Why are xmm registers being pxor'ed

Asked Apr 26 '22 at 08:45

Active Apr 26 '22 at 10:17

Viewed 44 times

In the above code, I can't figure out why pxor xmm0, xmm0 takes place.

edited Apr 26 '22 at 10:17

Peter Cordes

asked Apr 26 '22 at 08:45

InexactRex69

2

Please do not post images of code, just share the link that godbolt example. – Quimby Apr 26 '22 at 08:50
1

Okay.. I'll keep that in mind. I'm updating it now – InexactRex69 Apr 26 '22 at 08:54
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That's there to clear the bits of `xmm0`, but it is a good question as to why that's needed. From what I can tell neither `cvtsi2sd`, `divsd` nor `movsd` cares about the most significant 64 bits, everything else is set by `cvtsi2sd` or `divsd`. – Lala5th Apr 26 '22 at 09:03
@Lala5th: It's there to break the false dependency from the short-signed design of `cvtsi2sd` merging into the destination, instead of zero-extending to the full XMM reg. – Peter Cordes Apr 26 '22 at 09:53

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