I read the manual description of those two operations but don't understand the difference yet. Can someone explain with an example how shufpd compares to pshufd?

CodePudding user response：

1. pshufd shuffles 32 bits as a unit. shufpd shuffles 64 bits as a unit.
2. pshufd shuffles within a single register. shufpd can merge-shuffle 2 registers.
3. They can be used to do the same task, but mixing integer and floating point instructions (pshufd with floating points, or shufpd with integers) may cause a bypass delay.

Below is a copy paste from the Intel docs explaining each operation with pseudocode. The difference is very clear when you read carefully.

pshufd a, a, imm8
  DEFINE SELECT4(src, control) {
    CASE(control[1:0]) OF
    0:  tmp[31:0] := src[31:0]
    1:  tmp[31:0] := src[63:32]
    2:  tmp[31:0] := src[95:64]
    3:  tmp[31:0] := src[127:96]
    ESAC
    RETURN tmp[31:0]
  }
  dst[31:0] := SELECT4(a[127:0], imm8[1:0])
  dst[63:32] := SELECT4(a[127:0], imm8[3:2])
  dst[95:64] := SELECT4(a[127:0], imm8[5:4])
  dst[127:96] := SELECT4(a[127:0], imm8[7:6])

shufpd a, b, imm8
  dst[63:0] := (imm8[0] == 0) ? a[63:0] : a[127:64]
  dst[127:64] := (imm8[1] == 0) ? b[63:0] : b[127:64]

Examples?

a = [1, 1, 2, 2]
b = [3, 3, 4, 4]

shufpd a, b, 1 -> [2, 2, 3, 3]

You cannot do this with pshufd, but sometimes both can be used for the same task.

a = [1, 1, 2, 2]

pshufd a, a, 0x4e -> [2, 2, 1, 1]
shufpd a, a, 1 -> [2, 2, 1, 1]