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In-Reply-To: <5ec6e4bf-614d-647b-f68c-5970516fd00e@linaro.org>
From: David CARLIER <devnexen@gmail.com>
Date: Thu, 28 Jun 2018 18:38:00 -0000
Message-ID: <CA+XhMqzhsFwUzmyV_ofv5enkP6srWo+0zDb31sC4P2ba+c4SQQ@mail.gmail.com>
Subject: Re: [patch] explicit_bzero constraint on the destination buffer
To: adhemerval.zanella@linaro.org
Cc: libc-alpha@sourceware.org
Content-Type: text/plain; charset="UTF-8"
X-SW-Source: 2018-06/txt/msg00915.txt.bz2

Ah right indeed it has effect only with older compilers on gcc 7.3
theree is still the additional instruction, default clang 4.0 has but
not clang 6.0 for example. And I ran with those older versions sorry
for the noise.
On Thu, 28 Jun 2018 at 18:16, Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 28/06/2018 13:05, David CARLIER wrote:
> > Hi,
> >
> > This is my first message but here a little patch proposal for
> > explicit_bzero function.
> >
> > Hope it s good.
> >
> > Thanks.
> >
> > Kind regards.
> >
>
> Does it improve the generate code and/or compile support in any meaningful
> way? What are you trying to accomplish by passing the memory pointer as an
> input operand?
>
> At least for x86_64 and aarch64 I am not seeing any code generation
> difference. For powerpc64le it seems it forces the frame-pointer to be
> saved/restored on GCC 6.2.1:
>
> $ diff -u original.S patched.S
> --- original.S  2018-06-28 14:09:55.576497556 -0300
> +++ patched.S   2018-06-28 14:11:26.914362203 -0300
> @@ -12,18 +12,21 @@
>         addi 2,2,.TOC.-.LCF0@l
>         .localentry     explicit_bzero,.-explicit_bzero
>         mflr 0
> +       std 31,-8(1)
>         mr 5,4
>         li 4,0
> +       mr 31,3
>         std 0,16(1)
> -       stdu 1,-32(1)
> +       stdu 1,-48(1)
>         bl memset
>         nop
> -       addi 1,1,32
> +       addi 1,1,48
>         ld 0,16(1)
> +       ld 31,-8(1)
>         mtlr 0
>         blr
>         .long 0
> -       .byte 0,0,0,1,128,0,0,0
> +       .byte 0,0,0,1,128,1,0,0
>         .size   explicit_bzero,.-explicit_bzero
>         .ident  "GCC: (GNU) 6.2.1 20161129 [gcc-6-branch revision 242956]"
>         .section        .note.GNU-stack,"",@progbits
>
> I do not see it on GCC 8.1.1, but I sceptical this change brings any advantage.
>
> [1] https://sourceware.org/ml/libc-alpha/2016-12/msg00537.html