User Guide

154 128-Bit Media and Scientific Programming

AMD64 Technology 24592—Rev. 3.15—November 2009

Figure 4-30. PCMPEQB Compare Operation

For the PCMPEQx instructions, if the compared values are equal, the result mask is all 1s. If the values

are not equal, the result mask is all 0s. For the PCMPGTx instructions, if the signed value in the first

operand is greater than the signed value in the second operand, the result mask is all 1s. If the value in

the first operand is less than or equal to the value in the second operand, the result mask is all 0s.

By specifying the same register for both operands, PCMPEQx can be used to set the bits in an XMM

Figure 4-10 on page 115 shows an example of a non-branching sequence that implements a two-way

multiplexer—one that is equivalent to the following sequence of ternary operators in C or C++:

r0=a0>b0?a0:b0

r1=a1>b1?a1:b1

r2=a2>b2?a2:b2

r3=a3>b3?a3:b3

r4=a4>b4?a4:b4

r5=a5>b5?a5:b5

r6=a6>b6?a6:b6

r7=a7>b7?a7:b7

Assuming xmm0 contains the vector a, and xmm1 contains the vector b, the above C sequence can be

implemented with the following assembler sequence:

MOVQ xmm3, xmm0

PCMPGTW xmm3, xmm1 ;a>b?0xffff : 0

PAND xmm0, xmm3 ;a>b?a:0

PANDN xmm3, xmm1 ;a>b?0:b

POR xmm0, xmm3 ;r=a>b?a:b

513-168.eps

result

operand 1

127 0

operand 2

127 0

imm8

compare

all 1s or 0s all 1s or 0s

compare

. . . . . . . . . . . . . .