[llvm-bugs] [Bug 43688] New: optimize ctpop codegen by width and narrow ctpop intrinsic in IR

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43688

            Bug ID: 43688
           Summary: optimize ctpop codegen by width and narrow ctpop
                    intrinsic in IR
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org

Spinning this off from bug 43656:

define i32 @zpop(i8 %x) {
  %z = zext i8 %x to i32
  %pop = tail call i32 @llvm.ctpop.i32(i32 %z)
  ret i32 %pop
}

define i32 @popz(i8 %x) {
  %pop = tail call i8 @llvm.ctpop.i8(i8 %x)
  %z = zext i8 %pop to i32
  ret i32 %z
}

declare i8 @llvm.ctpop.i8(i8)
declare i32 @llvm.ctpop.i32(i32)

--------------------------------------------------------------------------

These are equivalent, so we should try to canonicalize them in IR. The narrow
call is likely better for vectorization and would line up with our transforms
of most math/logic ops. 

But we don't have DAGCombiner and/or legalization to ensure that the narrow
call is optimized in codegen.

For example on base x86-64:
_zpop:                                  ## @zpop
        movzbl  %dil, %eax
        movl    %eax, %ecx
        shrl    %ecx
        andl    $-43, %ecx
        subl    %ecx, %eax
        movl    %eax, %ecx
        andl    $858993459, %ecx        ## imm = 0x33333333
        shrl    $2, %eax
        andl    $858993459, %eax        ## imm = 0x33333333
        addl    %ecx, %eax
        movl    %eax, %ecx
        shrl    $4, %ecx
        addl    %eax, %ecx
        andl    $252645135, %ecx        ## imm = 0xF0F0F0F
        imull   $16843009, %ecx, %eax   ## imm = 0x1010101
        shrl    $24, %eax
        retq

_popz:                                  ## @popz
        movl    %edi, %eax
        shrb    %al
        andb    $85, %al
        subb    %al, %dil
        movl    %edi, %eax
        andb    $51, %al
        shrb    $2, %dil
        andb    $51, %dil
        addb    %al, %dil
        movl    %edi, %eax
        shrb    $4, %al
        addb    %dil, %al
        andb    $15, %al
        movzbl  %al, %eax
        retq

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