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Re: gcc -S vs clang -S


On Tue, May 12, 2015 at 6:36 PM, Fei Ding <fdingiit@gmail.com> wrote:
> I think Thiago and Eric just want to know which code-gen is better and why...


You need to understand for a complex process (CISC ISAs) like x86,
there is no one right answer sometimes.  You need to look at each
micro-arch and understand the pipeline.  Sometimes different code
stream will performance the same but it also depends on the code size
too.

Thanks,
Andrew Pinski

>
> 2015-05-12 23:29 GMT+08:00 Eric Botcazou <ebotcazou@libertysurf.fr>:
>>> Note that at -O3 there is a difference still:
>>> clang (3.6.0):
>>>         addl    %esi, %edi
>>>         movl    %edi, %eax
>>>         retq
>>>
>>> gcc (4.9.2)
>>>         leal    (%rdi,%rsi), %eax
>>>         ret
>>>
>>> Can't tell which is best, if any.
>>
>> But what's your point exactly here?  You cannot expect different compilers to
>> generate exactly the same code on a given testcase for non-toy architectures.
>>
>> Note that this kind of discussion is more appropriate for gcc-help@gcc.gnu.org
>>
>> --
>> Eric Botcazou


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