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Re: [PATCH] Convert manual unsigned +/- overflow checking into {ADD,SUB}_OVERFLOW (PR target/67089)

From: Marc Glisse <marc dot glisse at inria dot fr>
To: Jakub Jelinek <jakub at redhat dot com>
Cc: gcc-patches at gcc dot gnu dot org, Richard Biener <rguenther at suse dot de>, Richard Henderson <rth at redhat dot com>
Date: Wed, 25 Nov 2015 12:19:04 +0100 (CET)
Subject: Re: [PATCH] Convert manual unsigned +/- overflow checking into {ADD,SUB}_OVERFLOW (PR target/67089)
Authentication-results: sourceware.org; auth=none
References: <20151124205352 dot GS5675 at tucnak dot redhat dot com> <alpine dot DEB dot 2 dot 20 dot 1511250817160 dot 9949 at laptop-mg dot saclay dot inria dot fr> <20151125083627 dot GU5675 at tucnak dot redhat dot com> <alpine dot DEB dot 2 dot 20 dot 1511250944440 dot 9949 at laptop-mg dot saclay dot inria dot fr> <20151125090425 dot GW5675 at tucnak dot redhat dot com>
Reply-to: gcc-patches at gcc dot gnu dot org

On Wed, 25 Nov 2015, Jakub Jelinek wrote:

On Wed, Nov 25, 2015 at 09:58:15AM +0100, Marc Glisse wrote:

I guess it got lost in my text, but if a user writes:

unsigned diff = a - b;
if (b > a) { /* overflow */ ... }
else { ... }

Your patch will not detect it. It seems that replacing x-y>x with y>x could


Sorry, already committed the patch (without incremental that hasn't been
tested anyway).

Sorry, I never meant to imply that your patch was wrong in any way orshould be blocked, I like it. I was only discussing possible futureimprovements...

It is true that the patch does not detect this, but it is harder that way.
What if it is
if (b > a) ...
// Huge amount of code
r = a - b;
?  Trying to emit the subtraction above the comparison would then very
likely increase register preassure.

The same is true whether we write it b > a or (a - b) > a (I don't thinkPRE + SCCVN avoid increasing register pressure).

So, I'd really prefer doing x-y>x to y>x only for single use.


Ok (for now).

--
Marc Glisse

Follow-Ups:
- Re: [PATCH] Convert manual unsigned +/- overflow checking into {ADD,SUB}_OVERFLOW (PR target/67089)
  - From: Jakub Jelinek

References:
- [PATCH] Convert manual unsigned +/- overflow checking into {ADD,SUB}_OVERFLOW (PR target/67089)
  - From: Jakub Jelinek
- Re: [PATCH] Convert manual unsigned +/- overflow checking into {ADD,SUB}_OVERFLOW (PR target/67089)
  - From: Marc Glisse
- Re: [PATCH] Convert manual unsigned +/- overflow checking into {ADD,SUB}_OVERFLOW (PR target/67089)
  - From: Jakub Jelinek
- Re: [PATCH] Convert manual unsigned +/- overflow checking into {ADD,SUB}_OVERFLOW (PR target/67089)
  - From: Marc Glisse
- Re: [PATCH] Convert manual unsigned +/- overflow checking into {ADD,SUB}_OVERFLOW (PR target/67089)
  - From: Jakub Jelinek

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