[llvm-dev] imm COPY generated by PHI elim not propagated

[Ryan Taylor via llvm-dev]

This would require getting the reaching definition which requires live
intervals analysis.


On Thu, Nov 14, 2019 at 12:15 PM Quentin Colombet <qcolombet at apple.com>
wrote:

> That sounds like the folding could be done when you expand the copy in
> expand pseudo after regalloc.
>
> > On Nov 14, 2019, at 12:20 AM, Arsenault, Matthew <
> Matthew.Arsenault at amd.com> wrote:
> >
> > In this case the load imm is foldable into the copy, once converted to a
> mov. Directly folding this would be 4 v_mov_b32 instead of 5 produced
> currently
> >
> > -Matt
> >
> > On 11/14/19, 07:20, "llvm-dev on behalf of Quentin Colombet via
> llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of
> llvm-dev at lists.llvm.org> wrote:
> >
> >    Hi Ryan,
> >
> >    Unless you can fold your immediate directly in an instruction, it is
> actually not profitable to propagate them. Indeed you will end up with a
> bunch of load imm instead of reusing a register that already hold this
> value.
> >
> >    The way it works right now is, if holding this value in a register is
> too expensive, i.e., it triggers a spill, then we rematerialize the
> immediate instead of holding a register for it.
> >
> >    Cheers,
> >    -Quentin
> >
> >> On Nov 13, 2019, at 7:36 AM, Ryan Taylor via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> I have some code such that:
> >>
> >> vgpr1 = mov 0
> >> branch bb
> >> bb:
> >> PHI vgpr2 = vgpr1, ….
> >> PHI vgpr3 = vgpr1, ….
> >> PHI vgpr4 = vgpr1, ….
> >> PHI vgpr5 = vgpr1, ….
> >>
> >> PHI node elimination is generating copies for all these PHIs (and
> hoisting them) as such:
> >>
> >> vgpr1 = 0
> >> vgpr20 = COPY vgpr1 // old vgpr2
> >> vgpr30 = COPY vgpr1 // old vgpr3
> >> vgpr40 = COPY vgpr1 // old vgpr4
> >> vgpr 50 = COPY vgprt1 // old vgpr5
> >>
> >> I expect the zero to get propagated in a later phase but it's not. I
> was looking at adding immediate folding to the register coalescer but this
> doesn't really seem like the right place. Any suggestions?
> >>
> >> I'm sort of surprised that other targets haven't run into this issue.
> >>
> >> -Ryan
> >>
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >    _______________________________________________
> >    LLVM Developers mailing list
> >    llvm-dev at lists.llvm.org
> >    https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20191115/4de64fec/attachment-0001.html>