[Bug libstdc++/99876] New: std::filesystem::absolute is inefficient

["terra at gnome dot org" <gcc-bugzilla@gcc.gnu.org>]

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)