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

[Quentin Colombet via llvm-dev]

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
>> 
>> 
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