[all-commits] [llvm/llvm-project] 5c7c1f: [InstCombine] Try the flipped strictness of predi...

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 5c7c1f6aba467b1c5a51d58fc591314ecd7147ce
      https://github.com/llvm/llvm-project/commit/5c7c1f6aba467b1c5a51d58fc591314ecd7147ce
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2024-05-28 (Tue, 28 May 2024)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
    M llvm/test/Transforms/InstCombine/icmp.ll

  Log Message:
  -----------
   [InstCombine] Try the flipped strictness of predicate in `foldICmpShlConstant` (#92773)

This patch extends the transform `(icmp pred iM (shl iM %v, N), C) ->
(icmp pred i(M-N) (trunc %v iM to i(M-N)), (trunc (C>>N))` to handle
icmps with the flipped strictness of predicate.

See the following case:
```
icmp ult i64 (shl X, 32), 8589934593 ->
icmp ule i64 (shl X, 32), 8589934592 ->
icmp ule i32 (trunc X, i32), 2 ->
icmp ult i32 (trunc X, i32), 3
```

Fixes the regression introduced by
https://github.com/llvm/llvm-project/pull/86111#issuecomment-2098203152.

Alive2 proofs: https://alive2.llvm.org/ce/z/-sp5n3

`nuw` cannot be propagated as we always use `ashr` here. I don't see the
value of fixing this (see the test `test_icmp_shl_nuw`).



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