[PATCH] D138353: [Passes][VectorCombine] enable early run generally and try load folds

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: llvm/lib/Passes/PassBuilderPipelines.cpp:618-619
 
-  // The matrix extension can introduce large vector operations early, which can
-  // benefit from running vector-combine early on.
-  if (EnableMatrix)
-    FPM.addPass(VectorCombinePass(/*TryEarlyFoldsOnly=*/true));
+  // Try vectorization/scalarization transforms that are likely to be reduced by
+  // GVN and InstCombine.
+  FPM.addPass(VectorCombinePass(/*TryEarlyFoldsOnly=*/true));
----------------
spatel wrote:
> lebedev.ri wrote:
> > What does "reduced" here mean? "obscured"?
> No, that was supposed to mean "enable more folds". 
> In the motivating example from #17113, we have:
> 
> ```
>   %2 = load float, ptr %0, align 16
>   %3 = insertelement <4 x float> undef, float %2, i64 0
>   %4 = getelementptr inbounds [4 x float], ptr %0, i64 0, i64 1
>   %5 = load float, ptr %4, align 4
> 
> ```
> VectorCombine can widen the first load (with legality/profitability constraints):
> 
> ```
>   %2 = load <4 x float>, ptr %0, align 16
>   %3 = shufflevector <4 x float> %2, <4 x float> poison, <4 x i32> <i32 0, i32 undef, i32 undef, i32 undef>
>   %4 = getelementptr inbounds [4 x float], ptr %0, i64 0, i64 1
>   %5 = load float, ptr %4, align 4
> 
> ```
> And GVN then replaces the redundant 2nd load:
> 
> ```
>   %2 = load <4 x float>, ptr %0, align 16
>   %3 = shufflevector <4 x float> %2, <4 x float> poison, <4 x i32> <i32 0, i32 undef, i32 undef, i32 undef>
>   %4 = getelementptr inbounds [4 x float], ptr %0, i64 0, i64 1
>   %5 = bitcast <4 x float> %2 to i128
>   %6 = lshr i128 %5, 32
>   %7 = trunc i128 %6 to i32
>   %8 = bitcast i32 %7 to float
> 
> ```
> And then InstCombine manages to remove all of those extra instructions.
I would suggest something closer to
```
  // Try vectorization/scalarization transforms
  // that are both improvements themselves,
  // and can allow further GVN and InstCombine folds.
```
then, maybe, optionally.



CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D138353/new/

https://reviews.llvm.org/D138353