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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>,Marc Glisse <marc dot glisse at inria dot fr>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 16 Oct 2014 19:52:30 +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 DEB dot 2 dot 11 dot 1407271853200 dot 22972 at stedding dot saclay dot inria dot fr> <alpine dot DEB dot 2 dot 11 dot 1409071654310 dot 5483 at laptop-mg dot saclay dot inria dot fr> <alpine dot DEB dot 2 dot 11 dot 1410151621380 dot 8286 at stedding dot saclay dot inria dot fr> <543E9BED dot 4070905 at redhat dot com> <CAFiYyc27PjA1A_DZnfWGzU2y10Lp3rEoZs1m7hknq1eJDOqK_Q at mail dot gmail dot com> <alpine dot DEB dot 2 dot 11 dot 1410161321450 dot 27920 at stedding dot saclay dot inria dot fr> <543FFFD8 dot 4090406 at redhat dot com>
On October 16, 2014 7:26:48 PM CEST, Jeff Law <firstname.lastname@example.org> wrote:
>On 10/16/14 08:11, Marc Glisse wrote:
>> I am looking into that, it must be doable. It seems not too hard, in
>> maybe_register_def, to push all results from make_ssa_name to some
>> data-structure (I don't think new_ssa_names gives me that list, but
>> there may be other ways to get it without introducing yet another
>> and either mark them as used in maybe_replace_use or
>> or better loop through them at the end, checking has_zero_uses (it is
>> bit wasteful, only those coming from clobbers may have 0 uses (or we
>> missed a dce/dse earlier), but it should be fast enough, even walking
>> all ssa_names should be fast enough). If we go to that much trouble,
>> may as well clean them while we are there. It isn't obvious to me how
>> notice unused new ssa_names more easily, to trigger a DCE.
>I'd walk the SSA_NAMEs at the end checking for zero uses. I'm curious
>how often that will trigger :-)
Most often for the initial into SSA I guess. After that only for the cases we rename a variable which does not happen often. SRA and update-address-taken cone to my mind.
BTW, I dislike having multiple DCE implementations...