Re: [PR64164] drop copyrename, integrate into expand

[Alexandre Oliva <aoliva@redhat.com>]

On Sep 29, 2015, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:

> this commit

> commit 33cc9081157a8c90460e4c0bdda2ac461a3822cc
> Author: aoliva <aoliva@138bc75d-0d04-0410-961f-82ee72b054a4>
> Date:   2015-09-27 09:02:00 +0000

>     revert to assign_parms assignments using default defs
>     ...

> introduced a test failure on arm-none-eabi (using newlib, compiling
> with -mthumb -march=armv8-a -mfpu=crypto-neon-fp-armv8 -mfloat-abi=hard ):

> FAIL: gcc.target/arm/pr43920-2.c scan-assembler-times pop 2

Thanks for the report.

The problem here is that we don't allocate the pseudo assigned to
<retval> to r0.  That's because we coalesce <retval> versions with
another variable that crosses a function call.  We do that because
uncprop brings these unrelated variables, that happen to contain the
same -1 value we want to return, into the PHI node with the final
<retval> value.

We can't coalesce both start and end with <retval>, because start and
end conflict, but by chance we try start first, and that succeeds.  If
we tried end first (e.g., by giving it a higher coalesce priority,
because fewer calls are crossed by its value in the path to the relevant
edge), we could have got the coalesced variable assigned to r0, and that
would enable us to optimize out the copy to r0 before return, and so
merge the return-only basic block with other blocks.  But ATM we don't
take the definition point or path to the edge into account when
computing coalesce costs, so we can't deterministically do better for
this testcase, and I'm not sure using these additional information would
make it better overall.

Compiling with -fno-tree-dominator-opts skips uncprop so that we don't
even try to coalesce other variables with <retval>, so we get the code
expected by the testcase.  But we obviously don't want to disable this
optimization in general.

Any other thoughts, anyone?

-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer