[llvm-bugs] [Bug 28849] New: AVX512: invalid code generation for scalar code when AVX512 is enabled

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https://llvm.org/bugs/show_bug.cgi?id=28849

            Bug ID: 28849
           Summary: AVX512: invalid code generation for scalar code when
                    AVX512 is enabled
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: wenzel.jakob at epfl.ch
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

When AVX512 is enabled, Clang produces incorrect code for very basic scalar
programs which don't involve any vector/intrinsic code at all. This only occurs
at the lowest optimization level.

Consider the following snippet:

#include <assert.h>
int main(int argc, char *argv[]) {
    float a = 1;
    float b = 1;
    assert(a == b);
    return 0;
}

Example usage (Clang HEAD):

$ clang test.cpp -o test -mavx512f -fomit-frame-pointer
$ ./test
./Assertion failed: (a == b), function main, file test.cpp, line 6.


Looking at the assembly, there are some weird sequences of moves from
traditional registers to AVX512-style mask registers and back. Probably
something is wrong there.

_main:                                  ## @main
    .cfi_startproc
## BB#0:
    subq    $40, %rsp
Ltmp0:
    .cfi_def_cfa_offset 48
    movb    $1, %al
    movl    $0, 36(%rsp)
    movl    %edi, 32(%rsp)
    movq    %rsi, 24(%rsp)
    movl    $1065353216, 20(%rsp)   ## imm = 0x3F800000
    movl    $1065353216, 16(%rsp)   ## imm = 0x3F800000
    vmovss    20(%rsp), %xmm0         ## xmm0 = mem[0],zero,zero,zero
    vmovss    LCPI0_0(%rip), %xmm1    ## xmm1 = mem[0],zero,zero,zero
    vucomiss    %xmm1, %xmm0
    setnp    %cl
    sete    %dl
    setne    %r8b
    setp    %r9b
    andb    %cl, %dl
    andb    $1, %dl
    movb    %dl, %r10b
    kmovw    %r10d, %k0
    orb    %r9b, %r8b
    andb    $1, %r8b
    movb    %r8b, %r10b
    kmovw    %r10d, %k1
    testb    $1, %k1
    movb    %al, 15(%rsp)           ## 1-byte Spill
    kmovw    %k0, 12(%rsp)           ## 2-byte Spill
    jne    LBB0_1
    jmp    LBB0_2
LBB0_1:
    leaq    L___func__.main(%rip), %rdi
    leaq    L_.str(%rip), %rsi
    movl    $6, %edx
    leaq    L_.str.1(%rip), %rcx
    callq    ___assert_rtn
LBB0_2:
    jmp    LBB0_3
LBB0_3:
    xorl    %eax, %eax
    addq    $40, %rsp
    retq
    .cfi_endproc

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