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Re: [PATCH v2] Add uaddv_optab, usubv4_optab

From: Richard Henderson <rth at redhat dot com>
To: Segher Boessenkool <segher at kernel dot crashing dot org>
Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, Eric Botcazou <ebotcazou at libertysurf dot fr>, Jakub Jelinek <jakub at redhat dot com>, Uros Bizjak <ubizjak at gmail dot com>
Date: Mon, 23 Nov 2015 13:10:56 +0100
Subject: Re: [PATCH v2] Add uaddv_optab, usubv4_optab
Authentication-results: sourceware.org; auth=none
References: <56519B27 dot 7080704 at redhat dot com> <20151122165740 dot GA32568 at gate dot crashing dot org>

On 11/22/2015 05:57 PM, Segher Boessenkool wrote:

Hi Richard,

On Sun, Nov 22, 2015 at 11:38:31AM +0100, Richard Henderson wrote:

One of which I believe I've worked around in the i386 backend, but I
believe to be a latent problem within combine.

With the following patch, disable the add<mode>3_*_overflow_2 patterns.
Then compile c-c++-common/torture/builtin-arith-overflow-4.c with -O2 and
you'll see

  t151_2add:
        testb   %dil, %dil
        leal    -1(%rdi), %eax
        jne     .L644


0xff + x < 0xff  (everything as unsigned char) is the same as  x != 0 .

You'd think yes. But certainly something right there triggered the abort thatfails the test case. Perhaps I simply mis-identified the error, but the "fix"for this fixed the other as well.

r~

References:
- [PATCH v2] Add uaddv_optab, usubv4_optab
  - From: Richard Henderson
- Re: [PATCH v2] Add uaddv_optab, usubv4_optab
  - From: Segher Boessenkool

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