[all-commits] [llvm/llvm-project] 191a6e: optimize icmp-ugt-ashr

[Nadav Rotem via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 191a6e9dfa1a54b616e12bde2efa849ad8e03f48
      https://github.com/llvm/llvm-project/commit/191a6e9dfa1a54b616e12bde2efa849ad8e03f48
  Author: Nadav Rotem <nadav256 at gmail.com>
  Date:   2022-01-20 (Thu, 20 Jan 2022)

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

  Log Message:
  -----------
  optimize icmp-ugt-ashr

This diff optimizes the sequence icmp-ugt(ashr,C_1) C_2. InstCombine
already implements this optimization for sgt, and this patch adds
support ugt. This patch adds the check for UGT.

@craig.topper came up with the idea and proof:

  define i1 @src(i8 %x, i8 %y, i8 %c) {
    %cp1 = add i8 %c, 1
    %i = shl i8 %cp1, %y
    %i.2 = ashr i8 %i, %y
    %cmp = icmp eq i8 %cp1, %i.2
    ;Assume: C + 1 == (((C + 1) << y) >> y)
    call void @llvm.assume(i1 %cmp)

    ; uncomment for the sgt case
    %j = shl i8 %cp1, %y
    %j.2 = sub i8 %j, 1
    %cmp2 = icmp ne i8 %j.2, 127
    ;Assume (((c + 1 ) << y) - 1) != 127
    call void @llvm.assume(i1 %cmp2)

    %s = ashr i8 %x, %y
    %r = icmp sgt i8 %s, %c
    ret i1 %r
  }

  define i1 @tgt(i8 %x, i8 %y, i8 %c) {
    %cp1 = add i8 %c, 1
    %j = shl i8 %cp1, %y
    %j.2 = sub i8 %j, 1

    %r = icmp sgt i8 %x, %j.2
    ret i1 %r
  }

  declare void @llvm.assume(i1)

  This change is related to the optimizations in D117252.

  Differential Revision: https://reviews.llvm.org/D117365