[llvm-bugs] [Bug 49895] New: optimize smax(x, -1) with bit-hack

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49895

            Bug ID: 49895
           Summary: optimize smax(x, -1) with bit-hack
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org

Noticed while investigating idioms for signum functions...

If we have an smax with -1:

declare i32 @llvm.smax.i32(i32, i32)
define i32 @smax(i32 %x) {
  %m = call i32 @llvm.smax.i32(i32 %x, i32 -1)
  ret i32 %m
}

or:

define i32 @cmpsel(i32 %x) {
  %a = icmp sgt i32 %x, -1
  %m = select i1 %a, i32 %x, i32 -1
  ret i32 %m
}

It seems most targets would do better to convert to arithmetic shift + logic:

define i32 @tgt(i32 %x) {
  %a = ashr i32 %x, 31
  %m = or i32 %x, %a
  ret i32 %m
}

https://alive2.llvm.org/ce/z/AJYAmp

AArch64:
        cmp     w0, #0                     
        csinv   w0, w0, wzr, ge
vs.
        orr     w0, w0, w0, asr #31

PowerPC64:
        li 4, -1
        cmpwi   3, -1
        rldic 4, 4, 0, 32
        iselgt  3, 3, 4
vs.
        srawi 4, 3, 31
        or 3, 3, 4

x86:
        testl   %edi, %edi
        movl    $-1, %eax
        cmovnsl %edi, %eax

vs.
        movl    %edi, %eax
        sarl    $31, %eax
        orl     %edi, %eax

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