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

Fri Nov 15 11:58:15 PST 2019

You could do it after RA and before rewrite, when you still have the live intervals around.

> On Nov 15, 2019, at 11:16 AM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> 
> 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 <mailto: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 <mailto: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 <mailto:llvm-dev-bounces at lists.llvm.org> on behalf of llvm-dev at lists.llvm.org <mailto: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 <mailto: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
> >> 
> >> 
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> > 
> 

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