Ternary operator vs array?

Question

In C, is indexing an array faster than the ?: operator?

For example, would (const int[]){8, 14}[N > 10] be faster than N > 10? 14 : 8?

Answer 1

Use of the compound literal (and array in general) will be much less efficient as arrays are created (by current real-world compilers) despite the optimization level. Worse, they're created on the stack, not just indexing static constant data (which would still be slower, at least higher latency, than an ALU select operation like x86 cmov or AArch64 csel which most modern ISAs have).

I have tested it using all compilers I use (including Keil and IAR) and some I don't use (icc and clang).

int foo(int N)
{
    return (const int[]){8, 14}[N > 10]; 
}

int bar(int N)
{
    return  N > 10? 14 : 8;
}

foo:
        mov     rax, QWORD PTR .LC0[rip]     # load 8 bytes from .rodata
        mov     QWORD PTR [rsp-8], rax       # store both elements to the stack
        xor     eax, eax                      # prepare a zeroed reg for setcc
        cmp     edi, 10
        setg    al                            # materialize N>10 as a 0/1 integer
        mov     eax, DWORD PTR [rsp-8+rax*4]  # index the array with it
        ret

bar:
        cmp     edi, 10
        mov     edx, 8               # set up registers with both constants
        mov     eax, 14
        cmovle  eax, edx             # ALU select operation on FLAGS from CMP
        ret
.LC0:
        .long   8
        .long   14

https://godbolt.org/z/qK65Gv