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Re: update address taken: don't drop clobbers
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Jeff Law <law at redhat dot com>
- Cc: Michael Matz <matz at suse dot de>, Marc Glisse <marc dot glisse at inria dot fr>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Fri, 11 Jul 2014 10:10:28 +0200
- Subject: Re: update address taken: don't drop clobbers
- Authentication-results: sourceware.org; auth=none
- References: <alpine dot DEB dot 2 dot 02 dot 1406282350110 dot 31815 at stedding dot saclay dot inria dot fr> <CAFiYyc0fRhV09A3C2WT8yQ1ndp9dcyWntCVSPHzhwHb3tgNZLg at mail dot gmail dot com> <alpine dot LNX dot 2 dot 00 dot 1407101743300 dot 31863 at wotan dot suse dot de> <53BED9FE dot 1040701 at redhat dot com>
On Thu, Jul 10, 2014 at 8:22 PM, Jeff Law <law@redhat.com> wrote:
> On 07/10/14 09:48, Michael Matz wrote:
>>
>> Hi,
>>
>> On Thu, 10 Jul 2014, Richard Biener wrote:
>>
>>> Apart from the out-of-SSA patch you proposed elsewhere a possibility
>>> is to simply never mark undefined SSA names as
>>> SSA_NAME_OCCURS_IN_ABNORMAL_PHI ... (or not mark those
>>> as must-coalesce).
>>
>>
>> The insight to note is, that undefined SSA names should really be
>> coalesced with something (otherwise you lost an optimization opportunity),
>> but it doesn't matter with _what_ each use of the undefined name is
>> coalesced, you can even identify different uses of them with different SSA
>> names (e.g. the LHS of each using stmt). Requires some change in the
>> order things are done in out-of-ssa.
>
> The last part is what I hinted might be problematical. If some undefined
> SSA_NAME appears on the RHS of two PHIs and we want to coalesce that
> undefined SSA_NAME with the LHS of each of those PHIs, then the LHS of those
> two PHIs must coalesce as well. At least that's my recollection of how all
> that stuff worked.
Yes, coalescing doesn't do "live-range splitting" to avoid coalescing the
two PHI results. But they have to be coalesced anyway.
I still think simply never recording conflicts for undefined SSA names
is a proper "hack" to avoid this issue.
> It was that realization that made me wonder if we should have a unique
> SSA_NAME at each undefined use point.
That would be unnecessarily expensive.
Richard.
> jeff
>