Yingwei Zheng 5fe146672d [InstCombine] Simplify switch with selects (#84143) ...

An example from https://github.com/image-rs/image:
```
define void @test_ult_rhsc(i8 %x) {
  %val = add nsw i8 %x, -2
  %cmp = icmp ult i8 %val, 11
  %cond = select i1 %cmp, i8 %val, i8 6
  switch i8 %cond, label %bb1 [
  i8 0, label %bb2
  i8 10, label %bb3
  ]

bb1:
  call void @func1()
  unreachable
bb2:
  call void @func2()
  unreachable
bb3:
  call void @func3()
  unreachable
}
```

When `%cmp` evaluates to false, we can prove that the range of `%val` is
[11, umax]. Thus we can safely replace `%cond` with `%val` since both
`switch 6` and `switch %val` go to the default dest `%bb1`.

Alive2: https://alive2.llvm.org/ce/z/uSTj6w
Godbolt: https://godbolt.org/z/MGrG84bzr

This patch will benefit many rust applications and some C/C++
applications (e.g., cvc5).

2024-04-15 16:40:16 +08:00

3.9 KiB

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