https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89071
--- Comment #3 from Peter Cordes <peter at cordes dot ca> --- (In reply to H.J. Lu from comment #1) I have a patch for PR 87007: > > https://gcc.gnu.org/ml/gcc-patches/2019-01/msg00298.html > > which inserts a vxorps at the last possible position. vxorps > will be executed only once in a function. That's talking about the mem,reg case, which like I said is different. I reported Bug 80571 a while ago about the mem,reg case (or gp-reg for si2ss/d), so it's great that you have a fix for that, doing one xor-zeroing and reusing that as a merge target for a whole function / loop. But this bug is about the reg,reg case, where I'm pretty sure there's nothing to be gained from xor-zeroing anything. We can fully avoid any false dep just by choosing both source registers = src, making the destination properly write-only. If you *have* an xor-zeroed register, there's no apparent harm in using it as the merge-target for a reg-reg vcvt, vsqrt, vround, or whatever, but there's no benefit either vs. just setting both source registers the same. So whichever is easier to implement, but ideally we want to avoid introducing a vxorps into functions / blocks that don't need it at all.
