On 12/02/2015 02:05 PM, Paolo Bonzini wrote: > > > 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. :( Yes its indeed the aliasing. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
