[Bug c/56083] New: Vectorizer uses xor/movlps/movhps rather than movups

[ljkarkk2 at cc dot hut.fi]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56083



             Bug #: 56083

           Summary: Vectorizer uses xor/movlps/movhps rather than movups

    Classification: Unclassified

           Product: gcc

           Version: unknown

            Status: UNCONFIRMED

          Severity: minor

          Priority: P3

         Component: c

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: ljkar...@cc.hut.fi





Unnecessarily complex machine code is generated on x86-64. Perhaps there is a

reason for this but to me it seems like the compiler is failing to optimize

properly. Asm code labels changed and comments added, other than that they are

are produced by the respective compilers for this C code:



void negate(float* fvec) {

    fvec[0] *= -1;

    fvec[1] *= -1;

    fvec[2] *= -1;

    fvec[3] *= -1;

}



.LC0:

    .long    2147483648              # float -0.000000e+00

    .long    2147483648              # float -0.000000e+00

    .long    2147483648              # float -0.000000e+00

    .long    2147483648              # float -0.000000e+00

.LC1:

        .long   3212836864              # float -1.000000e+00

        .long   3212836864              # float -1.000000e+00

        .long   3212836864              # float -1.000000e+00

        .long   3212836864              # float -1.000000e+00



# Hand optimized

    movups    (%rdi), %xmm0       # Load fvec into xmm0

    xorps    .LC0(%rip), %xmm0   # Invert sign bits of each float (xor -0.0f)

    movups    %xmm0, (%rdi)       # Store xmm0 to fvec



# Clang -O3 -mllvm -vectorize

        movups  (%rdi), %xmm0       # Load fvec

        mulps   .LC1(%rip), %xmm0   # Multiply by negative ones

        movups  %xmm0, (%rdi)       # Store fvec



# GCC 4.7.2 -O3

    xorps    %xmm0, %xmm0        # Zero xmm0 (unnecessary)

    movlps    (%rdi), %xmm0       # Load two lower floats

    movhps    8(%rdi), %xmm0      # Load two higher floats

    xorps    .LC0(%rip), %xmm0   # Invert sign bits

    movlps    %xmm0, (%rdi)       # Store two lower floats

    movhps    %xmm0, 8(%rdi)      # Store two higher floats