[wasm][refactor] Change signature of DecodeLocals.

We would like DecodeLocals to allow inserting new locals
in any position. This is useful for the upcoming 'let' instruction.

Bug: v8:7748
Change-Id: Ic7f2a7fba0f69ee76b0ace46bb0cecee9d047306
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2208859
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67936}

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

@@ -952,24 +952,28 @@ class WasmDecoder : public Decoder {
     }
   }
 
-  enum DecodeLocalsMode { kUpdateLocals, kNoUpdateLocals };
-
   // Decodes local definitions in the current decoder.
   // Returns true iff locals are found.
-  // Write the total length of decoded locals in 'total_length'.
-  // If 'mode' is kPrependLocals, the 'local_types_' of this decoder
-  // will be updated. Otherwise, this is used just to check legality and
-  // measure the size of the locals.
-  // The decoder's pc is not advanced.
-  // If no locals are found (i.e., no compressed uint32 is found at pc),
-  // this will exit as 'false' and without an error.
+  // Writes the total length of decoded locals in 'total_length'.
+  // If insert_postion is present, the decoded locals will be inserted into the
+  // 'local_types_' of this decoder. Otherwise, this function is used just to
+  // check validity and determine the encoding length of the locals in bytes.
+  // The decoder's pc is not advanced. If no locals are found (i.e., no
+  // compressed uint32 is found at pc), this will exit as 'false' and without an
+  // error.
   bool DecodeLocals(const byte* pc, uint32_t* total_length,
-                    DecodeLocalsMode mode) {
+                    const base::Optional<uint32_t> insert_position) {
     DCHECK_NOT_NULL(local_types_);
 
     uint32_t length;
     *total_length = 0;
 
+    // The 'else' value is useless, we pass it for convenience.
+    ZoneVector<ValueType>::iterator insert_iterator =
+        insert_position.has_value()
+            ? local_types_->begin() + insert_position.value()
+            : local_types_->begin();
+
     // Decode local declarations, if any.
     uint32_t entries = read_u32v<kValidate>(pc, &length, "local decls count");
     if (failed()) {
@@ -1005,8 +1009,9 @@ class WasmDecoder : public Decoder {
         return false;
       }
       *total_length += length;
-      if (mode == kUpdateLocals) {
-        local_types_->insert(local_types_->end(), count, type);
+      if (insert_position.has_value()) {
+        insert_iterator =
+            local_types_->insert(insert_iterator, count, type) + count;
       }
     }
     DCHECK(ok());
@@ -1800,7 +1805,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
     this->InitializeLocalsFromSig();
     uint32_t locals_length;
     this->DecodeLocals(this->pc(), &locals_length,
-                       WasmDecoder<validate>::kUpdateLocals);
+                       static_cast<uint32_t>(this->local_types_->size()));
     this->consume_bytes(locals_length);
 
     CALL_INTERFACE(StartFunction);

src/wasm/function-body-decoder.cc

@@ -24,16 +24,21 @@ bool DecodeLocalDecls(const WasmFeatures& enabled, BodyLocalDecls* decls,
   WasmFeatures no_features = WasmFeatures::None();
   WasmDecoder<Decoder::kValidate> decoder(nullptr, enabled, &no_features,
                                           nullptr, start, end, 0);
+  // The decoded functions need to be inserted into &decls->type_list,
+  // so we pass a pointer to it to local_types_ which will be updated
+  // in DecodeLocals.
   decoder.local_types_ = &decls->type_list;
   uint32_t length;
-  if (decoder.DecodeLocals(decoder.pc(), &length,
-                           decoder.DecodeLocalsMode::kUpdateLocals)) {
+  if (decoder.DecodeLocals(
+          decoder.pc(), &length,
+          static_cast<uint32_t>(decoder.local_types_->size()))) {
     DCHECK(decoder.ok());
-    decoder.consume_bytes(length);
-    decls->encoded_size = decoder.pc_offset();
+    decls->encoded_size = length;
     return true;
+  } else {
+    decls->encoded_size = 0;
+    return false;
   }
-  return false;
 }
 
 BytecodeIterator::BytecodeIterator(const byte* start, const byte* end,