Re: Using POPCNT and other advanced bit manipulation instructions

Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Hah, I just realized you have to add -mlzcnt in order for these builtins
> to use the lzcnt instructions.  It goes from something like

>     bsrq    %rax, %rax
>     xorq    $63, %rax

> to
>     lzcntq    %rax, %rax

> Significant?

I'd bet a fair amount of money that we'd be better off *not* using
lzcnt, even if available, because then we could just expose things
along this line:

static inline int
pg_clz(...)
{
#ifdef HAVE__BUILTIN_CLZ
    return __builtin_clz(x);
#else
    handwritten implementation;
#endif
}

Avoiding a function call (that has to indirect through a pointer) probably
saves much more than the difference between lzcnt and the other way.

The tradeoff might be different for popcount, though, especially since
it looks like __builtin_popcount() is not nearly as widely available
as the clz/ctz builtins.

            regards, tom lane