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Summary: [[ https://bugs.llvm.org/show_bug.cgi?id=38149 | PR38149 ]] As discussed in https://reviews.llvm.org/D49179#1158957 and later, the IR for 'check for [no] signed truncation' pattern can be improved: https://rise4fun.com/Alive/gBf ^ that pattern will be produced by Implicit Integer Truncation sanitizer, https://reviews.llvm.org/D48958 https://bugs.llvm.org/show_bug.cgi?id=21530 in signed case, therefore it is probably a good idea to improve it. The DAGCombine will reverse this transform, see https://reviews.llvm.org/D49266 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: https://bugs.llvm.org/show_bug.cgi?id=38204 and https://crbug.com/864832 Proofs that the fixed folds are ok: https://rise4fun.com/Alive/VYM Differential Revision: https://reviews.llvm.org/D49320 llvm-svn: 337376
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