[all-commits] [llvm/llvm-project] fd0ffb: [ValueTracking] Recognize `LShr(UINT_MAX, Y) + 1` ...

[Monad via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: fd0ffb74389d6bf52c5c2afc8cd0a4e26cfa6c2f
      https://github.com/llvm/llvm-project/commit/fd0ffb74389d6bf52c5c2afc8cd0a4e26cfa6c2f
  Author: Monad <yanwqmonad at gmail.com>
  Date:   2024-05-07 (Tue, 07 May 2024)

  Changed paths:
    M llvm/lib/Analysis/ValueTracking.cpp
    M llvm/test/Analysis/ValueTracking/known-power-of-two-urem.ll

  Log Message:
  -----------
  [ValueTracking] Recognize `LShr(UINT_MAX, Y) + 1` as a power-of-two (#91171)

There is a missed optimization in
``` llvm
define i8 @known_power_of_two_rust_next_power_of_two(i8 %x, i8 %y) {
  %2 = add i8 %x, -1
  %3 = tail call i8 @llvm.ctlz.i8(i8 %2, i1 true)
  %4 = lshr i8 -1, %3
  %5 = add i8 %4, 1
  %6 = icmp ugt i8 %x, 1
  %p = select i1 %6, i8 %5, i8 1

  %r = urem i8 %y, %p
  ret i8 %r
}
```
which is extracted from the Rust code
``` rust
fn func(x: usize, y: usize) -> usize {
    let z = x.next_power_of_two();
    y % z
}
```
Here `%p` (a.k.a `z`) is semantically a power-of-two, so `y urem p` can
be optimized to `y & (p - 1)`. (Alive2 proof:
https://alive2.llvm.org/ce/z/H3zooY)

---

It could be generalized to recognizing `LShr(UINT_MAX, Y) + 1` as a
power-of-two, which is what this PR does.
Alive2 proof: https://alive2.llvm.org/ce/z/zUPTbc



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