SpillBetweenUntil(LiveRange* range,
     // The split result intersects with [start, end[.
     // Split it at position between ]start+1, end[, spill the middle part
     // and put the rest to unhandled.
+    auto third_part_end = end.PrevInstruction().InstructionEnd();
+    if (IsBlockBoundary(end.InstructionStart())) {
+      third_part_end = end.InstructionStart();
+    }
     auto third_part = SplitBetween(
         second_part, Max(second_part->Start().InstructionEnd(), until),
-        end.PrevInstruction().InstructionEnd());
+        third_part_end);
 
     DCHECK(third_part != second_part);