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Improve code in scm_gcd for inum/inum case

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* libguile/numbers.c (scm_gcd): Improve implementation of inum/inum case
  to be more clear and efficient.
This commit is contained in:
Mark H Weaver 2013-03-03 04:34:17 -05:00
parent f57ea23ac8
commit a2dead1b0f
1 changed files with 30 additions and 24 deletions
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54
libguile/numbers.c
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@ -3889,52 +3889,58 @@ SCM_PRIMITIVE_GENERIC (scm_i_gcd, "gcd", 0, 2, 1,
SCM SCM
scm_gcd (SCM x, SCM y) scm_gcd (SCM x, SCM y)
{ {
if (SCM_UNBNDP (y)) if (SCM_UNLIKELY (SCM_UNBNDP (y)))
return SCM_UNBNDP (x) ? SCM_INUM0 : scm_abs (x); return SCM_UNBNDP (x) ? SCM_INUM0 : scm_abs (x);
if (SCM_I_INUMP (x)) if (SCM_LIKELY (SCM_I_INUMP (x)))
{ {
if (SCM_I_INUMP (y)) if (SCM_LIKELY (SCM_I_INUMP (y)))
{ {
scm_t_inum xx = SCM_I_INUM (x); scm_t_inum xx = SCM_I_INUM (x);
scm_t_inum yy = SCM_I_INUM (y); scm_t_inum yy = SCM_I_INUM (y);
scm_t_inum u = xx < 0 ? -xx : xx; scm_t_inum u = xx < 0 ? -xx : xx;
scm_t_inum v = yy < 0 ? -yy : yy; scm_t_inum v = yy < 0 ? -yy : yy;
scm_t_inum result; scm_t_inum result;
if (xx == 0) if (SCM_UNLIKELY (xx == 0))
result = v; result = v;
else if (yy == 0) else if (SCM_UNLIKELY (yy == 0))
result = u; result = u;
else else
{ {
scm_t_inum k = 1; int k = 0;
scm_t_inum t;
/* Determine a common factor 2^k */ /* Determine a common factor 2^k */
while (!(1 & (u | v))) while (((u | v) & 1) == 0)
{ {
k <<= 1; k++;
u >>= 1; u >>= 1;
v >>= 1; v >>= 1;
} }
/* Now, any factor 2^n can be eliminated */ /* Now, any factor 2^n can be eliminated */
if (u & 1) if ((u & 1) == 0)
t = -v; while ((u & 1) == 0)
u >>= 1;
else
while ((v & 1) == 0)
v >>= 1;
/* Both u and v are now odd. Subtract the smaller one
from the larger one to produce an even number, remove
more factors of two, and repeat. */
while (u != v)
{
if (u > v)
{
u -= v;
while ((u & 1) == 0)
u >>= 1;
}
else else
{ {
t = u; v -= u;
b3: while ((v & 1) == 0)
t = SCM_SRS (t, 1); v >>= 1;
} }
if (!(1 & t)) }
goto b3; result = u << k;
if (t > 0)
u = t;
else
v = -t;
t = u - v;
if (t != 0)
goto b3;
result = u * k;
} }
return (SCM_POSFIXABLE (result) return (SCM_POSFIXABLE (result)
? SCM_I_MAKINUM (result) ? SCM_I_MAKINUM (result)