Re: [patch] explicit_bzero constraint on the destination buffer

[David CARLIER <devnexen@gmail.com>]

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