[llvm-dev] Suggested BasicAA improvements

[Markus Lavin via llvm-dev]

We recently noticed some deficiencies in BasicAA that result in poor scheduling for our downstream VLIW target. It boils down to BasicAA not being able to tell that accesses to %G1 and %G2 below would not alias.

%1 = mul i16 %X, 6
%2 = shl i16 %X, 3
%G1 = getelementptr i16, i16* %base, i16 %1
%G2 = getelementptr i16, i16* %base, i16 %2

In general that would be correct but in the special case where it is known that %X != 0 it seems reasonable to conclude that they cannot alias.

In https://reviews.llvm.org/D55107 (not at all ready, just for discussion) there is an attempt to teach BasicAA about this in the presence of a @llvm.assume on %X.

Now I am curious if adding such additional analysis to BasicAA would be considered a good idea and if so what would be the proper way to integrate it as right now my implementation feels mostly bolted on top of the existing one.

-Markus

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