[PATCH] D64432: [InstCombine] try to narrow a truncated load

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert added a comment.

In D64432#1582980 <https://reviews.llvm.org/D64432#1582980>, @reames wrote:

> I'm not sure that doing this at the IR level is the best idea.  The problem is that when we narrow, we loose the dereferenceable fact about part of the memory access.  This can in turn limit other transforms which would have been profitable.  As an example:
>  a = load <2 x i8>* p
>  b = load <2 x i8>* (p+1)
>  sum = a[0] + a[1] + b[1]
>
> Narrowing the b load to i8 looses the fact that the memory location corresponding to b[0] is dereferenceable, which would prevent transforms such as:
>  a = load <4 x i8>* p
>  a[2] = 0;
>  sum = horizontal_sum(a);
>
> (Note: I'm not saying this alternate transform is always profitable.  I'm just making a point about lost opportunity.)


Could we check here if the base pointer has `dereferenceable` annotation and use that as a condition for this transformation? (It's more complicated to be completely lossless but this seems to be an easy to test starting point).


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https://reviews.llvm.org/D64432