On 12/02/2015 01:20 PM, Paolo Bonzini wrote:
>
>
> On 02/12/2015 12:06, Christian Borntraeger wrote:
>> + memcpy(&vcpu->run->s.regs.gprs[14], &vcpu->arch.sie_block->gg14, 16);
>
> This is preexisting but... boy it's ugly. :)
>
> 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.
the memcpy gives
memcpy(&vcpu->arch.sie_block->gg14, &vcpu->run->s.regs.gprs[14], 16);
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: d2 0f 10 a0 21 b8 mvc 160(16,%r1),440(%r2)
I will prepare a patch and do my usual micro benchmark. Unless
things get much worse I will schedule this for the next pull.
Christian
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