[wasm] Refactor the Result object

Instead of storing {start} and {error_pc} we now store the
{error_offset}, which is anyways the only value we use.

R=clemensh@chromium.org

Change-Id: Ifd9791eff5c9efce2e7e2a1989bf3b5eaa464a02
Reviewed-on: https://chromium-review.googlesource.com/471527
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44510}

src/wasm/decoder.h

@@ -206,8 +206,8 @@ class Decoder {
     if (failed()) {
       TRACE("Result error: %s\n", error_msg_.get());
       result.error_code = kError;
-      result.start = start_;
-      result.error_pc = error_pc_;
+      DCHECK_GE(error_pc_, start_);
+      result.error_offset = static_cast<uint32_t>(error_pc_ - start_);
       // transfer ownership of the error to the result.
       result.error_msg.reset(error_msg_.release());
     } else {

src/wasm/module-decoder.cc

@@ -215,7 +215,6 @@ class ModuleDecoder : public Decoder {
       : Decoder(module_start, module_end),
         module_zone(zone),
         origin_(FLAG_assume_asmjs_origin ? kAsmJsOrigin : origin) {
-    result_.start = start_;
     if (end_ < start_) {
       error(start_, "end is less than start");
       end_ = start_;

src/wasm/wasm-result.h

@@ -28,7 +28,7 @@ enum ErrorCode {
 // The overall result of decoding a function or a module.
 template <typename T>
 struct Result {
-  Result() : val(), error_code(kSuccess), start(nullptr), error_pc(nullptr) {}
+  Result() : val(), error_code(kSuccess), error_offset(0) {}
   Result(Result&& other) { *this = std::move(other); }
   Result& operator=(Result&& other) {
     MoveFrom(other);
@@ -38,8 +38,7 @@ struct Result {
 
   T val;
   ErrorCode error_code;
-  const byte* start;
-  const byte* error_pc;
+  uint32_t error_offset;
   std::unique_ptr<char[]> error_msg;
 
   bool ok() const { return error_code == kSuccess; }
@@ -48,8 +47,7 @@ struct Result {
   template <typename V>
   void MoveFrom(Result<V>& that) {
     error_code = that.error_code;
-    start = that.start;
-    error_pc = that.error_pc;
+    error_offset = that.error_offset;
     error_msg = std::move(that.error_msg);
   }
 
@@ -67,12 +65,7 @@ std::ostream& operator<<(std::ostream& os, const Result<T>& result) {
       os << "success (no value)";
     }
   } else if (result.error_msg.get() != nullptr) {
-    ptrdiff_t offset = result.error_pc - result.start;
-    if (offset < 0) {
-      os << result.error_msg.get() << " @" << offset;
-    } else {
-      os << result.error_msg.get() << " @+" << offset;
-    }
+    os << result.error_msg.get() << " @" << result.error_offset;
   } else {
     os << result.error_code;
   }

test/cctest/wasm/wasm-run-utils.h

@@ -389,7 +389,7 @@ inline void TestBuildingGraph(Zone* zone, JSGraph* jsgraph, ModuleEnv* module,
       result = BuildTFGraph(zone->allocator(), &builder, sig, start, end);
     }
 
-    ptrdiff_t pc = result.error_pc - result.start;
+    uint32_t pc = result.error_offset;
     std::ostringstream str;
     str << "Verification failed: " << result.error_code << " pc = +" << pc
         << ", msg = " << result.error_msg.get();

test/unittests/wasm/function-body-decoder-unittest.cc

@@ -136,7 +136,7 @@ class FunctionBodyDecoderTest : public TestWithZone {
         start, end);
 
     if (result.error_code != expected) {
-      ptrdiff_t pc = result.error_pc - result.start;
+      uint32_t pc = result.error_offset;
       std::ostringstream str;
       if (expected == kSuccess) {
         str << "Verification failed: " << result.error_code << " pc = +" << pc