[wasm] Simplify SignatureMap

Minor simplifications and an additional overflow check.

R=mstarzinger@chromium.org

Bug: v8:8238
Change-Id: I169464319a0e70562f3a443f429e462d30dd2fa3
Reviewed-on: https://chromium-review.googlesource.com/c/1296482Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56929}

src/wasm/signature-map.cc

@@ -13,22 +13,18 @@ namespace wasm {
 uint32_t SignatureMap::FindOrInsert(const FunctionSig& sig) {
   CHECK(!frozen_);
   auto pos = map_.find(sig);
-  if (pos != map_.end()) {
-    return pos->second;
-  } else {
-    uint32_t index = next_++;
-    map_[sig] = index;
-    return index;
-  }
+  if (pos != map_.end()) return pos->second;
+  // Indexes are returned as int32_t, thus check against their limit.
+  CHECK_GE(kMaxInt, map_.size());
+  uint32_t index = static_cast<uint32_t>(map_.size());
+  map_.insert(std::make_pair(sig, index));
+  return index;
 }
 
 int32_t SignatureMap::Find(const FunctionSig& sig) const {
   auto pos = map_.find(sig);
-  if (pos != map_.end()) {
-    return static_cast<int32_t>(pos->second);
-  } else {
-    return -1;
-  }
+  if (pos == map_.end()) return -1;
+  return static_cast<int32_t>(pos->second);
 }
 
 }  // namespace wasm

src/wasm/signature-map.h

@@ -35,7 +35,6 @@ class V8_EXPORT_PRIVATE SignatureMap {
   void Freeze() { frozen_ = true; }
 
  private:
-  uint32_t next_ = 0;
   bool frozen_ = false;
   std::unordered_map<FunctionSig, uint32_t, base::hash<FunctionSig>> map_;
 };