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Re: [PATCH] Fix PR84986
- From: Jakub Jelinek <jakub at redhat dot com>
- To: Richard Biener <rguenther at suse dot de>
- Cc: gcc-patches at gcc dot gnu dot org, Jan Hubicka <jh at suse dot de>
- Date: Tue, 20 Mar 2018 11:16:31 +0100
- Subject: Re: [PATCH] Fix PR84986
- References: <alpine.LSU.2.20.1803201100430.18265@zhemvz.fhfr.qr>
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
On Tue, Mar 20, 2018 at 11:08:12AM +0100, Richard Biener wrote:
> With the x86 vectorizer cost-model rewrite we ended up costing
> scalar conversions as nothing. After my patch using the proper
> target cost estimates for the scalar version this now exposes
> underestimating scalar cost and thus no longer vectorizing
> the testcase in this PR. This fix is to restrict zero-costing
> to sign-conversions, all other conversions are possibly
> value-changing. I guess some zero-extensions are free as well
> but I didn't want to get too fancy as I'm not sure about
> QImode -> SImode conversions for example since whether
> that's free (can just use %eax instead of %ax) likely depends on
> context.
Aren't casts from integral to integral with smaller precision also always
zero cost, at least for scalar code? It just expands to using a SUBREG
of whatever holds the larger value.
Jakub