[all-commits] [llvm/llvm-project] 554feb: [InstSimplify] Simplify `select cond, undef, val` ...

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 554feb0058980ae3c6159a61ffdae2c2d345bf7a
      https://github.com/llvm/llvm-project/commit/554feb0058980ae3c6159a61ffdae2c2d345bf7a
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2023-12-28 (Thu, 28 Dec 2023)

  Changed paths:
    M llvm/lib/Analysis/InstructionSimplify.cpp
    M llvm/test/Transforms/InstCombine/select.ll
    M llvm/test/Transforms/InstSimplify/select-inseltpoison.ll
    M llvm/test/Transforms/InstSimplify/select.ll

  Log Message:
  -----------
  [InstSimplify] Simplify `select cond, undef, val` to `val` if `val = poison` implies `cond = poison` (#76465)

This patch folds:
```
select cond, undef, val -> val
select cond, val, undef -> val
```
iff `impliesPoison(val, cond)` returns true.

Example:
```
define i32 @src1(i32 %retval.0.i.i) {
  %cmp.i = icmp sgt i32 %retval.0.i.i, -1
  %spec.select.i = select i1 %cmp.i, i32 %retval.0.i.i, i32 undef
  ret i32 %spec.select.i
}

define i32 @tgt1(i32 %retval.0.i.i) {
  ret i32 %retval.0.i.i
}
```
Alive2: https://alive2.llvm.org/ce/z/okJW3G

Compile-time impact:
http://llvm-compile-time-tracker.com/compare.php?from=38c9390b59c4d2b9181614d6a909887497d3692f&to=e146f51ba278aa3bb6879a9ec651831ac8938e91&stat=instructions%3Au