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Date: Sun, 12 Apr 2020 00:27:49 -0700
Message-ID: <CAJn6YFC-brTm=nkC9_MdgZp3rj+NeMSctUYxk-8wsUThxQmRfQ@mail.gmail.com>
Subject: g++ with -fprofile-dir flag has a bug (backslash instead of
 forward-slash issue)
To: cygwin AT cygwin DOT com
From: John Selbie via Cygwin <cygwin AT cygwin DOT com>
Reply-To: John Selbie <jselbie AT gmail DOT com>
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TLDR: With gcc/g++ 9.2.0 and 9.30 on Cygwin, when you use
-fprofile-generate and -fprofile-dir together, the target path for the
.gcda file is corrupted with a backslash instead of having a forward slash
used.

Here's a sample run where profile guided optimization is getting enabled
for a simple little C++ program:

    jselbie AT IRONMAIDEN ~/bench
    $ g++ bench.cpp -o bench -fprofile-generate
-fprofile-dir=/home/jselbie/bench/profile -O3

    jselbie AT IRONMAIDEN ~/bench
    $ ./bench.exe

profiling:/home/jselbie/bench/profile/#home#jselbie#bench\bench.gcda:Skip

That "profiling...Skip" line is an error indicating that the profiling
library can't save the gcda file. If you look closely,there's backslash
between the encoded path and the filename (bench.gcda).

Doing a strings search on the binary reveals the same thing:

    jselbie AT IRONMAIDEN ~/bench
    $ strings bench.exe | grep jselbie
    /home/jselbie/bench/profile/#home#jselbie#bench\bench.gcda

If I switch back to Linux and do the same steps, it not only works, but you
can see there is no backslash embedded into the binary either

    jselbie AT IRONMAIDEN:~/bench$ g++ bench.cpp -o bench -fprofile-generate
-fprofile-dir=/home/jselbie/bench/profile -O3

    jselbie AT IRONMAIDEN:~/bench$ ./bench

    jselbie AT IRONMAIDEN:~/bench$ ls profile/
    bench.gcda

    jselbie AT IRONMAIDEN:~/bench$ strings bench | grep jselbie
    /home/jselbie/bench/profile/bench.gcda


The workaround is to just skip the -fprofile-dir flag altogether.

Seems like there is a bug in Cygwin's build of gcc/g++ that puts the wrong
path separator into the executable

Can this be fixed?  Or a place where I can file a bug?

jrs
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