------- Comment #2 from vda dot linux at googlemail dot com 2006-07-18 12:06
-------
Pseudo-code to help in reading above code:
void fastdiv_params(unsigned B, unsigned max_A)
{
L = (max_unsigned_long_long/max_unsigned - 1) * B;
L = largest_pow2_smaller_or_eq_to(L);
bits = log2(L);
while ((K = L/B + 1) > max_unsigned)
bits--, L >>= 1;
// There is not much point in multiplying by even 'K'
// and then shifting right (dividing by 2*n). Reduce K & L to avoid it:
while (!(K & 1) && bits > 32)
bits--, L >>= 1, K = L/B + 1;
d_LB = ((L/B) * B + B - L);
bad_A = L / d_LB;
bad_A = (bad_A / B) * B + B-1;
if (bad_A <= max_A)
bail_out_no_params_exist;
// 'K' and 'bits' params are already determined, can return them...
/* return_params(K,bits); */
// ...but if max_A is smaller then max_unsigned, we can
// optionally use smaller 'K' and 'bits'. Find smallest which
// still work:
max_bits = bits;
max_bad_A = bad_A;
while(1) {
uint64_t last_L = L;
unsigned last_K = K;
bits--;
L >>= 1;
K = L/B + 1;
d_LB = ((L/B) * B + B - L);
bad_A = L / d_LB;
bad_A = (bad_A / B) * B + B-1;
if (bad_A <= max_A || bits < 32)
// new params are bad, report last ones and bail out
return_params(last_K,bits+1);
}
}
--
vda dot linux at googlemail dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|middle-end |c
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28417
