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Remove the "quick check no cancellation" optimization for

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9pi/2 < |x| < 32pi/2 since it is only a small or negative optimation
and it gets in the way of further optimizations.  It did one more
branch to avoid some integer operations and to use a different
dependency on previous results.  The branches are fairly predictable
so they are usually not a problem, so whether this is a good
optimization depends mainly on the timing for the previous results,
which is very machine-dependent.  On amd64 (A64), this "optimization"
is a pessimization of about 1 cycle or 1%; on ia64, it is an
optimization of about 2 cycles or 1%; on i386 (A64), it is an
optimization of about 5 cycles or 4%; on i386 (Celeron P2) it is an
optimization of about 4 cycles or 3% for cos but a pessimization of
about 5 cycles for sin and 1 cycle for tan.  I think the new i386
(A64) slowness is due to an pipeline stall due to an avoidable
load-store mismatch (so the old timing was better), and the i386
(Celeron) variance is due to its branch predictor not being too good.
This commit is contained in:
Bruce Evans 2008-02-22 17:26:24 +00:00
parent 43590b1517
commit 7c1b5e7953
1 changed files with 1 additions and 12 deletions
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13
lib/msun/src/e_rem_pio2.c
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@ -26,15 +26,6 @@ __FBSDID("$FreeBSD$");
#include "math.h"
#include "math_private.h"
static const int32_t npio2_hw[] = {
0x3FF921FB, 0x400921FB, 0x4012D97C, 0x401921FB, 0x401F6A7A, 0x4022D97C,
0x4025FDBB, 0x402921FB, 0x402C463A, 0x402F6A7A, 0x4031475C, 0x4032D97C,
0x40346B9C, 0x4035FDBB, 0x40378FDB, 0x403921FB, 0x403AB41B, 0x403C463A,
0x403DD85A, 0x403F6A7A, 0x40407E4C, 0x4041475C, 0x4042106C, 0x4042D97C,
0x4043A28C, 0x40446B9C, 0x404534AC, 0x4045FDBB, 0x4046C6CB, 0x40478FDB,
0x404858EB, 0x404921FB,
};
/*
* invpio2: 53 bits of 2/pi
* pio2_1: first 33 bit of pi/2
@ -148,9 +139,7 @@ __ieee754_rem_pio2(double x, double *y)
#endif
r = t-fn*pio2_1;
w = fn*pio2_1t; /* 1st round good to 85 bit */
if(n<32&&ix!=npio2_hw[n-1]) {
y[0] = r-w; /* quick check no cancellation */
} else {
{
u_int32_t high;
j = ix>>20;
y[0] = r-w;