Richard and Eric,

I see you have objected and indicated the additional cost. Have you quantified 
how much more expensive the pass is?

-----Original Message-----
From: Richard Biener [mailto:richard.guent...@gmail.com] 
Sent: Tuesday, October 17, 2017 4:45 AM
To: Eric Botcazou <ebotca...@adacore.com>
Cc: Jeff Law <l...@redhat.com>; GCC Patches <gcc-patches@gcc.gnu.org>; Michael 
Collison <michael.colli...@arm.com>; Segher Boessenkool 
<seg...@kernel.crashing.org>; Kyrill Tkachov <kyrylo.tkac...@foss.arm.com>; nd 
<n...@arm.com>
Subject: Re: [PATCH][compare-elim] Merge zero-comparisons with normal ops

On Sat, Oct 14, 2017 at 10:39 AM, Eric Botcazou <ebotca...@adacore.com> wrote:
>> This looks good.  OK for the trunk.
>
> FWIW I disagree.  The patch completely shuns the existing 
> implementation of the pass, which is based on a forward scan within 
> basic blocks to identify the various interesting instructions and 
> record them, and uses full-blown def-use and use-def chains instead, 
> which are much more costly to compute.  It's not clear to me why the existing 
> implementation couldn't have been extended.
>
> The result is that, for targets for which the pass was initially written, i.e.
> targets for which most (all) arithmetic instructions clobber the 
> flags, the pass will be slower for absolutely no benefits, as the 
> existing implementation would already have caught all the interesting cases.
>
> So it's again a case of a generic change made for a specific target 
> without consideration for other, admittedly less mainstream, targets...

I agree with Eric here.

Richard.

> --
> Eric Botcazou

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