SSE2 instruction to convert a 8x16 register to two 4x32 registers having the even and odd indexed elements

Question

Is there any SSE2 instruction to convert a 8x16 register to two 4x32 registers,one 4x32 register having the odd indexed elements from the 8x16 register and the other having the even indexed elements? Please suggest.

Answer 1

Untested:

movdqa xmm1, xmm0
pslld xmm0, 16
psrad xmm1, 16  ; odd words
psrad xmm0, 16  ; even words

Should be easy enough to convert to intrinsics.

There is no single instruction for this, not even in later versions of SSE. Multiple-outputs is very rare, mostly reserved for old instructions.

pmovsxwd from SSE4.1 uses the (for this problem) wrong subset of elements, namely the bottom 4.

Answer 2

Note sure if there's a single instruction for this, but something like this ought to work (untested):

; Assume that the 8 16-bit values are in xmm0
PSHUFLW xmm1,xmm0,0D8h  ; Change word order to 3120 in the low qword
PSHUFHW xmm1,xmm1,0D8h  ; Change word order to 3120 in the high qword
PSHUFD xmm1,xmm1,0D8h   ; Change dword order to 3120
MOVAPD xmm0,xmm1        ; Copy to xmm0
PUNPCKLWD xmm0,xmm0     ; Expand even words to dwords
PUNPCKHWD xmm1,xmm1     ; Expand odd words to dwords
PSLLD xmm0,16           ; Sign-extend
PSRAD xmm0,16           ; ...
PSLLD xmm1,16
PSRAD xmm1,16

xmm0 should now contain the 4 even words sign-extended to 32 bits, and xmm1 should contain the odd words.

If you can use SSE4.1 instructions it's possible to simplify the sign-extension part a bit. For the even words (xmm0) you could replace the unpack and the two shifts with:

PMOVSXWD xmm0,xmm0