[llvm-bugs] [Bug 43815] New: Load folding defeated by VPCMP to VTESTNM optimization

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43815

            Bug ID: 43815
           Summary: Load folding defeated by VPCMP to VTESTNM optimization
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: dave at znu.io
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

While trying to create an AVX512BW version of strlen() using the X86 clang
intrinsics, I noticed that load folding was being defeated by a VPCMP to
VTESTNM optimization. For example:

unsigned long avx512_strlen(const char *_ptr) {
    __m512i zero = { 0 };
    //asm("" : "+v" (zero));
    const char *ptr = reinterpret_cast<const char
*>(reinterpret_cast<uintptr_t>(_ptr) & ~63ul);
    uint64_t mask = ~0ul << (ptr - _ptr);
    auto result = _mm512_cmp_epi8_mask(zero, *reinterpret_cast<const
__m512i*>(ptr), 0);
    result &= mask;
    if (!result) for (;;) {
        ptr += 64;
        result = _mm512_cmp_epi8_mask(zero, *reinterpret_cast<const
__m512i*>(ptr), 0);
        if (result)
            break;
    }
    ptr += __builtin_ctzll(result);
    return ptr - _ptr;
}

Generates:

_ZL13avx512_strlenPKc:                  # @_ZL13avx512_strlenPKc
        .cfi_startproc
# %bb.0:
        movq    %rdi, %rcx
        andq    $-64, %rcx
        movl    %ecx, %eax
        subl    %edi, %eax
        vmovdqa64       (%rcx), %zmm0
        vptestnmb       %zmm0, %zmm0, %k0
        kmovq   %k0, %rdx
        shrxq   %rax, %rdx, %rdx
        shlxq   %rax, %rdx, %rax
        testq   %rax, %rax
        je      .LBB0_1
# %bb.3:
        tzcntq  %rax, %rax
        addq    %rcx, %rax
        subq    %rdi, %rax
        vzeroupper
        retq
        .p2align        4, 0x90
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
        vmovdqa64       64(%rcx), %zmm0
        addq    $64, %rcx
        vptestnmb       %zmm0, %zmm0, %k0
        kortestq        %k0, %k0
        je      .LBB0_1
# %bb.2:
        kmovq   %k0, %rax
        tzcntq  %rax, %rax
        addq    %rcx, %rax
        subq    %rdi, %rax
        vzeroupper
        retq

But with the inline asm() uncommented, the desired code is emitted:

_ZL13avx512_strlenPKc:                  # @_ZL13avx512_strlenPKc
        .cfi_startproc
# %bb.0:
        vxorps  %xmm0, %xmm0, %xmm0
        #APP
        #NO_APP
        movq    %rdi, %rcx
        andq    $-64, %rcx
        movl    %ecx, %eax
        subl    %edi, %eax
        vpcmpeqb        (%rcx), %zmm0, %k0
        kmovq   %k0, %rdx
        shrxq   %rax, %rdx, %rdx
        shlxq   %rax, %rdx, %rax
        testq   %rax, %rax
        je      .LBB0_1
# %bb.3:
        tzcntq  %rax, %rax
        addq    %rcx, %rax
        subq    %rdi, %rax
        vzeroupper
        retq
        .p2align        4, 0x90
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
        vpcmpeqb        64(%rcx), %zmm0, %k0
        addq    $64, %rcx
        kortestq        %k0, %k0
        je      .LBB0_1
# %bb.2:
        kmovq   %k0, %rax
        tzcntq  %rax, %rax
        addq    %rcx, %rax
        subq    %rdi, %rax
        vzeroupper
        retq

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