user manual

60 Replace Branches with Computation in 3DNow!™ Code

AMD Athlon™ Processor x86 Code Optimization

22007E/0—November 1999

Replace Branches with Computation in 3DNow!™ Code

Branches negatively impact the performance of 3DNow! code.

Branches can operate only on one data item at a time, i.e., they

are inherently scalar and inhibit the SIMD processing that

makes 3DNow! code superior. Also, branches based on 3DNow!

comparisons require data to be passed to the integer units,

which requires either transport through memory, or the use of

“MOVD reg, MMreg” instructions. If the body of the branch is

small, one can achieve higher performance by replacing the

branch with computation. The computation simulates

predicated execution or conditional moves. The principal tools

for this are the following instructions: PCMPGT, PFCMPGT,

PFCMPGE, PFMIN, PFMAX, PAND, PANDN, POR, PXOR.

Muxing Constructs

The most important construct to avoiding branches in

3DNow!™ and MMX™ code is a 2-way muxing construct that is

equivalent to the ternary operator “?:” in C and C++. It is

implemented using the PCMP/PFCMP, PAND, PANDN, and

POR instructions. To maximize performance, it is important to

apply the PAND and PANDN instructions in the proper order.

Example 1 (Avoid):

; r = (x < y) ? a : b

;

; in: mm0 a

; mm1 b

; mm2 x

; mm3 y

; out: mm1 r

PCMPGTD MM3, MM2 ; y > x ? 0xffffffff : 0

MOVQ MM4, MM3 ; duplicate mask

PANDN MM3, MM0 ; y > x ? 0 : a

PAND MM1, MM4 ; y > x ? b : 0

POR MM1, MM3 ; r = y > x ? b : a

Because the use of PANDN destroys the mask created by PCMP,

the mask needs to be saved, which requires an additional

chain, and increases register pressure. Therefore 2-way muxing

constructs should be written as follows.