Map code without a DWARF line to no location in wasm source maps (#28033)

`wasm-sourcemap.py` drops DWARF rows with line 0, which LLVM uses for
code that does not come from a single source line, for example when it
merges several calls to `abort()` into one. The previous mapping then
covers that code too, so it is reported at an unrelated line.

This writes an entry with only the wasm offset at these rows instead. An
entry without a source location ends the previous mapping, so the code
gets no location. Binaryen does the same when it writes source maps
(WebAssembly/binaryen#5906): one such entry for each run of code without
a location, and none before the first mapping. Binaryen and emsymbolizer
already read these entries. When this tool was written, Binaryen could
not, which is likely why the rows were dropped.

Each entry adds about two bytes. The new test checks that code at line 0
maps to no location and that consecutive line 0 rows share one entry.

Author: JesusFileto

src/source_map_support.js

@@ -62,7 +62,8 @@ class WasmSourceMap {
   lookup(offset) {
     const normalized = this.normalizeOffset(offset);
     const info = this.mapping[normalized];
-    if (!info) {
+    // A segment with only an offset marks code that has no source location.
+    if (!info || info.source === undefined) {
       return null;
     }
     return {

test/test_other.py

@@ -10112,6 +10112,41 @@ def test_wasm_sourcemap_dead(self):
     # has only two entries
     self.assertRegex(output, r'"mappings":\s*"[A-Za-z0-9+/]+,[A-Za-z0-9+/]+"')
 
+  def test_wasm_sourcemap_line_zero(self):
+    # code at DWARF line 0 should map to no location, not to the row before it
+    wasm_sourcemap = importlib.import_module('tools.wasm-sourcemap')
+    emsymbolizer = importlib.import_module('tools.emsymbolizer')
+    entries = [
+      {'address': 0x10, 'line': 5, 'column': 3, 'file': 'a.c', 'eos': False},
+      {'address': 0x18, 'line': 0, 'column': 0, 'file': 'a.c', 'eos': False},
+      {'address': 0x1a, 'line': 0, 'column': 0, 'file': 'a.c', 'eos': False},
+      {'address': 0x20, 'line': 6, 'column': 1, 'file': 'a.c', 'eos': False},
+    ]
+    options = wasm_sourcemap.parse_args(['unused.wasm'])
+    write_file('a.wasm.map', json.dumps(wasm_sourcemap.build_sourcemap(entries, [], 0, options)))
+    source_map = emsymbolizer.WasmSourceMap()
+    source_map.parse('a.wasm.map')
+    # consecutive line 0 rows share one segment
+    self.assertEqual(source_map.offsets, [0x10, 0x18, 0x20])
+    self.assertEqual(source_map.lookup(0x14).line, 5)
+    self.assertIsNone(source_map.lookup(0x18).source)
+    self.assertIsNone(source_map.lookup(0x1c).source)
+    self.assertEqual(source_map.lookup(0x20).line, 6)
+    # the runtime's reader in source_map_support.js should agree
+    js = read_file(path_from_root('src/source_map_support.js'))
+    js = js[js.index('class WasmSourceMap'):js.index('let wasmSourceMap;')]
+    create_file('lookup.js', js + '''
+const map = JSON.parse(require('fs').readFileSync('a.wasm.map', 'utf8'));
+const sm = new WasmSourceMap(map);
+console.log(JSON.stringify([0x14, 0x18, 0x1c, 0x20].map((pc) => sm.lookup(pc))));
+''')
+    self.assertEqual(json.loads(self.run_js('lookup.js')), [
+      {'file': 'a.c', 'line': 5, 'column': 3},
+      None,
+      None,
+      {'file': 'a.c', 'line': 6, 'column': 1},
+    ])
+
   def test_wasm_sourcemap_relative_paths(self):
     ensure_dir('build')
 

tools/wasm-sourcemap.py

@@ -522,6 +522,7 @@ def build_sourcemap(entries, func_ranges, code_section_offset, options):
   last_line = 1
   last_column = 1
   last_func_id = 0
+  last_was_unmapped = False
 
   active_funcs = []
   next_func_range_id = 0
@@ -550,9 +551,17 @@ def get_function_id(address):
   for entry in entries:
     line = entry['line']
     column = entry['column']
-    # ignore entries with line 0
+    # line 0 means no source location; end the previous mapping with a
+    # segment that has only the offset, which the spec uses for unmapped
+    # code. The other fields are deltas, so they keep their last values.
     if line == 0:
+      if mappings and not last_was_unmapped:
+        address = entry['address'] + code_section_offset
+        mappings.append(encode_vlq(address - last_address))
+        last_address = address
+        last_was_unmapped = True
       continue
+    last_was_unmapped = False
     # start at least at column 1
     if column == 0:
       column = 1