Re: init_htab causes SIGFPE (or worse) due to miscalculation for large nbuckets

Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> It looks like all of the callers, except two, immediately shift the
> result. So perhaps it would be better to make a new function (something
> like "ceil_pow2") that returns the lowest power of two greater than or
> equal to the input, and it can return a long (bounded to +LONG_MAX).

That does seem like a good idea. We need one for an int-sized result
too, to fix the original problem in init_htab. So I propose these
functions:

/* calculate ceil(log base 2) of num */
int
my_log2(long num)
{
int i;
long limit;

/* guard against too-large input, which would put us into infinite loop */
if (num > LONG_MAX / 2)
num = LONG_MAX / 2;

for (i = 0, limit = 1; limit < num; i++, limit <<= 1)
;
return i;
}

/* calculate first power of 2 >= num, bounded to what will fit in a long */
long
next_power_of_two_long(long num)
{
/* my_log2's internal range check is sufficient */
return 1L << my_log2(num);
}

/* calculate first power of 2 >= num, bounded to what will fit in an int */
int
next_power_of_two_int(long num)
{
if (num > INT_MAX / 2)
num = INT_MAX / 2;
return 1 << my_log2(num);
}

regards, tom lane