Update WebAssembly Branch Hinting proposal

The main change is the section name, which is now 'metadata.code.branch_hint'.
The binary format has also a couple of minor changes.
Semantics remain unchanged.

Change-Id: I056c9f672ae494979e8ea55266fa766139b71d38
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3487788Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79292}

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

@@ -2234,6 +2234,7 @@ class WasmFullDecoder : public WasmDecoder<validate, decoding_mode> {
     DCHECK_LE(this->pc_, this->end_);
     DCHECK_EQ(this->num_locals(), 0);
 
+    locals_offset_ = this->pc_offset();
     this->InitializeLocalsFromSig();
     uint32_t params_count = static_cast<uint32_t>(this->num_locals());
     uint32_t locals_length;
@@ -2340,7 +2341,7 @@ class WasmFullDecoder : public WasmDecoder<validate, decoding_mode> {
   }
 
   uint32_t pc_relative_offset() const {
-    return this->pc_offset() - first_instruction_offset;
+    return this->pc_offset() - locals_offset_;
   }
 
   void DecodeFunctionBody() {
@@ -2374,7 +2375,6 @@ class WasmFullDecoder : public WasmDecoder<validate, decoding_mode> {
       CALL_INTERFACE_IF_OK_AND_REACHABLE(StartFunctionBody, c);
     }
 
-    first_instruction_offset = this->pc_offset();
     // Decode the function body.
     while (this->pc_ < this->end_) {
       // Most operations only grow the stack by at least one element (unary and
@@ -2407,7 +2407,7 @@ class WasmFullDecoder : public WasmDecoder<validate, decoding_mode> {
   }
 
  private:
-  uint32_t first_instruction_offset = 0;
+  uint32_t locals_offset_ = 0;
   Interface interface_;
 
   // The value stack, stored as individual pointers for maximum performance.

src/wasm/module-decoder.cc

@@ -36,7 +36,7 @@ namespace {
 constexpr char kNameString[] = "name";
 constexpr char kSourceMappingURLString[] = "sourceMappingURL";
 constexpr char kCompilationHintsString[] = "compilationHints";
-constexpr char kBranchHintsString[] = "branchHints";
+constexpr char kBranchHintsString[] = "metadata.code.branch_hint";
 constexpr char kDebugInfoString[] = ".debug_info";
 constexpr char kExternalDebugInfoString[] = "external_debug_info";
 
@@ -1345,11 +1345,6 @@ class ModuleDecoderImpl : public Decoder {
           break;
         }
         last_func_idx = func_idx;
-        uint8_t reserved = inner.consume_u8("reserved byte");
-        if (reserved != 0x0) {
-          inner.errorf("Invalid reserved byte: %#x", reserved);
-          break;
-        }
         uint32_t num_hints = inner.consume_u32v("number of hints");
         BranchHintMap func_branch_hints;
         TRACE("DecodeBranchHints[%d] module+%d\n", func_idx,
@@ -1357,13 +1352,18 @@ class ModuleDecoderImpl : public Decoder {
         // Keep track of the previous branch offset to validate the ordering
         int64_t last_br_off = -1;
         for (uint32_t j = 0; j < num_hints; ++j) {
-          uint32_t br_dir = inner.consume_u32v("branch direction");
           uint32_t br_off = inner.consume_u32v("branch instruction offset");
           if (int64_t(br_off) <= last_br_off) {
             inner.errorf("Invalid branch offset: %d", br_off);
             break;
           }
           last_br_off = br_off;
+          uint32_t data_size = inner.consume_u32v("data size");
+          if (data_size != 1) {
+            inner.errorf("Invalid data size: %#x. Expected 1.", data_size);
+            break;
+          }
+          uint32_t br_dir = inner.consume_u8("branch direction");
           TRACE("DecodeBranchHints[%d][%d] module+%d\n", func_idx, br_off,
                 static_cast<int>(inner.pc() - inner.start()));
           WasmBranchHint hint;

test/unittests/wasm/module-decoder-unittest.cc

@@ -76,9 +76,11 @@ namespace module_decoder_unittest {
                     'H', 'i', 'n', 't', 's'),                              \
           ADD_COUNT(__VA_ARGS__))
 
-#define SECTION_BRANCH_HINTS(...)                                           \
-  SECTION(Unknown,                                                          \
-          ADD_COUNT('b', 'r', 'a', 'n', 'c', 'h', 'H', 'i', 'n', 't', 's'), \
+#define SECTION_BRANCH_HINTS(...)                                          \
+  SECTION(Unknown,                                                         \
+          ADD_COUNT('m', 'e', 't', 'a', 'd', 'a', 't', 'a', '.', 'c', 'o', \
+                    'd', 'e', '.', 'b', 'r', 'a', 'n', 'c', 'h', '_', 'h', \
+                    'i', 'n', 't'),                                        \
           __VA_ARGS__)
 
 #define FAIL_IF_NO_EXPERIMENTAL_EH(data)                                 \
@@ -2228,10 +2230,10 @@ TEST_F(WasmModuleVerifyTest, BranchHinting) {
   WASM_FEATURE_SCOPE(branch_hinting);
   static const byte data[] = {
       TYPE_SECTION(1, SIG_ENTRY_v_v), FUNCTION_SECTION(2, 0, 0),
-      SECTION_BRANCH_HINTS(ENTRY_COUNT(2), 0 /*func_index*/, 0 /*reserved*/,
-                           ENTRY_COUNT(1), 1 /*likely*/, 2 /* if offset*/,
-                           1 /*func_index*/, 0 /*reserved*/, ENTRY_COUNT(1),
-                           0 /*unlikely*/, 4 /* br_if offset*/),
+      SECTION_BRANCH_HINTS(ENTRY_COUNT(2), 0 /*func_index*/, ENTRY_COUNT(1),
+                           3 /* if offset*/, 1 /*reserved*/, 1 /*likely*/,
+                           1 /*func_index*/, ENTRY_COUNT(1),
+                           5 /* br_if offset*/, 1 /*reserved*/, 0 /*unlikely*/),
       SECTION(Code, ENTRY_COUNT(2),
               ADD_COUNT(0, /*no locals*/
                         WASM_IF(WASM_I32V_1(1), WASM_NOP), WASM_END),
@@ -2243,9 +2245,9 @@ TEST_F(WasmModuleVerifyTest, BranchHinting) {
 
   EXPECT_EQ(2u, result.value()->branch_hints.size());
   EXPECT_EQ(WasmBranchHint::kLikely,
-            result.value()->branch_hints[0].GetHintFor(2));
+            result.value()->branch_hints[0].GetHintFor(3));
   EXPECT_EQ(WasmBranchHint::kUnlikely,
-            result.value()->branch_hints[1].GetHintFor(4));
+            result.value()->branch_hints[1].GetHintFor(5));
 }
 
 class WasmSignatureDecodeTest : public TestWithZone {