[Debug] Fix debug-compilation-dir for system libs (#20776)

Fixes #20775

Before this change:

$ llvm-dwarfdump -debug-info -debug-line --recurse-depth=0  emscripten/cache/sysroot/lib/wasm32-emscripten/libcompiler_rt.a
...
              DW_AT_comp_dir    ("/emsdk/emscripten")

After this change:

$ llvm-dwarfdump -debug-info -debug-line --recurse-depth=0  emscripten/cache/sysroot/lib/wasm32-emscripten/libcompiler_rt.a
...
              DW_AT_comp_dir    ("/emsdk/emscripten/cache/build/libcompiler_rt-tmp")

Author: kon72

test/test_other.py

@@ -9780,6 +9780,21 @@ def test_separate_dwarf_with_filename_and_path(self):
                       '-sSEPARATE_DWARF_URL=http://somewhere.com/hosted.wasm'])
     self.assertIn(b'somewhere.com/hosted.wasm', read_binary('a.out.wasm'))
 
+  @crossplatform
+  def test_dwarf_system_lib(self):
+    if config.FROZEN_CACHE:
+      self.skipTest("test doesn't work with frozen cache")
+    self.run_process([EMBUILDER, 'build', 'libemmalloc', '--force'])
+    libc = os.path.join(config.CACHE, 'sysroot', 'lib', 'wasm32-emscripten', 'libemmalloc.a')
+    self.assertExists(libc)
+
+    dwdump = self.run_process(
+        [LLVM_DWARFDUMP, libc, '-debug-info', '-debug-line', '--recurse-depth=0'],
+        stdout=PIPE).stdout
+    # Check that the embedded location of the source file is correct.
+    self.assertIn('DW_AT_name\t("system/lib/emmalloc.c")', dwdump)
+    self.assertIn('DW_AT_comp_dir\t("/emsdk/emscripten")', dwdump)
+
   @parameterized({
     'O0': (['-O0'],),
     'O1': (['-O1'],),

tools/system_libs.py

@@ -471,7 +471,9 @@ def generate_ninja(self, build_dir, libname):
     cflags = self.get_cflags()
     if self.deterministic_paths:
       source_dir = utils.path_from_root()
+      relative_source_dir = os.path.relpath(source_dir, build_dir)
       cflags += [f'-ffile-prefix-map={source_dir}=/emsdk/emscripten',
+                 f'-ffile-prefix-map={relative_source_dir}/=',
                  '-fdebug-compilation-dir=/emsdk/emscripten']
     asflags = get_base_cflags(preprocess=False)
     input_files = self.get_files()
@@ -491,7 +493,9 @@ def build_objects(self, build_dir):
     cflags = self.get_cflags()
     if self.deterministic_paths:
       source_dir = utils.path_from_root()
+      relative_source_dir = os.path.relpath(source_dir, build_dir)
       cflags += [f'-ffile-prefix-map={source_dir}=/emsdk/emscripten',
+                 f'-ffile-prefix-map={relative_source_dir}/=',
                  '-fdebug-compilation-dir=/emsdk/emscripten']
     case_insensitive = is_case_insensitive(build_dir)
     for src in self.get_files():
@@ -531,6 +535,7 @@ def build_objects(self, build_dir):
         # Use relative paths to reduce the length of the command line.
         # This allows to avoid switching to a response file as often.
         src = os.path.relpath(src, build_dir)
+        src = utils.normalize_path(src)
         batches.setdefault(tuple(cmd), []).append(src)
       objects.add(o)