https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112717
Bug ID: 112717
Summary: .gcda profiles compatibility guarantees between GCC
versions
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: gcov-profile
Assignee: unassigned at gcc dot gnu.org
Reporter: zamazan4ik at tut dot by
CC: marxin at gcc dot gnu.org
Target Milestone: ---
Hi. I have several questions regarding .gcda profiles re-usage between GCC
versions for Profile-Guided Optimization (PGO) purposes.
The first question goes about forward and backward guarantees .gcda profiles. I
didn't find related information in the GCC documentation. Are there guarantees
in this area? Like "it's guaranteed that .gcda profiles from GCC version N will
be always readable by GCC version N+1", where N is a minor/major GCC version.
For us it's an important question since we are thinking about caching .gcda
profiles in storage so PGO profiles can be reused later probably with a newer
compiler. This goes in another direction too in the case if we generated the
PGO profile with GCC 10 and some time later decided to revert the compiler to
GCC 9. If there are some guarantees in this area, would be great to see them
documented somewhere in the documentation (probably in a place like
https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html).
The second question is about PGO profiles reusability between GCC versions. As
far as I understand, PGO profiles track some "counters" about the code.
Possibly these counters can rely somehow on the performed by GCC optimizations
(it's just my guess). Let's imagine that GCC 11 added more optimization passes
that affect somehow generated code (e.g. much more aggressive inlining compared
to GCC 10). In this case, probably, PGO profiles from GCC 10 wouldn't be useful
anymore and we will need to regenerate them once again but with GCC 11. Is this
scenario real? If yes, are there ways to mitigate it somehow?
For LLVM I have the same questions that are discussed here:
https://discourse.llvm.org/t/profile-guided-optimization-pgo-related-questions-and-suggestions/75232
. As far as I understand, GCC also implements PGO on something like "IR" (don't
know how it's called properly in GCC - "Generic" or "GIMPLE"?), so probably
some answers from LLVM would be applicable to GCC as well.
