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Re: [patch] explicit_bzero constraint on the destination buffer
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