[wasm] [decoder] Minor interface change

The baseline compiler needs to know the depth in order to access the
parent block. This is a small CL, but anything that can land before the
big baseline CL reduced the complexity of the latter.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I2e29cc974908438266adb4301026dfe5fbfb1990
Reviewed-on: https://chromium-review.googlesource.com/677301Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48112}

src/wasm/function-body-decoder-impl.h

@@ -515,8 +515,8 @@ struct ControlWithNamedConstructors : public ControlBase<Value> {
   F(Unreachable)                                                               \
   F(Select, const Value& cond, const Value& fval, const Value& tval,           \
     Value* result)                                                             \
-  F(BreakTo, Control* block)                                                   \
-  F(BrIf, const Value& cond, Control* block)                                   \
+  F(BreakTo, uint32_t depth)                                                   \
+  F(BrIf, const Value& cond, uint32_t depth)                                   \
   F(BrTable, const BranchTableOperand<validate>& operand, const Value& key)    \
   F(Else, Control* if_block)                                                   \
   F(LoadMem, ValueType type, MachineType mem_type,                             \
@@ -1375,7 +1375,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
             BreakDepthOperand<validate> operand(this, this->pc_);
             if (this->Validate(this->pc_, operand, control_.size()) &&
                 TypeCheckBreak(operand.depth)) {
-              interface_.BreakTo(this, control_at(operand.depth));
+              interface_.BreakTo(this, operand.depth);
             }
             len = 1 + operand.length;
             EndControl();
@@ -1386,7 +1386,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
             auto cond = Pop(0, kWasmI32);
             if (this->Validate(this->pc_, operand, control_.size()) &&
                 TypeCheckBreak(operand.depth)) {
-              interface_.BrIf(this, cond, control_at(operand.depth));
+              interface_.BrIf(this, cond, operand.depth);
             }
             len = 1 + operand.length;
             break;
@@ -1431,7 +1431,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
                 this->error(pos, "improper branch in br_table");
                 break;
               }
-              interface_.BreakTo(this, control_at(target));
+              interface_.BreakTo(this, target);
             }
             len = 1 + iterator.length();
             EndControl();

src/wasm/function-body-decoder.cc

@@ -257,21 +257,22 @@ class WasmGraphBuildingInterface {
     ssa_env_->control = merge;
   }
 
-  void BreakTo(Decoder* decoder, Control* block) {
-    if (block->is_loop()) {
-      Goto(decoder, ssa_env_, block->end_env);
+  void BreakTo(Decoder* decoder, uint32_t depth) {
+    Control* target = decoder->control_at(depth);
+    if (target->is_loop()) {
+      Goto(decoder, ssa_env_, target->end_env);
     } else {
-      MergeValuesInto(decoder, block);
+      MergeValuesInto(decoder, target);
     }
   }
 
-  void BrIf(Decoder* decoder, const Value& cond, Control* block) {
+  void BrIf(Decoder* decoder, const Value& cond, uint32_t depth) {
     SsaEnv* fenv = ssa_env_;
     SsaEnv* tenv = Split(decoder, fenv);
     fenv->SetNotMerged();
     BUILD(BranchNoHint, cond.node, &tenv->control, &fenv->control);
     ssa_env_ = tenv;
-    BreakTo(decoder, block);
+    BreakTo(decoder, depth);
     ssa_env_ = fenv;
   }
 
@@ -290,7 +291,7 @@ class WasmGraphBuildingInterface {
       ssa_env_ = Split(decoder, copy);
       ssa_env_->control = (i == operand.table_count) ? BUILD(IfDefault, sw)
                                                      : BUILD(IfValue, i, sw);
-      BreakTo(decoder, decoder->control_at(target));
+      BreakTo(decoder, target);
     }
     DCHECK(decoder->ok());
     ssa_env_ = break_env;