[PATCH] D127115: [RFC][DAGCombine] Make sure combined nodes are added back to the worklist in topological order.

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added inline comments.


================
Comment at: llvm/test/CodeGen/RISCV/rvv/fixed-vectors-vxor-vp.ll:166
+; RV32-NEXT:    vxor.vv v8, v8, v10, v0.t
+; RV32-NEXT:    ret
+;
----------------
frasercrmck wrote:
> RKSimon wrote:
> > @craig.topper Might all of these vrgather regressions be due to a missing combine for splat cases?
> Something like that. I see that we have the initial pattern:
> ```
>           t9: v2i64 = insert_vector_elt undef:v2i64, Constant:i64<-1>, Constant:i64<0>
>         t10: v2i64 = vector_shuffle<0,0> t9, undef:v2i64
> ```
> 
> on RV64 that's combined:
> ```
> Combining: t9: v2i64 = insert_vector_elt undef:v2i64, Constant:i64<-1>, Constant:i64<0>
>  ... into: t14: v2i64 = BUILD_VECTOR Constant:i64<-1>, undef:i64
> 
> Combining: t14: v2i64 = BUILD_VECTOR Constant:i64<-1>, undef:i64
> 
> Combining: t10: v2i64 = vector_shuffle<0,0> t14, undef:v2i64
>  ... into: t15: v2i64 = BUILD_VECTOR Constant:i64<-1>, Constant:i64<-1>
> 
>         t15: v2i64 = BUILD_VECTOR Constant:i64<-1>, Constant:i64<-1>
>       t11: v2i64 = sub t5, t15
> ```
> 
> On RV32 it's:
> ```
> Combining: t8: v2i64 = insert_vector_elt undef:v2i64, Constant:i64<-1>, Constant:i32<0>
>  ... into: t15: v2i64 = BUILD_VECTOR Constant:i64<-1>, undef:i64
> 
> Combining: t15: v2i64 = BUILD_VECTOR Constant:i64<-1>, undef:i64
>  ... into: t16: v2i64 = splat_vector Constant:i64<-1>
> 
> Combining: t16: v2i64 = splat_vector Constant:i64<-1>
> 
> Combining: t9: v2i64 = vector_shuffle<0,0> t16, undef:v2i64
> 
>           t16: v2i64 = splat_vector Constant:i64<-1>
>         t9: v2i64 = vector_shuffle<0,0> t16, undef:v2i64
>       t10: v2i64 = sub t5, t9
> ```
> 
> I'm not able to say what //exactly// is going wrong, though. We're definitely missing combines for shuffles of splats, but do we really want to see `splat_vector` for fixed-length vectors here?
Something you might consider is just use SelectionDAG::getSplatSourceVector / isSplatValue - that way you aren't having to handle specific splat patterns.


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https://reviews.llvm.org/D127115