[llvm-bugs] [Bug 34730] New: Efficient check that all vector elements are zero

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34730

            Bug ID: 34730
           Summary: Efficient check that all vector elements are zero
           Product: libraries
           Version: 5.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm at henning-thielemann.de
                CC: llvm-bugs at lists.llvm.org

I want to check that all elements of a vector are zero.
I can use this e.g. for checking that two vectors are equal, by xor-ing them
and then check for a zero vector.
I have two implementations: The first one is generic and the second one is
tailored to a 64-bit architecture. Can I expect that the first one is optimized
to the second one? Currently, it is not.

$ cat null-vector.ll
target triple = "x86_64-pc-linux-gnu"

define i1 @test_zero0(<16 x i8>){
_L1:
  %1  = extractelement <16 x i8> %0, i32 0
  %2  = extractelement <16 x i8> %0, i32 1
  %3  = extractelement <16 x i8> %0, i32 2
  %4  = extractelement <16 x i8> %0, i32 3
  %5  = extractelement <16 x i8> %0, i32 4
  %6  = extractelement <16 x i8> %0, i32 5
  %7  = extractelement <16 x i8> %0, i32 6
  %8  = extractelement <16 x i8> %0, i32 7
  %9  = extractelement <16 x i8> %0, i32 8
  %10 = extractelement <16 x i8> %0, i32 9
  %11 = extractelement <16 x i8> %0, i32 10
  %12 = extractelement <16 x i8> %0, i32 11
  %13 = extractelement <16 x i8> %0, i32 12
  %14 = extractelement <16 x i8> %0, i32 13
  %15 = extractelement <16 x i8> %0, i32 14
  %16 = extractelement <16 x i8> %0, i32 15
  %17 = or i8 %1, %2
  %18 = or i8 %17, %3
  %19 = or i8 %18, %4
  %20 = or i8 %19, %5
  %21 = or i8 %20, %6
  %22 = or i8 %21, %7
  %23 = or i8 %22, %8
  %24 = or i8 %23, %9
  %25 = or i8 %24, %10
  %26 = or i8 %25, %11
  %27 = or i8 %26, %12
  %28 = or i8 %27, %13
  %29 = or i8 %28, %14
  %30 = or i8 %29, %15
  %31 = or i8 %30, %16
  %32 = icmp eq i8 %31, 0
  ret i1 %32
}

define i1 @test_zero1(<16 x i8>){
_L1:
  %vl = shufflevector <16 x i8> %0, <16 x i8> undef, <8 x i32> <i32 0, i32 1,
i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
  %vh = shufflevector <16 x i8> %0, <16 x i8> undef, <8 x i32> <i32 8, i32 9,
i32 10, i32 11, i32 12, i32 13, i32 14, i32 15>
  %wl = bitcast <8 x i8> %vl to i64
  %wh = bitcast <8 x i8> %vh to i64
  %w = or i64 %wl, %wh
  %c = icmp eq i64 %w, 0
  ret i1 %c
}

$ opt-5.0 -O3 <null-vector.ll | llc-5.0
        .text
        .file   "<stdin>"
        .globl  test_zero0              # -- Begin function test_zero0
        .p2align        4, 0x90
        .type   test_zero0, at function
test_zero0:                             # @test_zero0
# BB#0:                                 # %_L1
        movaps  %xmm0, -24(%rsp)
        movb    -24(%rsp), %al
        orb     -23(%rsp), %al
        orb     -22(%rsp), %al
        orb     -21(%rsp), %al
        orb     -20(%rsp), %al
        orb     -19(%rsp), %al
        orb     -18(%rsp), %al
        orb     -17(%rsp), %al
        orb     -16(%rsp), %al
        orb     -15(%rsp), %al
        orb     -14(%rsp), %al
        orb     -13(%rsp), %al
        orb     -12(%rsp), %al
        orb     -11(%rsp), %al
        orb     -10(%rsp), %al
        orb     -9(%rsp), %al
        sete    %al
        retq
.Lfunc_end0:
        .size   test_zero0, .Lfunc_end0-test_zero0
                                        # -- End function
        .globl  test_zero1              # -- Begin function test_zero1
        .p2align        4, 0x90
        .type   test_zero1, at function
test_zero1:                             # @test_zero1
# BB#0:                                 # %_L1
        movq    %xmm0, %rax
        pshufd  $78, %xmm0, %xmm0       # xmm0 = xmm0[2,3,0,1]
        movq    %xmm0, %rcx
        orq     %rax, %rcx
        sete    %al
        retq
.Lfunc_end1:
        .size   test_zero1, .Lfunc_end1-test_zero1
                                        # -- End function

        .section        ".note.GNU-stack","", at progbits

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