Re: use CLZ instruction in AllocSetFreeIndex()

John Naylor <john(dot)naylor(at)2ndquadrant(dot)com> writes:
> On Fri, Dec 27, 2019 at 9:54 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> ... but couldn't the
>> right shift be elided in favor of changing the constant we
>> subtract clz's result from? Shifting off those bits separately
>> made sense in the old implementation, but assuming that CLZ is
>> more or less constant-time, it doesn't with CLZ.

> That makes sense -- I'll look into doing that.

Actually, we could apply that insight to both code paths.
In the existing path, that requires assuming
ALLOCSET_NUM_FREELISTS+ALLOC_MINBITS <= 17, but that's OK.
(Nowadays I'd probably add a StaticAssert about that.)

regards, tom lane