Useless jp / jnp assembly instruction on x86_64

Question

I'm trying to figure out what purpose jp/jnp instructions serve in LLVM-generated C code. Sample:

int main(int argc, const char * argv[]) {
    double value = 1.5;

    if (value == 1.5) {
        value = 3.0;
    }

    return 0;
}

Assembly output:

Ltmp4:
    movsd   LCPI0_0(%rip), %xmm0
    movl    $0, -4(%rbp)
    movl    %edi, -8(%rbp)
    movq    %rsi, -16(%rbp)
Ltmp5:
    movsd   %xmm0, -24(%rbp)
Ltmp6:
    movsd   -24(%rbp), %xmm1
    ucomisd %xmm0, %xmm1
    jne LBB0_2
    jp  LBB0_2
## BB#1:
    movabsq $3, %rax
    cvtsi2sdq   %rax, %xmm0
Ltmp7:
    movsd   %xmm0, -24(%rbp)
Ltmp8:
LBB0_2:
    movl    $0, %eax
    popq    %rbp
    retq

The jne is checking if value != 1.5 and jumping over the assignment, but what is the jp doing in this context?

Answer 1

jne is jump if not equal, i.e. jump if the zero flag is not set. jp is jump if parity.

ucomisd is defined to compare two doubles. It will indicate that they are one of four things: unordered, equal, greater than or less than.

The zero flag is set if the numbers are unordered or equal. So the jne avoids the remaining cases of greater than or less than.

Parity is set only if the result is unordered. The jp catches that.

So the two together avoid: unordered, greater than, less than. Leaving only the fourth possibility, of equal.