[all-commits] [llvm/llvm-project] e8af13: [InstCombine] Generalize trunc-shift-icmp fold fro...

[Pedro Lobo via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e8af134bb7f891caa49178c8a04a8ca944c611df
      https://github.com/llvm/llvm-project/commit/e8af134bb7f891caa49178c8a04a8ca944c611df
  Author: Pedro Lobo <pedro.lobo at tecnico.ulisboa.pt>
  Date:   2025-11-22 (Sat, 22 Nov 2025)

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

  Log Message:
  -----------
  [InstCombine] Generalize trunc-shift-icmp fold from (1 << Y) to (Pow2 << Y) (#169163)

Extends the `icmp(trunc(shl))` fold to handle any power of 2 constant as
the shift base, not just 1. This generalizes the following patterns by
adjusting the comparison offsets by `log2(Pow2)`.

```llvm
(trunc (1 << Y) to iN) == 0    --> Y u>= N
(trunc (1 << Y) to iN) != 0    --> Y u<  N
(trunc (1 << Y) to iN) == 2**C --> Y ==  C
(trunc (1 << Y) to iN) != 2**C --> Y !=  C

; to

(trunc (Pow2 << Y) to iN) == 0    --> Y u>= N - log2(Pow2)
(trunc (Pow2 << Y) to iN) != 0    --> Y u<  N - log2(Pow2)
(trunc (Pow2 << Y) to iN) == 2**C --> Y ==  C - log2(Pow2)
(trunc (Pow2 << Y) to iN) != 2**C --> Y !=  C - log2(Pow2)
```

Proof: https://alive2.llvm.org/ce/z/2zwTkp



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