[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, 212 insertions, 0 deletions

diff --git a/src/wasm/source-map.cpp b/src/wasm/source-map.cpp
new file mode 100644
index 000000000..7ad26e898
--- /dev/null
+++ b/src/wasm/source-map.cpp
@@ -0,0 +1,212 @@
+/*
+ * 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.
+ */
+
+#include "source-map.h"
+#include "support/colors.h"
+
+namespace wasm {
+
+std::vector<char> defaultEmptySourceMap;
+
+void MapParseException::dump(std::ostream& o) const {
+  Colors::magenta(o);
+  o << "[";
+  Colors::red(o);
+  o << "map parse exception: ";
+  Colors::green(o);
+  o << text;
+  Colors::magenta(o);
+  o << "]";
+  Colors::normal(o);
+}
+
+void SourceMapReader::readHeader(Module& wasm) {
+  assert(pos == 0);
+  if (buffer.empty()) {
+    return;
+  }
+
+  auto skipWhitespace = [&]() {
+    while (pos < buffer.size() && (buffer[pos] == ' ' || buffer[pos] == '\n')) {
+      ++pos;
+    }
+  };
+
+  auto findField = [&](const char* name) {
+    bool matching = false;
+    size_t len = strlen(name);
+    size_t index = 0;
+    while (1) {
+      char ch = get();
+      if (ch == '\"') {
+        if (matching) {
+          if (index == len) {
+            // We matched a terminating quote.
+            break;
+          }
+          matching = false;
+        } else {
+          // Beginning of a new potential match.
+          matching = true;
+          index = 0;
+        }
+      } else if (matching && name[index] == ch) {
+        ++index;
+      } else if (matching) {
+        matching = false;
+      }
+    }
+    skipWhitespace();
+    expect(':');
+    skipWhitespace();
+    return true;
+  };
+
+  auto readString = [&](std::string& str) {
+    std::vector<char> vec;
+    skipWhitespace();
+    expect('\"');
+    while (1) {
+      if (maybeGet('\"')) {
+        break;
+      }
+      vec.push_back(get());
+    }
+    skipWhitespace();
+    str = std::string(vec.begin(), vec.end());
+  };
+
+  if (!findField("sources")) {
+    throw MapParseException("cannot find the 'sources' field in map");
+  }
+
+  skipWhitespace();
+  expect('[');
+  if (!maybeGet(']')) {
+    do {
+      std::string file;
+      readString(file);
+      wasm.debugInfoFileNames.push_back(file);
+    } while (maybeGet(','));
+    expect(']');
+  }
+
+  if (findField("names")) {
+    skipWhitespace();
+    expect('[');
+    if (!maybeGet(']')) {
+      do {
+        std::string symbol;
+        readString(symbol);
+        wasm.debugInfoSymbolNames.push_back(symbol);
+      } while (maybeGet(','));
+      expect(']');
+    }
+  }
+
+  if (!findField("mappings")) {
+    throw MapParseException("cannot find the 'mappings' field in map");
+  }
+
+  expect('\"');
+  if (maybeGet('\"')) {
+    // There are no mappings.
+    location = 0;
+    return;
+  }
+
+  // Read the location of the first debug location.
+  location = readBase64VLQ();
+}
+
+std::optional<Function::DebugLocation>
+SourceMapReader::readDebugLocationAt(size_t currLocation) {
+  if (pos >= buffer.size()) {
+    return std::nullopt;
+  }
+
+  while (location && location <= currLocation) {
+    do {
+      char next = peek();
+      if (next == ',' || next == '\"') {
+        // This is a 1-length entry, so the next location has no debug info.
+        hasInfo = false;
+        break;
+      }
+
+      hasInfo = true;
+      file += readBase64VLQ();
+      line += readBase64VLQ();
+      col += readBase64VLQ();
+
+      next = peek();
+      if (next == ',' || next == '\"') {
+        hasSymbol = false;
+        break;
+      }
+
+      hasSymbol = true;
+      symbol += readBase64VLQ();
+
+    } while (false);
+
+    // Check whether there is another record to read the position for.
+    char next = get();
+    if (next == '\"') {
+      // End of records.
+      location = 0;
+      break;
+    }
+    if (next != ',') {
+      throw MapParseException("Expected delimiter");
+    }
+
+    // Set up for the next record.
+    location += readBase64VLQ();
+  }
+
+  if (!hasInfo) {
+    return std::nullopt;
+  }
+
+  auto sym = hasSymbol ? symbol : std::optional<uint32_t>{};
+  return Function::DebugLocation{file, line, col, sym};
+}
+
+int32_t SourceMapReader::readBase64VLQ() {
+  uint32_t value = 0;
+  uint32_t shift = 0;
+  while (1) {
+    auto ch = get();
+    if ((ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch < 'g')) {
+      // last number digit
+      uint32_t digit = ch < 'a' ? ch - 'A' : ch - 'a' + 26;
+      value |= digit << shift;
+      break;
+    }
+    if (!(ch >= 'g' && ch <= 'z') && !(ch >= '0' && ch <= '9') && ch != '+' &&
+        ch != '/') {
+      throw MapParseException("invalid VLQ digit");
+    }
+    uint32_t digit =
+      ch > '9' ? ch - 'g' : (ch >= '0' ? ch - '0' + 20 : (ch == '+' ? 30 : 31));
+    value |= digit << shift;
+    shift += 5;
+  }
+  return value & 1 ? -int32_t(value >> 1) : int32_t(value >> 1);
+}
+
+} // namespace wasm