Make it possible to have different instrumented and feedback builds without copying gcda files around [pr93401]

Hi,
in GCC 8 we changed -fprofile-generate=<path> to use mangled absolute paths in
the <path> directory. This was necessary to avoid clashes of files when gcc is
executed from different directories to build different sources of same
filename.

However this made it difficult to build projects on setups where instrumented
build is done in one directory, feedback build in different and possibly
training happens in yet another directory structure.  This happens i.e. for
Firefox builds for month or two.

This patch adds -fprofile-prefix-path that can be used to inform gcc where the
root of build directory is and strip it form the gcda filenames.
This is similar to exisitng debug-prefix-map but without the map feature since
it seems useless for profile data.

We spent quite some time with Maritn Liska discussing options and found no
better solution.  I was looking how this work on LLVM and they produce single
profdata file which is then transformed into kind of simple database by
llvmprofdata tool.  This database keys functions by filename and symbol name.
If you arrane two files with same name define static variable with same symbol
name this gets messedup and result in wrong info. So I think this is not very
good solution and preffer the extra option.

Bootstrapped/regtested x86_64-linux. I plan to commit it later today if there
are no complains.

I suppose our manual could have some central section on profile feedback
explaining the whole setup at one place.

Honza

	PR gcov-profile/93401
	* common.opt (profile-prefix-path): New option.
	* coverae.c: Include diagnostics.h.
	(coverage_init): Strip profile prefix path.
	* doc/invoke.texi (-fprofile-prefix-path): Document.

1 files changed, 17 insertions, 2 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 3bb95ffd220..8b5cd8246a9 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -549,8 +549,9 @@ Objective-C and Objective-C++ Dialects}.
 @gccoptlist{-p  -pg  -fprofile-arcs  --coverage  -ftest-coverage @gol
 -fprofile-abs-path @gol
 -fprofile-dir=@var{path}  -fprofile-generate  -fprofile-generate=@var{path} @gol
--fprofile-note=@var{path}  -fprofile-update=@var{method} @gol
--fprofile-filter-files=@var{regex}  -fprofile-exclude-files=@var{regex} -fprofile-reproducibility @gol
+-fprofile-note=@var{path} -fprofile-prefix-path=@var{path} @gol
+-fprofile-update=@var{method} -fprofile-filter-files=@var{regex} @gol
+-fprofile-exclude-files=@var{regex} -fprofile-reproducibility @gol
 -fsanitize=@var{style}  -fsanitize-recover  -fsanitize-recover=@var{style} @gol
 -fasan-shadow-offset=@var{number}  -fsanitize-sections=@var{s1},@var{s2},... @gol
 -fsanitize-undefined-trap-on-error  -fbounds-check @gol
@@ -13400,6 +13401,20 @@ If @var{path} is specified, GCC saves @file{.gcno} file into @var{path}
 location.  If you combine the option with multiple source files,
 the @file{.gcno} file will be overwritten.
 
+@item -fprofile-prefix-path=@var{path}
+
+This option can be used in combination with
+@option{profile-generate=}@var{profile_dir} and
+@option{profile-use=}@var{profile_dir} to inform GCC where is the base
+directory of built source tree.  By default @var{profile_dir} will contain
+files with mangled absolute paths of all object files in the built project.
+This is not desirable when directory used to build the instrumented binary
+differs from the directory used to build the binary optimized with profile
+feedback because the profile data will not be found during the optimized build.
+In such setups @option{-fprofile-prefix-path=}@var{path} with @var{path}
+pointing to the base directory of the build can be used to strip the irrelevant
+part of the path and keep all file names relative to the main build directory.
+
 @item -fprofile-update=@var{method}
 @opindex fprofile-update