[asm.js] [wasm] Store function start position for stack check

src/wasm/module-decoder.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/wasm/module-decoder.h"
 
 #include "src/base/functional.h"
 #include "src/base/platform/platform.h"
 #include "src/flags.h"
 #include "src/macro-assembler.h"
@@ skipping 1184 matching lines @@
   for (uint32_t i = 0; i < functions_count && decoder.ok(); ++i) {
     uint32_t size = decoder.consume_u32v("table size");
     if (size == 0) {
       table.push_back(std::vector<AsmJsOffsetEntry>());
       continue;
     }
     if (!decoder.checkAvailable(size)) {
       decoder.error("illegal asm function offset table size");
     }
     const byte* table_end = decoder.pc() + size;
-    uint32_t locals_size = decoder.consume_u32("locals size");
+    uint32_t locals_size = decoder.consume_u32v("locals size");
+    int function_start_position = decoder.consume_u32v("function start pos");
     int last_byte_offset = locals_size;
-    int last_asm_position = 0;
+    int last_asm_position = function_start_position;
     std::vector<AsmJsOffsetEntry> func_asm_offsets;
     func_asm_offsets.reserve(size / 4);  // conservative estimation
+    // Add an entry for the stack check, associated with position 0.
+    func_asm_offsets.push_back(
+        {0, function_start_position, function_start_position});
     while (decoder.ok() && decoder.pc() < table_end) {
       last_byte_offset += decoder.consume_u32v("byte offset delta");
       int call_position =
           last_asm_position + decoder.consume_i32v("call position delta");
       int to_number_position =
           call_position + decoder.consume_i32v("to_number position delta");
       last_asm_position = to_number_position;
       func_asm_offsets.push_back(
           {last_byte_offset, call_position, to_number_position});
     }
     if (decoder.pc() != table_end) {
       decoder.error("broken asm offset table");
     }
     table.push_back(std::move(func_asm_offsets));
   }
   if (decoder.more()) decoder.error("unexpected additional bytes");
 
   return decoder.toResult(std::move(table));
 }
 
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8