[PATCH] D46278: [AArch64] Fold B = csel A, A into B = COPY A

Wed Apr 28 11:25:13 PDT 2021

jroelofs added inline comments.

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Comment at: llvm/test/CodeGen/AArch64/srem-seteq.ll:272
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    cmp w0, #0 // =0
-; CHECK-NEXT:    csel w8, w0, w0, lt
-; CHECK-NEXT:    cmp w0, w8
+; CHECK-NEXT:    cmp w0, w0
 ; CHECK-NEXT:    cset w0, eq
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jroelofs wrote:
> dmgreen wrote:
> > I think it's worth fixing this in ISel. The whole thing should be `mov x0, 1`, which the further simplification during DAG combine will simplify to so long as it doesn't get caught up on the CSEL.
> > 
> > I presume the same sort of thing could easily be done in GlobalISel.
> > 
> > Are there other cases motivating this patch?
> Yes, I have seen a few of the trivial matching-operands-fcsel case in the test-suite. Let me try to reduce one of them.
https://clang.godbolt.org/z/WvnrcrGEn
`fcsel   d4, d3, d3, eq`

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D46278/new/

https://reviews.llvm.org/D46278