Improving performance of rav1d video decoder

The rust issue has people trying this with c code and the compiler generates the same issue. This will get fixed and it’ll help c and Rust code

Out of curiosity just clang or gcc as well?

I just tried it, and the problem is even worse in gcc.

Given this C code:

    typedef struct { uint16_t a, b; } pair;

    int eq_copy(pair a, pair b) {
        return a.a == b.a && a.b == b.b;
    }
    int eq_ref(pair *a, pair *b) {
        return a->a == b->a && a->b == b->b;
    }

Clang generates clean code for the eq_copy variant, but complex code for the eq_ref variant. Gcc emits pretty complex code in both variants.

For example, here's eq_ref from gcc -O2:

    eq_ref:
        movzx   edx, WORD PTR [rsi]
        xor     eax, eax
        cmp     WORD PTR [rdi], dx
        je      .L9
        ret
    .L9:
        movzx   eax, WORD PTR [rsi+2]
        cmp     WORD PTR [rdi+2], ax
        sete    al
        movzx   eax, al
        ret

Have a play around: https://c.godbolt.org/z/79Eaa3jYf