[wasm] Fix --dump-wasm-module

The generated file only contained the last section of the module, since
the decoder's range is reset in DecodeSection.
This CL fixes this by remembering the original module bytes and dumping
them instead.

R=ahaas@chromium.org

Change-Id: I5e0f97279886817d58caac6114994bb9c1a147fc
Reviewed-on: https://chromium-review.googlesource.com/892980Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50953}

src/wasm/module-decoder.cc

@@ -270,7 +270,7 @@ class ModuleDecoderImpl : public Decoder {
     pc_ = end_;  // On error, terminate section decoding loop.
   }
 
-  void DumpModule(const ModuleResult& result) {
+  void DumpModule(const Vector<const byte> module_bytes) {
     std::string path;
     if (FLAG_dump_wasm_module_path) {
       path = FLAG_dump_wasm_module_path;
@@ -280,12 +280,13 @@ class ModuleDecoderImpl : public Decoder {
       }
     }
     // File are named `HASH.{ok,failed}.wasm`.
-    size_t hash = base::hash_range(start_, end_);
+    size_t hash = base::hash_range(module_bytes.start(), module_bytes.end());
     EmbeddedVector<char, 32> buf;
-    SNPrintF(buf, "%016zx.%s.wasm", hash, result.ok() ? "ok" : "failed");
+    SNPrintF(buf, "%016zx.%s.wasm", hash, ok() ? "ok" : "failed");
     std::string name(buf.start());
     if (FILE* wasm_file = base::OS::FOpen((path + name).c_str(), "wb")) {
-      if (fwrite(start_, end_ - start_, 1, wasm_file) != 1) {
+      if (fwrite(module_bytes.start(), module_bytes.length(), 1, wasm_file) !=
+          1) {
         OFStream os(stderr);
         os << "Error while dumping wasm file" << std::endl;
       }
@@ -848,7 +849,6 @@ class ModuleDecoderImpl : public Decoder {
       // Copy error code and location.
       result.MoveErrorFrom(intermediate_result_);
     }
-    if (FLAG_dump_wasm_module) DumpModule(result);
     return result;
   }
 
@@ -856,6 +856,7 @@ class ModuleDecoderImpl : public Decoder {
   ModuleResult DecodeModule(Isolate* isolate, bool verify_functions = true) {
     StartDecoding(isolate);
     uint32_t offset = 0;
+    Vector<const byte> orig_bytes(start(), end() - start());
     DecodeModuleHeader(Vector<const uint8_t>(start(), end() - start()), offset);
     if (failed()) {
       return FinishDecoding(verify_functions);
@@ -878,6 +879,8 @@ class ModuleDecoderImpl : public Decoder {
       section_iter.advance(true);
     }
 
+    if (FLAG_dump_wasm_module) DumpModule(orig_bytes);
+
     if (decoder.failed()) {
       return decoder.toResult<std::unique_ptr<WasmModule>>(nullptr);
     }