Clang uses the first value for -ffile-prefix-map, unlike GCC #157721
Description
Usually, I would expect the last value of compiler flags to win, and GCC agrees with me. This is on macOS 26, using Clang and GCC from Nixpkgs.
shion:/v/f/1/j/T/tmp.kqJvZSlwwK
❭ echo __FILE__ > test.c
shion:/v/f/1/j/T/tmp.kqJvZSlwwK
❭ clang -E -ffile-prefix-map=$PWD=/a -ffile-prefix-map=$PWD=/b $PWD/test.c
# 1 "/var/folders/1v/jtp_4pzx7xq371f8j_xdnrvm0000gn/T/tmp.kqJvZSlwwK/test.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 469 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "/var/folders/1v/jtp_4pzx7xq371f8j_xdnrvm0000gn/T/tmp.kqJvZSlwwK/test.c" 2
"/a/test.c"
shion:/v/f/1/j/T/tmp.kqJvZSlwwK
❭ gcc -E -ffile-prefix-map=$PWD=/a -ffile-prefix-map=$PWD=/b $PWD/test.c
# 0 "/var/folders/1v/jtp_4pzx7xq371f8j_xdnrvm0000gn/T/tmp.kqJvZSlwwK/test.c"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/var/folders/1v/jtp_4pzx7xq371f8j_xdnrvm0000gn/T/tmp.kqJvZSlwwK/test.c"
"/b/test.c"
shion:/v/f/1/j/T/tmp.kqJvZSlwwK
❭ clang -v
clang version 21.1.0
Target: arm64-apple-darwin25.0.0
Thread model: posix
InstalledDir: /nix/store/5rqijcr32h4c0cmv43fl0pc7ysryyq5n-clang-21.1.0/bin
shion:/v/f/1/j/T/tmp.kqJvZSlwwK
❭ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/nix/store/54v1wz1xc9vs2s6742lk6s8n0sl6vvj0-gcc-14.3.0/libexec/gcc/arm64-apple-darwin/14.3.0/lto-wrapper
Target: arm64-apple-darwin
Configured with: ../gcc-14.3.0/configure --prefix=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0 --with-gmp-include=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gmp-with-cxx-6.3.0-dev/include --with-gmp-lib=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gmp-with-cxx-6.3.0/lib --with-mpfr-include=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-mpfr-4.2.2-dev/include --with-mpfr-lib=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-mpfr-4.2.2/lib --with-mpc=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-libmpc-1.3.1 --with-native-system-header-dir=/usr/include --with-build-sysroot=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-apple-sdk-11.3/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk --with-gxx-include-dir=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/ --program-prefix= --enable-lto --disable-libstdcxx-pch --without-included-gettext --with-system-zlib --enable-static --enable-languages=c,c++,objc,obj-c++ --disable-multilib --enable-plugin --build=arm64-apple-darwin --host=arm64-apple-darwin --target=arm64-apple-darwin
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 14.3.0 (GCC)