[all-commits] [llvm/llvm-project] e100d2: [DAGCombiner] Fold subtraction if above a constant...

[Piotr Fusik via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e100d2bf9ad89139efe5b3edc441c2c67cc42bf4
      https://github.com/llvm/llvm-project/commit/e100d2bf9ad89139efe5b3edc441c2c67cc42bf4
  Author: Piotr Fusik <p.fusik at samsung.com>
  Date:   2025-04-11 (Fri, 11 Apr 2025)

  Changed paths:
    M llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    M llvm/test/CodeGen/RISCV/rv32zbb.ll
    M llvm/test/CodeGen/RISCV/rv64zbb.ll

  Log Message:
  -----------
  [DAGCombiner] Fold subtraction if above a constant threshold to `umin` (#135194)

Like #134235, but with a constant.
It's a pattern in Adler-32 checksum calculation in zlib.

Example:

    unsigned adler32_mod(unsigned x) {
      return x >= 65521u ? x - 65521u : x;
    }

Before, on RISC-V:

    lui     a1, 16
    lui     a2, 1048560
    addiw   a1, a1, -16
    sltu    a1, a1, a0
    negw    a1, a1
    addi    a2, a2, 15
    and     a1, a1, a2
    addw    a0, a0, a1

Or, with Zicond:

    lui     a1, 16
    lui     a2, 1048560
    addiw   a1, a1, -16
    sltu    a1, a1, a0
    addi    a2, a2, 15
    czero.eqz  a1, a2, a1
    addw    a0, a0, a1

After, with Zbb:

    lui     a1, 1048560
    addi    a1, a1, 15
    addw    a1, a0, a1
    minu    a0, a1, a0



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications