The select intrinsic selects between the first set of lanes or the
second one according to the value of the select
parameter. If the lane corresponding bit in select
is zero, it returns the value in the first set of lanes. If the bit is one, it
returns the value in the second set of lanes. For example, a select16 intrinsic function has the following function prototype.
v16int32 select16 ( unsigned int select,
v16int32 xbuff,
int xstart,
unsigned int xoffsets,
unsigned int xoffsets_hi,
v16int32 ybuff,
int ystart,
unsigned int yoffsets,
unsigned int yoffsets_hi
) For each bit of select (from low to high), it
will select a lane either from xbuff (if the
select parameter bit is 0) or from ybuff (if the select
parameter bit is 1). Data permute on the resulting lane of xbuff or ybuff is achieved by a
shuffle with corresponding bits in xoffsets or yoffsets.
Following is the pseudo C-style code for select.
for (int i = 0; i < 16; i++){
idx = f( xstart, xoffsets[i]); //i'th 4 bits of offsets
idy = f( ystart, yoffsets[i]);
o[i] = select[i] ? y[idy]:x[idx];
}For information about how f works in
previous code, refer to the regular lane selection scheme equation listed at the
beginning of this section.
When working on the int16 data type, the select intrinsic has an additional xsquare parameter
which allows a further 16-bit granularity mini permute after main permute. For
example, a select32 intrinsic function has the
following function prototype.
v32int16 select32 ( unsigned int select,
v64int16 xbuff,
int xstart,
unsigned int xoffsets,
unsigned int xoffsets_hi,
unsigned int xsquare,
int ystart,
unsigned int yoffsets,
unsigned int yoffsets_hi,
unsigned int ysquare
) Following is the pseudo C-style code for select.
for (int i = 0; i < 32; i++){
idx = f( xstart, xoffsets[i], xsquare);
idy = f( ystart, yoffsets[i], ysquare);
o[i] = select[i] ? y[idy]:x[idx];
}The following example uses select32
to interleave first 16 elements of A and B (A first).
int16 A[32]={0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,
16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31
};
int16 B[32]={32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,
48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63
};
v32int16 *pA=(v32int16*)A;
v32int16 *pB=(v32int16*)B;
v32int16 C = select32(0xAAAAAAAA, concat(*pA,*pB),
0, 0x03020100, 0x07060504, 0x1100,
32, 0x03020100, 0x07060504, 0x1100);The output C for the previous code is as follows.
{0,32,1,33,2,34,3,35,4,36,5,37,6,38,7,39,8,40,9,41,10,42,11,43,12,44,13,45,14,46,15,47
}This can also be done using the shuffle32 intrinsic.
v32int16 C = shuffle32(concat(*pA,*pB),
0, 0xF3F2F1F0, 0xF7F6F5F4, 0x3120);The following figure shows how the previous select32 intrinsic works.
