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Use /emsdk/emscripten for relative path substitution
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When `deterministic_paths` is set, we are currently using
`-ffile-prefix-map` to produce the same path in data and debug info. In
the case of absolute paths, their emscripten path is replaced with a
fake path `/emsdk/emscripten`, and in the case of relative paths, all
path relative to the emscripten directory is removed, so
`../../system/lib/somefile.c` becomes `system/lib/somefiles.c`.
https://github.com/emscripten-core/emscripten/blob/f66b5d706e174d9e5cc6122c06ea29dcd2735cd0/tools/system_libs.py#L472-L477
https://github.com/emscripten-core/emscripten/blob/f66b5d706e174d9e5cc6122c06ea29dcd2735cd0/tools/system_libs.py#L495-L501

So this does not make relative paths and absolute paths the same, which
can lead to different builds depending on whether the command line uses
absolute paths vs. relative ones. Currently we use relative paths when
`EMCC_BATCH_BUILD` is set. And Ninja builds cannot use relative paths.

This is also what was suggested in
#23195 (comment)
while discussins `__FILE__` problem in #23915.

This does not change any code size tests because no code size tests
happens to contain `__FILE__` at the moment. (One will be added in
 #22994)
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aheejin committed Dec 19, 2024
1 parent b330159 commit 0e98c20
Showing 1 changed file with 10 additions and 6 deletions.
16 changes: 10 additions & 6 deletions tools/system_libs.py
Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,10 @@
# link time.
USE_NINJA = int(os.environ.get('EMCC_USE_NINJA', '0'))

# A fake emscripten path to use in __FILE__ macro and debug info to produce
# reproducible builds across platforms.
FAKE_EMSCRIPTEN_PATH = '/emsdk/emscripten'


def files_in_path(path, filenames):
srcdir = utils.path_from_root(path)
Expand Down Expand Up @@ -472,9 +476,9 @@ def generate_ninja(self, build_dir, libname):
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']
cflags += [f'-ffile-prefix-map={source_dir}={FAKE_EMSCRIPTEN_PATH}',
f'-ffile-prefix-map={relative_source_dir}={FAKE_EMSCRIPTEN_PATH}',
'-fdebug-compilation-dir={FAKE_EMSCRIPTEN_PATH}']
asflags = get_base_cflags(preprocess=False)
input_files = self.get_files()
ninja_file = os.path.join(build_dir, 'build.ninja')
Expand All @@ -496,9 +500,9 @@ def build_objects(self, build_dir):
source_dir = utils.path_from_root()
if batch_inputs:
relative_source_dir = os.path.relpath(source_dir, build_dir)
cflags += [f'-ffile-prefix-map={relative_source_dir}/=']
cflags += [f'-ffile-prefix-map={source_dir}=/emsdk/emscripten',
'-fdebug-compilation-dir=/emsdk/emscripten']
cflags += [f'-ffile-prefix-map={relative_source_dir}={FAKE_EMSCRIPTEN_PATH}']
cflags += [f'-ffile-prefix-map={source_dir}={FAKE_EMSCRIPTEN_PATH}',
'-fdebug-compilation-dir={FAKE_EMSCRIPTEN_PATH}']
case_insensitive = is_case_insensitive(build_dir)
for src in self.get_files():
ext = shared.suffix(src)
Expand Down

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