llvm dev - Nov 2019 - imm COPY generated by PHI elim not propagated

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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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

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
    
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