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

Thu Nov 14 00:20:53 PST 2019

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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