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

[Fraser Cormack via Phabricator via llvm-commits]

frasercrmck 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
+;
----------------
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?


Repository:
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  https://reviews.llvm.org/D127115/new/

https://reviews.llvm.org/D127115