As discussed in https://reviews.llvm.org/D49179#1158957 and later,
the IR for 'check for [no] signed truncation' pattern can be improved:
^ that pattern will be produced by Implicit Integer Truncation sanitizer,
in signed case, therefore it is probably a good idea to improve it.
The DAGCombine will reverse this transform, see
This transform is surprisingly frustrating.
This does not deal with non-splat shift amounts, or with undef shift amounts.
I've outlined what i think the solution should be:
// Potential handling of non-splats: for each element: // * if both are undef, replace with constant 0. // Because (1<<0) is OK and is 1, and ((1<<0)>>1) is also OK and is 0. // * if both are not undef, and are different, bailout. // * else, only one is undef, then pick the non-undef one.
This is a re-commit, as the original patch, committed in rL337190
was reverted in rL337344 as it broke chromium build:
Proofs that the fixed folds are ok: https://rise4fun.com/Alive/VYM