Re: [PATCH] Custom code int(32|64) => text conversions out of performance reasons

On Sat, Nov 20, 2010 at 6:31 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Sat, Nov 20, 2010 at 12:34 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> what about building it working backwards from the end of the buffer
>>> and then memmove'ing it down to the start of the buffer?
>
>> I think that might be more clever than is really warranted.  I get
>> your point about buffer overrun, but I don't think it's that hard for
>> callers to do the right thing, so I'm inclined to think that's not
>> worth much in this case.

It also seems wrong that a caller might happen to know that their
argument will never be more than n digits but still has to allocate a
buffer large enough to hold 2^64.



>
> Fair enough --- it was just a passing thought.
>
>> I had given some thought to whether it might make sense to try to
>> figure out how long the string will be before we actually start
>> generating it, so that we can just start in the exactly right space
>> and have to clean up afterward.  But the obvious implementation seems
>> like it could be more expensive than just doing the copy.
>
> Yeah.  You certainly don't want to do the division sequence twice,
> and a log() call wouldn't be cheap either, and there don't seem to
> be many other alternatives.

There are bittwiddling hacks for computing log based 2. I'm not sure
it's worth worrying about to this degree though.



--
greg