https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128483
--- Comment #6 from vekumar at gcc dot gnu.org ---
(In reply to Reshma Roy from comment #5)
> (In reply to Hongtao Liu from comment #4)
> > (In reply to Reshma Roy from comment #3)
> > > (In reply to Richard Biener from comment #2)
> > > > IMO a better fix would be to use LCM to common/place the CSEd loads
> > > > since
> > > > that would avoid placing loads where it would be possibly not executed
> > > > before (so it avoids speculation). Unless speculating the loads was
> > > > intended.
> > >
> > > I agree. But from my understanding from the pass, it looks like the
> > > speculative load was intended. There is the following comment in the pass
> > > right before the function where the cse happens as below:
> > > "NB: We want to generate a pattern, which is executed only once, to
> > > cover the whole function. The LCM algorithm isn't appropriate here
> > > since it may place a pattern inside the loop. */"
> > > Hence, I am confused how we should approach this problem.
> > >
> > The comments is copied from remove_partial_avx_dependency.
> > When we were implementing remove_partial_avx_dependency, we had two
> > approaches on the table: using LCM to insert pxor instructions, or hoisting
> > them up to the dominator. Looking back, we should have caught that the LCM
> > approach wasn't hoisting some pxor instructions out of loops — probably
> > because LCM saw those branches as low-probability and decided not to bother.
> > For x86_cse, LCM is likely the better fit, and at the very least we should
> > be factoring in profile data.
> >
> > 3563 NB: We want to generate only a single vxorps to cover the whole
> > 3564 function. The LCM algorithm isn't appropriate here since it may
> > 3565 place a vxorps inside the loop. */
> > 3566
> > 3567static unsigned int
> > 3568remove_partial_avx_dependency (void)
> > 3569{
>
> Thanks for explaining. I will try a patch that uses LCM to place the
> broadcasts (X86_CSE_VEC_DUP and X86_CSE_CONST_VECTOR) in x86_cse.
Fix for PR92080 helps reduce the redundant vector xor by moving it nearest
dominator. They are zero latency instructions. Using LCM could make the tests
fail.
Fix for PR120941 stops X86_CSE_VEC_DUP from moving out of loop. If we move to
LCM, we should check the issue is also not resurfacing.
