From: Eric Botcazou <ebotcazou@adacore.com>
To: Jeff Law <law@redhat.com>
Cc: gcc-patches@gcc.gnu.org,
Michael Collison <Michael.Collison@arm.com>,
Segher Boessenkool <segher@kernel.crashing.org>,
Kyrill Tkachov <kyrylo.tkachov@foss.arm.com>, nd <nd@arm.com>
Subject: Re: [PATCH][compare-elim] Merge zero-comparisons with normal ops
Date: Sat, 14 Oct 2017 08:45:00 -0000 [thread overview]
Message-ID: <1650196.elqVzIhKdR@polaris> (raw)
In-Reply-To: <07ed2aa1-263d-a8f4-5603-47ef5b96e425@redhat.com>
> 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...
--
Eric Botcazou
next prev parent reply other threads:[~2017-10-14 8:39 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-10 22:03 Michael Collison
2017-08-28 18:43 ` Jeff Law
2017-08-29 9:25 ` Kyrill Tkachov
2017-09-01 23:07 ` Segher Boessenkool
2017-09-06 15:56 ` Michael Collison
2017-10-13 18:04 ` Jeff Law
2017-10-14 8:45 ` Eric Botcazou [this message]
2017-10-17 11:57 ` Richard Biener
2017-10-17 19:29 ` Michael Collison
2017-10-17 20:05 ` Eric Botcazou
2017-10-17 20:12 ` Richard Biener
2017-10-17 20:29 ` Michael Collison
2017-10-18 14:14 ` Eric Botcazou
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