[wasm] Simplify type checking for merges

Improve readability by avoiding a helper method. Just read from the
stack directly.

R=titzer@chromium.org

Change-Id: I38c944fac45c721f328a2b7bec3a3f4602f05c05
Reviewed-on: https://chromium-review.googlesource.com/c/1360572
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58023}

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

@@ -1508,12 +1508,10 @@ class WasmFullDecoder : public WasmDecoder<validate> {
     return static_cast<uint32_t>(stack_.size());
   }
 
-  inline Value& GetMergeValueFromStack(
-      Control* c, Merge<Value>* merge, uint32_t i) {
-    DCHECK(merge == &c->start_merge || merge == &c->end_merge);
-    DCHECK_GT(merge->arity, i);
-    DCHECK_GE(stack_.size(), c->stack_depth + merge->arity);
-    return stack_[stack_.size() - merge->arity + i];
+  inline Value* stack_value(uint32_t depth) {
+    DCHECK_LT(0, depth);
+    DCHECK_GE(stack_.size(), depth);
+    return &*(stack_.end() - depth);
   }
 
  private:
@@ -2645,10 +2643,13 @@ class WasmFullDecoder : public WasmDecoder<validate> {
   bool TypeCheckMergeValues(Control* c, Merge<Value>* merge) {
     DCHECK(merge == &c->start_merge || merge == &c->end_merge);
     DCHECK_GE(stack_.size(), c->stack_depth + merge->arity);
+    // The computation of {stack_values} is only valid if {merge->arity} is >0.
+    DCHECK_LT(0, merge->arity);
+    Value* stack_values = &*(stack_.end() - merge->arity);
     // Typecheck the topmost {merge->arity} values on the stack.
     for (uint32_t i = 0; i < merge->arity; ++i) {
-      auto& val = GetMergeValueFromStack(c, merge, i);
-      auto& old = (*merge)[i];
+      Value& val = stack_values[i];
+      Value& old = (*merge)[i];
       if (val.type != old.type) {
         // If {val.type} is polymorphic, which results from unreachable, make
         // it more specific by using the merge value's expected type.
@@ -2680,6 +2681,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
           expected, startrel(c->pc), actual);
       return false;
     }
+    if (expected == 0) return true;  // Fast path.
 
     return TypeCheckMergeValues(c, &c->end_merge);
   }
@@ -2687,6 +2689,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
   bool TypeCheckBreak(Control* c) {
     // Breaks must have at least the number of values expected; can have more.
     uint32_t expected = c->br_merge()->arity;
+    if (expected == 0) return true;  // Fast path.
     DCHECK_GE(stack_.size(), control_.back().stack_depth);
     uint32_t actual =
         static_cast<uint32_t>(stack_.size()) - control_.back().stack_depth;

src/wasm/graph-builder-interface.cc

@@ -649,12 +649,16 @@ class WasmGraphBuildingInterface {
     const bool first = target->state == SsaEnv::kUnreachable;
     Goto(decoder, ssa_env_, target);
 
+    if (merge->arity == 0) return;
+
     uint32_t avail =
         decoder->stack_size() - decoder->control_at(0)->stack_depth;
+    DCHECK_GE(avail, merge->arity);
     uint32_t start = avail >= merge->arity ? 0 : merge->arity - avail;
+    Value* stack_values = decoder->stack_value(merge->arity);
     for (uint32_t i = start; i < merge->arity; ++i) {
-      auto& val = decoder->GetMergeValueFromStack(c, merge, i);
-      auto& old = (*merge)[i];
+      Value& val = stack_values[i];
+      Value& old = (*merge)[i];
       DCHECK_NOT_NULL(val.node);
       DCHECK(val.type == old.type || val.type == kWasmVar);
       old.node = first ? val.node