[Bug rtl-optimization/87600] Fix for PRs 86939 and 87479 causes build issues for several targets

[Bug rtl-optimization/87600] Fix for PRs 86939 and 87479 causes build issues for several targets

[aoliva at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87600

Alexandre Oliva <aoliva at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |aoliva at gcc dot gnu.org

--- Comment #8 from Alexandre Oliva <aoliva at gcc dot gnu.org> ---
I've noticed regressions caused by make_more_copies, in scenarios that used
subreg s for the low part of promoted incoming parms.  With hard regs, the
substitution into a subreg became a reg, but with a pseudo, it remains a
subreg, which prevents further combines in some cases, as in e.g.
gcc_target/powerpc/20050603-3.c on ppc64.

I thought one way to go about it could be to scan for subregs of pseudos copied
from hard regs before introducing the additional copies, and introduce the
intermediate pseudo with the widest subreg mode if there aren't uses of the
full pseudo.

[Bug rtl-optimization/87600] Fix for PRs 86939 and 87479 causes build issues for several targets

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87600

--- Comment #9 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Alexandre Oliva from comment #8)
> I've noticed regressions caused by make_more_copies, in scenarios that used
> subreg s for the low part of promoted incoming parms.  With hard regs, the
> substitution into a subreg became a reg, but with a pseudo, it remains a
> subreg, which prevents further combines in some cases, as in e.g.
> gcc_target/powerpc/20050603-3.c on ppc64.

That happens because of the zero_extract problems.  The only thing
make_more_copies has to do with this is that this change exposes this
problem on this testcase.

> I thought one way to go about it could be to scan for subregs of pseudos
> copied from hard regs before introducing the additional copies, and
> introduce the intermediate pseudo with the widest subreg mode if there
> aren't uses of the full pseudo.

Usually we want the *smallest* possible mode for pseudos.  This is
important to get good optimisation.

[Bug rtl-optimization/87600] Fix for PRs 86939 and 87479 causes build issues for several targets

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87600

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |siarhei.siamashka at gmail dot com

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 46164 has been marked as a duplicate of this bug. ***

[Bug rtl-optimization/87600] Fix for PRs 86939 and 87479 causes build issues for several targets

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87600

--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #4)
> Oh btw, the #c3 problem isn't new at all, it happens with 4.8 already for
> example.

Yes it was originally reported as PR 46164 against GCC 4.5.1 even.