[wasm] Provide better stack traces for asm.js code

src/wasm/wasm-debug.cc

Index: src/wasm/wasm-debug.cc
diff --git a/src/wasm/wasm-debug.cc b/src/wasm/wasm-debug.cc
index aaca8622b88b4b962d9963cd4fe437c738ffeb51..4361b8d5906a8c7536c02a75dfdd8d034f8887f5 100644
--- a/src/wasm/wasm-debug.cc
+++ b/src/wasm/wasm-debug.cc
@@ -21,6 +21,7 @@ enum {
   kWasmDebugInfoWasmBytesHash,
   kWasmDebugInfoFunctionByteOffsets,
   kWasmDebugInfoFunctionScripts,
+  kWasmDebugInfoAsmJsOffsets,
   kWasmDebugInfoNumEntries
 };
 
@@ -236,3 +237,72 @@ Handle<FixedArray> WasmDebugInfo::GetFunctionOffsetTable(
 
   return offset_table;
 }
+
+int WasmDebugInfo::GetAsmJsSourcePosition(Handle<WasmDebugInfo> debug_info,
+                                          int func_index, int byte_offset) {
+  Object *offset_tables = debug_info->get(kWasmDebugInfoAsmJsOffsets);
+  Isolate *isolate = debug_info->GetIsolate();
+  if (offset_tables->IsUndefined(isolate)) {
+    AsmJsOffsetsResult asm_offsets;
+    {
+      Handle<JSObject> wasm_object(debug_info->wasm_object(), isolate);
+      uint32_t num_imported_functions =
+          static_cast<uint32_t>(wasm::GetNumImportedFunctions(wasm_object));
+      Handle<SeqOneByteString> wasm_bytes = wasm::GetWasmBytes(wasm_object);
+      DisallowHeapAllocation no_gc;
+      const byte *bytes_start = wasm_bytes->GetChars();
+      const byte *bytes_end = bytes_start + wasm_bytes->length();
+      asm_offsets = wasm::DecodeAsmJsOffsets(bytes_start, bytes_end,
+                                             num_imported_functions);
+    }
+    // Wasm bytes must be valid and must contain asm.js offset table.
+    DCHECK(asm_offsets.ok());
+    DCHECK_GE(static_cast<size_t>(kMaxInt), asm_offsets.val.size());
+    int num_functions = static_cast<int>(asm_offsets.val.size());
+    DCHECK_EQ(wasm::GetNumberOfFunctions(handle(debug_info->wasm_object())),
+              num_functions);
+    Handle<FixedArray> all_tables =
+        isolate->factory()->NewFixedArray(num_functions);
+    debug_info->set(kWasmDebugInfoAsmJsOffsets, *all_tables);
+    for (int func = 0; func < num_functions; ++func) {
+      std::vector<std::pair<int, int>> &func_asm_offsets =
+          asm_offsets.val[func];
+      if (func_asm_offsets.empty()) continue;
+      size_t array_size = 2 * kIntSize * func_asm_offsets.size();
+      CHECK_LE(array_size, static_cast<size_t>(kMaxInt));
+      ByteArray *arr =
+          *isolate->factory()->NewByteArray(static_cast<int>(array_size));
+      all_tables->set(func, arr);
+      int idx = 0;
+      for (std::pair<int, int> p : func_asm_offsets) {
+        // Byte offsets must be strictly monotonously increasing:
+        DCHECK(idx == 0 || p.first > arr->get_int(idx - 2));
+        arr->set_int(idx++, p.first);
+        arr->set_int(idx++, p.second);
+      }
+      DCHECK_EQ(arr->length(), idx * kIntSize);
+    }
+    offset_tables = *all_tables;
+  }
+
+  FixedArray *offset_tables_arr = FixedArray::cast(offset_tables);
+  DCHECK_LT(func_index, offset_tables_arr->length());
+  ByteArray *offset_table = ByteArray::cast(offset_tables_arr->get(func_index));
+
+  // Binary search for the current byte offset.
+  int left = 0;                                       // inclusive
+  int right = offset_table->length() / kIntSize / 2;  // exclusive
+  DCHECK_LT(left, right);
+  while (right - left > 1) {
+    int mid = left + (right - left) / 2;
+    if (offset_table->get_int(2 * mid) < byte_offset) {
+      left = mid;
+    } else {
+      right = mid;
+    }
+  }
+  // There should be an entry for each position that could show up on the stack
+  // trace:
+  DCHECK_EQ(byte_offset, offset_table->get_int(2 * left));
+  return offset_table->get_int(2 * left + 1);
+}