[all-commits] [llvm/llvm-project] 1220c9: [InstCombine] Fold the `log2_ceil` idiom (#76661)

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 1220c9bafc11a3edf96921a8ab892d777b7ed06b
      https://github.com/llvm/llvm-project/commit/1220c9bafc11a3edf96921a8ab892d777b7ed06b
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2024-01-10 (Wed, 10 Jan 2024)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    A llvm/test/Transforms/InstCombine/fold-log2-ceil-idiom.ll

  Log Message:
  -----------
  [InstCombine] Fold the `log2_ceil` idiom (#76661)

This patch folds the `log2_ceil` idiom:
```
(BW - ctlz(A)) + (is_power2(A) ? 0 : 1) ->
zext(ctpop(A) >u/!= 1) + (ctlz(A, true) ^ (BW - 1)) (canonical form) ->
BW - ctlz(A - 1, false)
```

Alive2: https://alive2.llvm.org/ce/z/6mSbdi