[NFC] Encapsulate source map reader state (#7132)

Move all state relevant to reading source maps out of WasmBinaryReader
and into a new utility, SourceMapReader. This is a prerequisite for
parallelizing the parsing of function bodies, since the source map
reader state is different at the beginning of each function.

Also take the opportunity to simplify the way we read source maps, for
example by deferring the reading of anything but the position of a debug
location until it will be used and by using `std::optional` instead of
singleton `std::set`s to store function prologue and epilogue debug
locations.

1 files changed, 101 insertions, 0 deletions

diff --git a/src/source-map.h b/src/source-map.h
new file mode 100644
index 000000000..d8c50b5e1
--- /dev/null
+++ b/src/source-map.h
@@ -0,0 +1,101 @@
+/*
+ * Copyright 2024 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_source_map_h
+#define wasm_source_map_h
+
+#include <optional>
+#include <unordered_map>
+
+#include "wasm.h"
+
+namespace wasm {
+
+struct MapParseException {
+  std::string text;
+
+  MapParseException(std::string text) : text(text) {}
+  void dump(std::ostream& o) const;
+};
+
+class SourceMapReader {
+  const std::vector<char>& buffer;
+
+  // Current position in the source map buffer.
+  size_t pos = 0;
+
+  // The location in the binary the next debug location will correspond to. 0
+  // iff there are no more debug locations.
+  size_t location = 0;
+
+  // The file index, line, column, and symbol index the next debug location will
+  // be offset from.
+  uint32_t file = 0;
+  uint32_t line = 1;
+  uint32_t col = 0;
+  uint32_t symbol = 0;
+
+  // Whether the last read record had position and symbol information.
+  bool hasInfo = false;
+  bool hasSymbol = false;
+
+public:
+  SourceMapReader(const std::vector<char>& buffer) : buffer(buffer) {}
+
+  void readHeader(Module& wasm);
+
+  std::optional<Function::DebugLocation>
+  readDebugLocationAt(size_t currLocation);
+
+  // Do not reuse debug info across function boundaries.
+  void finishFunction() { hasInfo = false; }
+
+private:
+  char peek() {
+    if (pos >= buffer.size()) {
+      throw MapParseException("unexpected end of source map");
+    }
+    return buffer[pos];
+  }
+
+  char get() {
+    char c = peek();
+    ++pos;
+    return c;
+  }
+
+  bool maybeGet(char c) {
+    if (pos < buffer.size() && peek() == c) {
+      ++pos;
+      return true;
+    }
+    return false;
+  }
+
+  void expect(char c) {
+    using namespace std::string_literals;
+    char got = get();
+    if (got != c) {
+      throw MapParseException("expected '"s + c + "', got '" + got + "'");
+    }
+  }
+
+  int32_t readBase64VLQ();
+};
+
+} // namespace wasm
+
+#endif // wasm_source_map_h