THe initial intent of the change was to only affect
`-fmacro-prefix-map=` option.
Due to the bug of `if (maps == macro_prefix_maps)` condition of initial
setting all three of:
static file_prefix_map *macro_prefix_maps; /* -fmacro-prefix-map */
static file_prefix_map *debug_prefix_maps; /* -fdebug-prefix-map */
static file_prefix_map *profile_prefix_maps; /* -fprofile-prefix-map */
matches the comparison and applied the mangling (as long as on options
were passed into those before).
As a result not only (intended) `__FILE__` embedding happened in `.data`
section, but also (unintended) debugging symbols (`-fdebug-prefix-map`)
and profiling data (`-fprofile-prefix-map`) were broken by mangling.
The patch update fixes it by explicitly passing a boolean that controls
the mangling in a single call site relevant to `-fmacro-prefix-map`.
While at it fixed `int / size_t` mismatch that caused build failure on
upcoming `gcc-14`.
Tested as:
- `nix` still has no `nlohmann_json` retention
- `gdb` can now resolve `stdc++` debugging symbols in templates
- `--coverage` has working source file paths
the mangling done by mangle-NIX_STORE-in-__FILE__.patch also applies to
source paths embedded in debug symbols. When putting a breakpoint in a
template instanciation from another lib, the path that gdb looks for is
therefore mangled (/nix/store/eeeeeee;...-the-lib-dev/include/foo.h)
This severely degrades the debugging experience. To alleviate that, it's
possible to make the mangling reversible: a debuginfod server can then
reverse the mangling. I plan to implement that in nixseparatedebuginfod.
The reversible mangling that was chosen in making the hash of the store
path uppercase.
Without the change `__FILE__` used in static inline functions in headers
embed paths to header files into executable images. For local headers
it's not a problem, but for headers in `/nix/store` this causes `-dev`
inputs to be retained in runtime closure.
Typical examples are `nix` -> `nlonhmann_json` and `pipewire` ->
`lttng-ust.dev`.
Ideally we would like to use `-fmacro-prefix-map=` feature of `gcc` as:
-fmacro-prefix-map=/nix/store/$hash1-nlohmann-json-ver=/nix/store/eeee.eee-nlohmann-json-ver
-fmacro-prefix-map=/nix/...
In practice it quickly exhausts argument lengtth limit due to `gcc`
deficiency: https://gcc.gnu.org/PR111527
Until it;s fixed let's hardcode header mangling if $NIX_STORE variable
is present in the environment.
Tested as:
$ printf "# 0 \"/nix/store/01234567890123456789012345678901-pppppp-vvvvvvv\" \nconst char * f(void) { return __FILE__; }" | NIX_STORE=/nix/store ./gcc/xgcc -Bgcc -x c - -S -o -
...
.string "/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-pppppp-vvvvvvv"
...
Mangled successfully.
