explicit_bzero constraint on the destination buffer

  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 

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
  

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
  

On 28/06/2018 15:37, David CARLIER wrote:
> 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.

Which version exactly are you seeing any code generation differences and
which kind of differences?

> 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