[all-commits] [llvm/llvm-project] da1938: [RISCV] Fold (X & -4096) == 0 -> (X >> 12) == 0 (#...

[Craig Topper via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: da19383ae76d8483492cd52c26349ade8bfa4fbe
      https://github.com/llvm/llvm-project/commit/da19383ae76d8483492cd52c26349ade8bfa4fbe
  Author: Craig Topper <craig.topper at sifive.com>
  Date:   2025-08-18 (Mon, 18 Aug 2025)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVISelLowering.cpp
    M llvm/lib/Target/RISCV/RISCVInstrInfo.td
    M llvm/test/CodeGen/RISCV/and-negpow2-cmp.ll
    M llvm/test/CodeGen/RISCV/bittest.ll

  Log Message:
  -----------
  [RISCV] Fold (X & -4096) == 0 -> (X >> 12) == 0 (#154233)

This is a more general form of the recently added isel pattern
(seteq (i64 (and GPR:$rs1, 0x8000000000000000)), 0)
  -> (XORI (i64 (SRLI GPR:$rs1, 63)), 1)

We can use a shift right for any AND mask that is a negated power
of 2. But for every other constant we need to use seqz instead of
xori. I don't think there is a benefit to xori over seqz as neither
are compressible.

We already do this transform from target independent code when the setcc
constant is a non-zero subset of the AND mask that is not a legal icmp
immediate.

I don't believe any of these patterns comparing MSBs to 0 are
canonical according to InstCombine. The canonical form is (X < 4096).
I'm curious if these appear during SelectionDAG and if so, how.

My goal here was just to remove the special case isel patterns.



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