swap xmm0 hi/lo

[jack]

suppose that xmm0 has two doubles, how do you swap the values so that I can move it to a scalar variable, I hope that it makes sense
ok, I think SHUFPD is the instruction to use, it's just a bit confusing http://qcd.phys.cmu.edu/QCDcluster/intel/vtune/reference/vc292.htm

[jack]

the following seems to work but I don't understand why

Code Select Expand


pshufd  xmm0, xmm0, 1110b

you can get some really messed-up numbers if the select operand is wrong

[jj2007]

It's tricky indeed

Each 2-bit field in the order operand selects the contents of one doubleword location in the destination operand. For example, bits 0 and 1 of the order operand selects the contents of doubleword 0 of the destination operand. The encoding of bits 0 and 1 of the order operand determines which doubleword from the source operand will be copied to doubleword 0 of the destination operand.

include \masm32\MasmBasic\MasmBasic.inc         ; download
MyOword         OWORD 0DDDDDDDDCCCCCCCCBBBBBBBBAAAAAAAAh
  Init
  Cls
  PrintLine "AAAAAAAAh is the least significant DWORD"
  movups xmm0, MyOword
  deb 4, "the original", x:xmm0
  PrintLine "x_x_x_x_b values indicate which source DWORD should be moved"
  PrintLine "3_2_1_0_b are the destination positions"
  pshufd xmm1, xmm0, 0b
  deb 4, "00000000b", x:xmm1
  pshufd xmm1, xmm0, 00000001b
  deb 4, "00000001b", x:xmm1
  pshufd xmm1, xmm0, 01000001b
  deb 4, "01000001b", x:xmm1
  pshufd xmm1, xmm0, 01010101b
  deb 4, "01010101b", x:xmm1
  pshufd xmm1, xmm0, 10101010b
  deb 4, "10101010b", x:xmm1
  pshufd xmm1, xmm0, 00011011b
  deb 4, "00011011b", x:xmm1
  PrintLine "The last one moves element 3 (11b) to position 0, element 2 (10b) to position 1, etc"
  Inkey
EndOfCode

Code Select Expand

AAAAAAAAh is the least significant DWORD
the original    x:xmm0          DDDDDDDD CCCCCCCC BBBBBBBB AAAAAAAA
x_x_x_x_b values indicate which source DWORD should be moved
3_2_1_0_b are the destination positions
00000000b       x:xmm1          AAAAAAAA AAAAAAAA AAAAAAAA AAAAAAAA
00000001b       x:xmm1          AAAAAAAA AAAAAAAA AAAAAAAA BBBBBBBB
01000001b       x:xmm1          BBBBBBBB AAAAAAAA AAAAAAAA BBBBBBBB
01010101b       x:xmm1          BBBBBBBB BBBBBBBB BBBBBBBB BBBBBBBB
10101010b       x:xmm1          CCCCCCCC CCCCCCCC CCCCCCCC CCCCCCCC
00011011b       x:xmm1          AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
The last one moves element 3 (11b) to position 0, element 2 (10b) to position 1, etc

Attached an interactive version for playing around with the immediate values.

[jack]

thank you for the example jj :)

[jj2007]

My pleasure. With the new attachment above, you can play with the immediate argument (the string is editable with cursor left/right, delete and backspace; hex and decimal values are also accepted)

[nidud]

deleted

[jj2007]

To swap two doubles, use 01001110b in the interactive program above. As Nidud writes, SHUFPD is the dedicated instruction, but PSHUFD works, too.

[jack]

thank you all
I like the simplicity of shufpd xmm0,xmm0,1 so will use that instead