AMD Athlon Processor x86 Optimization Manual page 80

AMD Athlon™ Processor x86 Code Optimization
Example 5:
64
C code:
#define PI 3.14159265358979323
float x,y,xa,ya,r,res;
int xs,df;
xs = x < 0 ? 1 : 0;
xa = fabs(x);
ya = fabs(y);
df = (xa < ya);
if (xs && df) {
res = PI/2 + r;
}
else if (xs) {
res = PI - r;
}
else if (df) {
res = PI/2 - r;
}
else {
res = r;
}
3DNow! code:
;in: MM0 = r
; MM1 = y
; MM2 = x
;out: MM0 = res
MOVQ MM7, sgn
MOVQ MM6, sgn
MOVQ MM5, mabs
PAND MM7, MM2
PAND MM1, MM5
PAND MM2, MM5
MOVQ MM6, MM1
PCMPGTD MM6, MM2
PSLLD MM6, 31
MOVQ MM5, MM7
PXOR MM7, MM6
MOVQ MM3, npio2
PXOR MM5, MM3
PSRAD MM6, 31
PANDN MM6, MM5
PFSUB MM6, MM3
POR MM0, MM7
PFADD MM0, MM6
;mask to extract sign bit
;mask to extract sign bit
;mask to clear sign bit
;xs = sign(x)
;ya = abs(y)
;xa = abs(x)
;y
;df = (xa < ya) ? 0xffffffff : 0
;df = bit<31>
;xs
;xs^df ? 0x80000000 : 0
;-pi/2
;xs ? pi/2 : -pi/2
;df ? 0xffffffff : 0
;xs ? (df ? 0 : pi/2) : (df ? 0 : -pi/2)
;pr = pi/2 + (xs ? (df ? 0 : pi/2) :
; (df ? 0 : -pi/2))
;ar = xs^df ? -r : r
;res = ar + pr
Replace Branches with Computation in 3DNow!™ Code
22007E/0—November 1999