[PATCH] D126533: [LAA] Relax pointer dependency with runtime pointer checks

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D126533#3546793 <https://reviews.llvm.org/D126533#3546793>, @dmgreen wrote:

> Is this example from the summary the motivating example?
>
>   struct Arrays {
>   double X[128];
>   double Y[128];
>   double Z[128];
>   double D;
>   } s1;
>   
>   void test(Arrays *S, int n) {
>     for ( int k=0 ; k<n ; k++ ) {
>       S->X[k] = S->D + S->Y[k] * S->Z[k];
>     }
>   }
>
> GCC seems to pull the invariant load out of the loop without needing any runtime checks: https://godbolt.org/z/PPn4jP1be
> Should clang be able to do the same? It seems that GCC thinks that S->X[k] cannot alias with S->D.

I think what's missing here is that Clang/LLVM doesn't support accesses to fixed-sizes arrays in structs in TBAA. There's been a lot of discussions about improving TBAA a couple of years ago that didn't make any progress unfortunately. I'm planning to summarize the status and see if we can find a way forward to address some of the current limitations.


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https://reviews.llvm.org/D126533