By SPEC, instruction vpermilps gets IMM bigger than 15.
Below, you can see that IMM can be any number between 0 to 255 include.
SELECT4(src, control){
CASE(control[1:0])
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]
}
tmp_dst[31:0] := SELECT4(a[127:0], imm8[1:0])
tmp_dst[63:32] := SELECT4(a[127:0], imm8[3:2])
tmp_dst[95:64] := SELECT4(a[127:0], imm8[5:4])
tmp_dst[127:96] := SELECT4(a[127:0], imm8[7:6])
FOR j := 0 to 3
i := j*32
IF k[j]
dst[i+31:i] := tmp_dst[i+31:i]
ELSE
dst[i+31:i] := src[i+31:i]
FI
ENDFOR
dst[MAX:128] := 0