[Bug middle-end/51663] Desirable/undesirable elimination of unused variables & functions at -O0, -O0 -flto and -O0 -fwhole-program

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51663

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #11)
> Cross-referencing PR gdb/25684 - "gdb testing with gcc -flto" (
> https://sourceware.org/bugzilla/show_bug.cgi?id=25684 ).
> 
> Ideally there would be a way to enable the lto infrastructure without
> actually optimizing, such that when running the gdb testsuite with and
> without flto and comparing results, any regression would indicate something
> that needs fixing.
> 
> In the current situation, each individual regression needs investigation
> whether something needs fixing or whether the failure is just an
> optimization artifact. And due to the fact there are optimizations, there
> are thousands of such regressions.

I suppose we're talking about -O0 -flto here.  What kind of transforms
are undesirable?  I think at -O0 you'll get

 - more aggressive unused variable/function removal
 - promotion of variables from global to local

some of the transforms are unavoidable due to partitioning(?) but we could
default to 1:1 partitioning at -O0 ...