Comment by xoranth | Hacker News Reader

Comment by xoranth | Hacker News Reader

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Clang is able to convert the code to a jump table, though GCC can't.

https://c.godbolt.org/z/Wq65j4rrM

> What would it take to make the compiler optimize this code meaningfully?

To make GCC compile the code meaningfully, it suffices to add a bunch of else statements:

https://c.godbolt.org/z/z4KvxnbfG

In fact, GCC gets very clever at -O3.

(what matters is whether the compiler can turn the sequence of ifs into a switch statement, then both compilers have special logic to generate a lookup table.)

Interesting. I only know a little assembler, so I can't tell what -03 is producing there, but I know enough to see that it's doing some sort of loop bouncing between even/odd label choice. Is that now a constant-size binary (if still linear time)?

What GCC is doing at -O3 is encoding even (resp. odd) numbers in a bitmap.

43690 = 0xAAAA = 0b1010101010101010

21845 = 0x5555 = 0b0101010101010101

Then

    sal     rax, cl
    test    eax, 43690
    jne     .L3

    // .L3
    mov     edi, OFFSET FLAT:.LC1 // This is a pointer to the string "odd"
    call    puts
    jmp     .L2

is equivalent to C:

    if ((1 << number) & 0xAAAA) {
        puts("odd"); // x64 is little endian
    }

So execution is constant time (it only needs to check against two constants), there's no loop.

It should be advantageous compared to a jump table because it doesn't have to do two indirect jumps (though you now have branches that might mispredict).

Edit: code formatting.

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