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Re: [PR64164] drop copyrename, integrate into expand
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Alexandre Oliva <aoliva at redhat dot com>
- Cc: Szabolcs Nagy <szabolcs dot nagy at arm dot com>, Alan Lawrence <Alan dot Lawrence at arm dot com>, Jeff Law <law at redhat dot com>, James Greenhalgh <James dot Greenhalgh at arm dot com>, "H.J. Lu" <hjl dot tools at gmail dot com>, Segher Boessenkool <segher at kernel dot crashing dot org>, GCC Patches <gcc-patches at gcc dot gnu dot org>, Christophe Lyon <christophe dot lyon at linaro dot org>, David Edelsohn <dje dot gcc at gmail dot com>, Eric Botcazou <ebotcazou at adacore dot com>
- Date: Thu, 8 Oct 2015 12:00:42 +0200
- Subject: Re: [PR64164] drop copyrename, integrate into expand
- Authentication-results: sourceware.org; auth=none
- References: <orio9cw10j dot fsf at livre dot home> <CAFiYyc3k9xh_6RGbTdHYOJ-aEmUWVeFRoWy=YzCHFDNSdtTXCA at mail dot gmail dot com> <orwpxqvqnp dot fsf at livre dot home> <20150723203112 dot GB27818 at gate dot crashing dot org> <CAMe9rOpR+2gPxo0tKaRPtcML_Q4=r-_=9iqk+_JZFPkM=eN=BQ at mail dot gmail dot com> <CAMe9rOpbLEyDexVJqJAFJ3W6o4AktNog-jwk2CY4GZkrmT+nfA at mail dot gmail dot com> <or4mkmhgc9 dot fsf at livre dot home> <CAMe9rOp=S5fu1N=i7waswCYqJeLBCrySqYdFYkVa7LV04vpQSg at mail dot gmail dot com> <CAMe9rOrq+ZBAg1nZ1twEcPqwBj4j9+XA+SXQJVWWzjfdvidjtw at mail dot gmail dot com> <or1tfkdjhj dot fsf at livre dot home> <20150810082355 dot GA31149 at arm dot com> <55C8BFC3 dot 3030603 at redhat dot com> <CAKQMxzRzMrGtf921vqXCno5uoBN+uzsnJ5wX2Twmvhp1ziAEcA at mail dot gmail dot com> <or37zlpujd dot fsf at livre dot home> <55E72D4C dot 40705 at arm dot com> <orfv2wxygm dot fsf at livre dot home> <55FC3171 dot 7040509 at arm dot com> <ord1x8nblu dot fsf at livre dot home> <560A6B26 dot 7010701 at arm dot com> <orvbai8fwl dot fsf at livre dot home>
On Thu, Oct 8, 2015 at 12:36 AM, Alexandre Oliva <aoliva@redhat.com> wrote:
> 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?
Bad luck? Add some heuristics that always help?
Ok, that wasn't really useful :/
Richard.
>
> --
> 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