Re: [GIT PULL 04/23] KVM: s390: rewrite vcpu_post_run and drop out early

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On 02/12/2015 14:04, Christian Borntraeger wrote:
>> > Do you gain much over the simpler
>> > 
>> > 	vcpu->run->s.regs.gprs[14] = vcpu->arch.sie_block->gg14;
>> > 	vcpu->run->s.regs.gprs[15] = vcpu->arch.sie_block->gg15;
>> > 
> Its just legacy code from the old days.
> There is a difference, but it seems to a missed opportunity from gcc
> 
>         vcpu->arch.sie_block->gg14 = vcpu->run->s.regs.gprs[14];
>     839c:       e3 30 f0 b8 00 04       lg      %r3,184(%r15)
>     83a2:       e3 10 32 40 00 04       lg      %r1,576(%r3)
>     83a8:       e3 20 30 80 00 04       lg      %r2,128(%r3)
>     83ae:       e3 20 21 b8 00 04       lg      %r2,440(%r2)
>     83b4:       e3 20 10 a0 00 24       stg     %r2,160(%r1)
>         vcpu->arch.sie_block->gg15 = vcpu->run->s.regs.gprs[15];
>     83ba:       e3 10 32 40 00 04       lg      %r1,576(%r3)
>     83c0:       e3 20 30 80 00 04       lg      %r2,128(%r3)
>     83c6:       e3 20 21 c0 00 04       lg      %r2,448(%r2)
>     83cc:       e3 20 10 a8 00 24       stg     %r2,168(%r1)
> 
> gcc seems to reuse and reload %r2 and %r3, maybe register pressure.

More likely to be -fno-strict-aliasing. :(

Paolo
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