Does GCC record optimization information into binary file?

[Qing Zhao]

> On Jan 29, 2026, at 11:39, Jakub Jelinek <jakub@redhat.com> wrote:
> 
> On Thu, Jan 29, 2026 at 04:08:46PM +0000, Qing Zhao wrote:
>>> Well, inlining is recorded in debug info.
>> 
>> Okay, that’s good to know. So, user can examine the debug section in binary to get the inlining information?
> 
> There is information on which routines have been inlined into what, just
> readelf -wi and look for DW_TAG_inlined_subroutine tags, with
> DW_AT_abstract_origin pointing to what has been inlined at that point.
> There are no details why the inliner chose to inline something and not
> something else.

Thanks a lot for the detailed information. This is helpful.
> 
>> How about the profiling information, especially which routines are optimized by using the profiling info? Is such info recorded in binary?
> 
> That is very fuzzy, we don't really track what exact optimizations we've
> done based on profile info and what for other reasons.
> You can look again at DW_AT_producer in debug info, if a function is in
> a TU compiled with -fprofile-use, then presumably it has been optimized
> using the profiling info.  Though, that just mostly means when reading the
> *.gcda files it initialized edge probabilities and bb counts from the
> observed counts rather than from heuristics.  And optimizations then just
> use those probabilities/counts to decide on optimizations.  Sure,
> -fprofile-use doesn't do just that, but still.  You can find out if
> probabilities/counts were *.gcda based or heuristics based, but not whether
> anything actually used that info in some way and for what.

So, you mean that the “DW_AT_producer” tag in the binary can distinguish 
whether a routine uses real profiling data or only heuristics?

If so, I guess that will be useful too. 

> Especially not
> whether the probabilities/counts were different enough from heuristic based
> ones to result in different optimization decisions.  You'd need to compile
> twice, once without -fprofile-use, and compare.

one question here: if one routine whose profiling data is available in *.gcda, but
the profiling data is bad due to some reason, as a result, the profiling data is ignored. 
Is there “DW_AT_producer” tag for such routine?

Qing
> 
> Jakub
>