Re: Using 128-bit integers for sum, avg and statistics aggregates

On 01/11/2015 02:36 AM, Andres Freund wrote:
> a) Afaics only __int128/unsigned __int128 is defined. See
>     https://gcc.gnu.org/onlinedocs/gcc/_005f_005fint128.html

Both GCC and Clang defines both of them. Which you use seems to just be 
a matter of preference.

> b) I'm doubtful that AC_CHECK_TYPES is a sufficiently good test on all
>     platforms. IIRC gcc will generate calls to functions to do the actual
>     arithmetic, and I don't think they're guranteed to be available on
>     platforms. That how it .e.g. behaves for atomic operations. So my
>     guess is that you'll need to check that a program that does actual
>     arithmetic still links.

I too thought some about this and decided to look at how other projects 
handled this. The projects I have looked at seems to trust that if 
__int128_t is defined it will also work.

https://github.com/python/cpython/blob/master/configure#L7778
http://cgit.freedesktop.org/cairo/tree/build/configure.ac.system#n88

But after some more searching now I notice that at least gstreamer does 
not trust this to be true.

https://github.com/ensonic/gstreamer/blob/master/configure.ac#L382

Should I fix it to actually compile some code which uses the 128-bit types?

> c) Personally I don't see the point of testing __uint128_t. That's just
>     a pointless test that makes configure run for longer.

Ok, will remove it in the next version of the patch.

>> @@ -3030,6 +3139,18 @@ int8_avg_accum(PG_FUNCTION_ARGS)
>>   Datum
>>   int2_accum_inv(PG_FUNCTION_ARGS)
>>   {
>> +#ifdef HAVE_INT128
>> +    Int16AggState *state;
>> +
>> +    state = PG_ARGISNULL(0) ? NULL : (Int16AggState *) PG_GETARG_POINTER(0);
>> +
>> +    /* Should not get here with no state */
>> +    if (state == NULL)
>> +        elog(ERROR, "int2_accum_inv called with NULL state");
>> +
>> +    if (!PG_ARGISNULL(1))
>> +        do_int16_discard(state, (int128) PG_GETARG_INT16(1));
>> +#else
>>       NumericAggState *state;
>>
>>       state = PG_ARGISNULL(0) ? NULL : (NumericAggState *) PG_GETARG_POINTER(0);
>> @@ -3049,6 +3170,7 @@ int2_accum_inv(PG_FUNCTION_ARGS)
>>           if (!do_numeric_discard(state, newval))
>>               elog(ERROR, "do_numeric_discard failed unexpectedly");
>>       }
>
> Hm. It might be nicer to move the if (!state) elog() outside the ifdef,
> and add curly parens inside the ifdef.

The reason I did so was because the type of the state differs and I did 
not feel like having two ifdef blocks. I have no strong opinion about 
this though.

> pg_config.h.win32 should be updated as well.

Is it possible to update it without running Windows?

-- 
Andreas Karlsson