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Re: [PATCH i386]: Enable push/pop in pro/epilogue for modern CPUs


On Sun, Dec 9, 2012 at 2:50 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
> Hello!
>
>> I noticed in prologue/epilogue, GCC prefers to use MOVs followed by a
>> SP adjustment instead of a sequence of pushes/pops. The preference to
>> the MOVs are good for old CPU micro-architectures (before pentium-4,
>> K10), because it breaks the data dependency.  In modern
>> micro-architecture, push/pop is implemented using a mechanism called
>> stack engine. The data dependency is removed by the hardware, and
>> push/pop becomes very cheap (1 uOp, 1 cycle latency), and they are
>> smaller. There is no longer the need to avoid using them.   This is
>> also what ICC does.
>
>> 2012-12-08  Xinliang David Li  <davidxl@google.com>
>>            * config/i386/i386.c: Eanble push/pop in pro/epilogue for moderen CPUs.
>
> s/moderen/modern
>
> OK for mainline SVN.

It's also more costly for unwind info in the prologue/epilogue.  Thus, did you
measure the effect on CFI size?

Thanks,
Richard.

> Thanks,
> Uros.


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