[Liftoff] Remove redundant stack_base field

This information is already stored in ControlBase::stack_depth.

R=titzer@chromium.org

Bug: v8:8423
Change-Id: Ie707878b5d7ee5180e44881086d05a590c0dea21
Reviewed-on: https://chromium-review.googlesource.com/c/1373786Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58199}

src/wasm/baseline/liftoff-assembler.cc

@@ -268,13 +268,15 @@ class StackTransferRecipe {
 // TODO(clemensh): Don't copy the full parent state (this makes us N^2).
 void LiftoffAssembler::CacheState::InitMerge(const CacheState& source,
                                              uint32_t num_locals,
-                                             uint32_t arity) {
+                                             uint32_t arity,
+                                             uint32_t stack_depth) {
+  uint32_t stack_base = stack_depth + num_locals;
   DCHECK(stack_state.empty());
   DCHECK_GE(source.stack_height(), stack_base);
   stack_state.resize(stack_base + arity, VarState(kWasmStmt));
 
-  // |------locals------|--(in between)--|--(discarded)--|----merge----|
-  //  <-- num_locals -->                 ^stack_base      <-- arity -->
+  // |------locals------|---(in between)----|--(discarded)--|----merge----|
+  //  <-- num_locals --> <-- stack_depth -->^stack_base      <-- arity -->
 
   // First, initialize merge slots and locals. Keep them in the registers which
   // are being used in {source}, but avoid using a register multiple times. Use

src/wasm/baseline/liftoff-assembler.h

@@ -121,8 +121,6 @@ class LiftoffAssembler : public TurboAssembler {
     LiftoffRegList used_registers;
     uint32_t register_use_count[kAfterMaxLiftoffRegCode] = {0};
     LiftoffRegList last_spilled_regs;
-    // TODO(clemensh): Remove stack_base; use ControlBase::stack_depth.
-    uint32_t stack_base = 0;
 
     bool has_unused_register(RegClass rc, LiftoffRegList pinned = {}) const {
       if (kNeedI64RegPair && rc == kGpRegPair) {
@@ -232,7 +230,7 @@ class LiftoffAssembler : public TurboAssembler {
 
     // TODO(clemensh): Don't copy the full parent state (this makes us N^2).
     void InitMerge(const CacheState& source, uint32_t num_locals,
-                   uint32_t arity);
+                   uint32_t arity, uint32_t stack_depth);
 
     void Steal(CacheState& source);
 

src/wasm/baseline/liftoff-compiler.cc

@@ -371,7 +371,6 @@ class LiftoffCompiler {
           UNIMPLEMENTED();
       }
     }
-    block->label_state.stack_base = __ num_locals();
 
     // The function-prologue stack check is associated with position 0, which
     // is never a position of any instruction in the function.
@@ -446,13 +445,9 @@ class LiftoffCompiler {
     DEBUG_CODE_COMMENT(WasmOpcodes::OpcodeName(opcode));
   }
 
-  void Block(FullDecoder* decoder, Control* block) {
-    block->label_state.stack_base = __ cache_state()->stack_height();
-  }
+  void Block(FullDecoder* decoder, Control* block) {}
 
   void Loop(FullDecoder* decoder, Control* loop) {
-    loop->label_state.stack_base = __ cache_state()->stack_height();
-
     // Before entering a loop, spill all locals to the stack, in order to free
     // the cache registers, and to avoid unnecessarily reloading stack values
     // into registers at branches.
@@ -489,7 +484,6 @@ class LiftoffCompiler {
     __ emit_cond_jump(kEqual, if_block->else_state->label.get(), kWasmI32,
                       value);
 
-    if_block->label_state.stack_base = __ cache_state()->stack_height();
     // Store the state (after popping the value) for executing the else branch.
     if_block->else_state->state.Split(*__ cache_state());
   }
@@ -501,7 +495,8 @@ class LiftoffCompiler {
       // Init the merge point from the else state, then merge the if state into
       // that.
       DCHECK_EQ(0, c->end_merge.arity);
-      c->label_state.InitMerge(c->else_state->state, __ num_locals(), 0);
+      c->label_state.InitMerge(c->else_state->state, __ num_locals(), 0,
+                               c->stack_depth);
       __ MergeFullStackWith(c->label_state);
     } else {
       c->label_state.Split(*__ cache_state());
@@ -1241,7 +1236,8 @@ class LiftoffCompiler {
   void BrImpl(Control* target) {
     if (!target->br_merge()->reached) {
       target->label_state.InitMerge(*__ cache_state(), __ num_locals(),
-                                    target->br_merge()->arity);
+                                    target->br_merge()->arity,
+                                    target->stack_depth);
     }
     __ MergeStackWith(target->label_state, target->br_merge()->arity);
     __ jmp(target->label.get());
@@ -1336,7 +1332,7 @@ class LiftoffCompiler {
     if (c->reachable()) {
       if (!c->end_merge.reached) {
         c->label_state.InitMerge(*__ cache_state(), __ num_locals(),
-                                 c->end_merge.arity);
+                                 c->end_merge.arity, c->stack_depth);
       }
       __ MergeFullStackWith(c->label_state);
       __ emit_jump(c->label.get());