[Liftoff][mips] Clean up "half stack slot" accesses

Bring mips in line with the ia32 and arm implementions. Instead of
computing the "half index" manually, pass the index and an enum
pointing to either half.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Id8812114df84fbddd4aa2570306526d291bface6
Reviewed-on: https://chromium-review.googlesource.com/c/1392188
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58514}

src/wasm/baseline/mips/liftoff-assembler-mips.h

@@ -39,6 +39,12 @@ inline MemOperand GetHalfStackSlot(uint32_t half_index) {
   return MemOperand(fp, -kFirstStackSlotOffset - offset);
 }
 
+inline MemOperand GetHalfStackSlot(uint32_t index, RegPairHalf half) {
+  STATIC_ASSERT(kLowWord == 0);
+  STATIC_ASSERT(kHighWord == 1);
+  return GetHalfStackSlot(2 * index + half);
+}
+
 inline MemOperand GetInstanceOperand() { return MemOperand(fp, -8); }
 
 inline void Load(LiftoffAssembler* assm, LiftoffRegister dst, Register base,
@@ -513,8 +519,8 @@ void LiftoffAssembler::Spill(uint32_t index, LiftoffRegister reg,
       sw(reg.gp(), dst);
       break;
     case kWasmI64:
-      sw(reg.low_gp(), dst);
-      sw(reg.high_gp(), liftoff::GetHalfStackSlot(2 * index + 1));
+      sw(reg.low_gp(), liftoff::GetHalfStackSlot(index, kLowWord));
+      sw(reg.high_gp(), liftoff::GetHalfStackSlot(index, kHighWord));
       break;
     case kWasmF32:
       swc1(reg.fp(), dst);
@@ -545,8 +551,8 @@ void LiftoffAssembler::Spill(uint32_t index, WasmValue value) {
       TurboAssembler::li(tmp.low_gp(), Operand(low_word));
       TurboAssembler::li(tmp.high_gp(), Operand(high_word));
 
-      sw(tmp.low_gp(), dst);
-      sw(tmp.high_gp(), liftoff::GetHalfStackSlot(2 * index + 1));
+      sw(tmp.low_gp(), liftoff::GetHalfStackSlot(index, kLowWord));
+      sw(tmp.high_gp(), liftoff::GetHalfStackSlot(index, kHighWord));
       break;
     }
     default:
@@ -564,8 +570,8 @@ void LiftoffAssembler::Fill(LiftoffRegister reg, uint32_t index,
       lw(reg.gp(), src);
       break;
     case kWasmI64:
-      lw(reg.low_gp(), src);
-      lw(reg.high_gp(), liftoff::GetHalfStackSlot(2 * index + 1));
+      lw(reg.low_gp(), liftoff::GetHalfStackSlot(index, kLowWord));
+      lw(reg.high_gp(), liftoff::GetHalfStackSlot(index, kHighWord));
       break;
     case kWasmF32:
       lwc1(reg.fp(), src);
@@ -1453,12 +1459,11 @@ void LiftoffStackSlots::Construct() {
         if (src.type() == kWasmF64) {
           DCHECK_EQ(kLowWord, slot.half_);
           asm_->lw(kScratchReg,
-                   liftoff::GetHalfStackSlot(2 * slot.src_index_ - 1));
+                   liftoff::GetHalfStackSlot(slot.src_index_, kHighWord));
           asm_->push(kScratchReg);
         }
         asm_->lw(kScratchReg,
-                 liftoff::GetHalfStackSlot(2 * slot.src_index_ +
-                                           (slot.half_ == kLowWord ? 0 : 1)));
+                 liftoff::GetHalfStackSlot(slot.src_index_, slot.half_));
         asm_->push(kScratchReg);
         break;
       }