https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99876
Bug ID: 99876 Summary: std::filesystem::absolute is inefficient Product: gcc Version: 10.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: terra at gnome dot org Target Milestone: --- Created attachment 50498 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50498&action=edit Preprocesses source code The documentation for std::filesystem::absolute states: "For POSIX-based operating systems, std::filesystem::absolute(p) is equivalent to std::filesystem::current_path() / p except for when p is the empty path. " g++ implements it is way -- that is correct, but wasteful. If the given filename is already absolute, it should be simply returned. There is no need to call current_path() which leads to a getcwd syscall. # /usr/local/products/gcc/10.1.0/bin/g++ -std=gnu++17 -Wall -O2 ttt.C # strace ./a.out 2>&1 | tail getcwd("/work/nova7/23232/src", 4096) = 22 getcwd("/work/nova7/23232/src", 4096) = 22 getcwd("/work/nova7/23232/src", 4096) = 22 getcwd("/work/nova7/23232/src", 4096) = 22 getcwd("/work/nova7/23232/src", 4096) = 22 getcwd("/work/nova7/23232/src", 4096) = 22 getcwd("/work/nova7/23232/src", 4096) = 22 getcwd("/work/nova7/23232/src", 4096) = 22 exit_group(0) = ? +++ exited with 0 +++ # cat ttt.C #include <filesystem> int main() { std::filesystem::path foo ("/home/welinder"); for (int i = 0; i < 10000; i++) (void)std::filesystem::absolute(foo); } # uname -a Linux monsterd03 5.3.18-lp152.66-default #1 SMP Tue Mar 2 13:18:19 UTC 2021 (73933a3) x86_64 x86_64 x86_64 GNU/Linux # /usr/local/products/gcc/10.1.0/bin/g++ -v Using built-in specs. COLLECT_GCC=/usr/local/products/gcc/10.1.0/bin/g++ COLLECT_LTO_WRAPPER=/usr/local/products/gcc/10.1.0/lib/gcc/x86_64-suse-linux/10.1.0/lto-wrapper Target: x86_64-suse-linux Configured with: ../../gcc-10.1.0/configure --enable-languages=c,c++,fortran --enable-targets=x86_64-suse-linux,i686-suse-linux --prefix=/usr/local/products/gcc/10.1.0 --with-gnu-as --with-as=/usr/local/products/gcc/binutils-2.32/bin/as --with-gnu-ld --with-ld=/usr/local/products/gcc/binutils-2.32/bin/ld --enable-threads=posix --enable-shared --enable-__cxa_atexit --enable-libstdcxx-allocator=pool x86_64-suse-linux Thread model: posix Supported LTO compression algorithms: zlib gcc version 10.1.0 (GCC)