In 4de3184f07fd8c548125d315dd306d4afa7c9698 we exposed BasicAA's
cross-iteration mode for use in LAA, so we can handle selects with equal
conditions correctly (where the select condition is not actually equal
across iterations).
However, if we replace the selects with equivalent phis, the issue still
exists. In the phi case, we effectively still have an assumption that
the condition(s) that control which phi arg is used will be the same
across iterations. Fix this by disabling this phi handling in
cross-iteration mode.
(I'm not entirely sure whether this is also needed when BasicAA enables
cross-iteration mode during internal phi recursion, but I wouldn't be
surprised if that's the case.)
Update getDependenceDistanceStrideAndSize to reason about different
combinations of strides directly and explicitly.
Update getPtrStride to return 0 for invariant pointers.
Then proceed by checking the strides.
If either source or sink are not strided by a constant (i.e. not a
non-wrapping AddRec) or invariant, the accesses may overlap
with earlier or later iterations and we cannot generate runtime
checks to disambiguate them.
Otherwise they are either loop invariant or strided. In that case, we
can generate a runtime check to disambiguate them.
If both are strided by constants, we proceed as previously.
This is an alternative to
https://github.com/llvm/llvm-project/pull/99239 and also replaces
additional checks if the underlying object is loop-invariant.
Fixes https://github.com/llvm/llvm-project/issues/87189.
PR: https://github.com/llvm/llvm-project/pull/99577
The test select-dependence.ll can be eliminated completely by dce, as it
returns a constant, and doesn't write any arguments. Lift out the local
allocas into arguments, so that it is less nonsensical. While at it,
rename the variables for greater readability, and regenerate the test
with UpdateTestChecks.
LAA analyzes cross-iteration memory dependencies, as such AA should
not make assumptions about equality of values inside the loop, as
they may come from different iterations.
Fix this by exposing the MayBeCrossIteration AA flag and enabling
it for LAA.
Differential Revision: https://reviews.llvm.org/D137958
