AMD 250 manual Sample Code Translated into AMD64 Code, Example 1 C Code, Example 1 3DNow! Code

Software Optimization Guide for AMD64 Processors 25112 Rev. 3.06 September 2005

Sample Code Translated into AMD64 Code

The following examples use scalar code translated into AMD64 code. Note that it is not recommended that you use 3DNow! SIMD instructions for scalar code, because the advantage of 3DNow! instructions lies in their “SIMDness.” These examples are meant to demonstrate general techniques for translating source code with branches into branchless 3DNow! code. Scalar source code was chosen to keep the examples simple. These techniques work identically for vector code.

Each example shows the C code and the resulting 3DNow! code.

Example 1: C Code

float x, y, z; if (x < y) {

z += 1.0; } else {

z -= 1.0;

}

Example 1: 3DNow!™ Code

;In: MM0 = x

;MM1 = y

;MM2 = z

;Out: MM0 = z

movq	mm3, mm0	; Save x.
movq	mm4, one	; 1.0
pfcmpge	mm0, mm1	; x < y ?	0	: 0xffffffff
pslld	mm0, 31	; x < y ?	0	:	0x80000000
pxor	mm0, mm4	; x < y ?	1.0		: -1.0
pfadd	mm0, mm2	; x < y ?	z +		1.0 : z - 1.0

Example 2: C Code

float x, z; z = abs(x); if (z >= 1) {

z = 1 / z;

}

Example 2: 3DNow!™ Code

;In: MM0 = x

;Out: MM0 = z

movq	mm5, mabs	; 0x7fffffff
pand	mm0, mm5	; z = abs(x)
pfrcp	mm2, mm0	; 1	/ z approximation
movq	mm1, mm0	; Save z.
pfrcpit1 mm0, mm2		; 1	/ z step
pfrcpit2	mm0, mm2	; 1	/ z final
pfmin	mm0, mm1	; z = z < 1 ? z : 1 / z

138 Branch Optimizations Chapter 6