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Extract the high and low words together. With gcc-3.4 on uniformly

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distributed non-large args, this saves about 14 of 134 cycles for
Athlon64s and about 5 of 199 cycles for AthlonXPs.

Moved the check for x == 0 inside the check for subnormals.  With
gcc-3.4 on uniformly distributed non-large args, this saves another
5 cycles on Athlon64s and loses 1 cycle on AthlonXPs.

Use INSERT_WORDS() and not SET_HIGH_WORD() when converting the first
approximation from bits to double.  With gcc-3.4 on uniformly distributed
non-large args, this saves another 4 cycles on both Athlon64s and and
AthlonXPs.

Accessing doubles as 2 words may be an optimization on old CPUs, but on
current CPUs it tends to cause extra operations and pipeline stalls,
especially for writes, even when only 1 of the words needs to be accessed.

Removed an unused variable.
This commit is contained in:
Bruce Evans 2005-12-20 01:21:30 +00:00
parent c505fe7a0f
commit 5776f433ab
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=153548
1 changed files with 6 additions and 8 deletions
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14
lib/msun/src/s_cbrt.c
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@ -43,17 +43,13 @@ cbrt(double x)
uint64_t bits;
} u;
double r,s,t=0.0,w;
uint64_t bits;
u_int32_t sign;
u_int32_t high,low;
GET_HIGH_WORD(hx,x);
EXTRACT_WORDS(hx,low,x);
sign=hx&0x80000000; /* sign= sign(x) */
hx ^=sign;
if(hx>=0x7ff00000) return(x+x); /* cbrt(NaN,INF) is itself */
GET_LOW_WORD(low,x);
if((hx|low)==0)
return(x); /* cbrt(0) is itself */
/*
* Rough cbrt to 5 bits:
@ -70,13 +66,15 @@ cbrt(double x)
* subtraction virtually to keep e >= 0 so that ordinary integer
* division rounds towards minus infinity; this is also efficient.
*/
if(hx<0x00100000) { /* subnormal number */
if(hx<0x00100000) { /* zero or subnormal? */
if((hx|low)==0)
return(x); /* cbrt(0) is itself */
SET_HIGH_WORD(t,0x43500000); /* set t= 2**54 */
t*=x;
GET_HIGH_WORD(high,t);
SET_HIGH_WORD(t,sign|((high&0x7fffffff)/3+B2));
INSERT_WORDS(t,sign|((high&0x7fffffff)/3+B2),0);
} else
SET_HIGH_WORD(t,sign|(hx/3+B1));
INSERT_WORDS(t,sign|(hx/3+B1),0);
/*
* New cbrt to 23 bits: