add asm dec128blk1

decode_amd64.s

@@ -0,0 +1,56 @@
+//go:build amd64
+// +build amd64
+
+#include "textflag.h"
+
+// dec128blk1(mask uint128be, in, out []byte)
+//    len(in)>0, len(in)%16==0, len(out)==len(in)/16
+TEXT ·dec128blk1(SB), NOSPLIT, $0-64
+    MOVQ ·mask+0(FP), DX
+    BSWAPQ DX
+    MOVQ ·in+16(FP), SI
+    MOVQ ·in+24(FP), CX
+    SHRQ $4, CX
+    MOVQ ·in+40(FP), DI
+    // go forward
+    CLD
+lop:
+    LODSQ
+    BSWAPQ AX
+    SUBQ DX, AX
+    MOVQ AX, BX
+    ANDB $1, AX
+    RORQ $1, AX
+    SHRQ $17, BX
+    SETCS AX
+    RORQ $1, AX
+    SHRQ $16, BX
+    SETCS AX
+    RORQ $1, AX
+    SHRQ $16, BX
+    SETCS AX
+
+    ROLQ $7, AX
+    MOVQ AX, R8
+
+    LODSQ
+    BSWAPQ AX
+    SUBQ DX, AX
+    MOVQ AX, BX
+    ANDB $1, AX
+    RORQ $1, AX
+    SHRQ $17, BX
+    SETCS AX
+    RORQ $1, AX
+    SHRQ $16, BX
+    SETCS AX
+    RORQ $1, AX
+    SHRQ $16, BX
+    SETCS AX
+
+    ROLQ $3, AX
+    ORQ  R8, AX
+
+    STOSB
+    LOOP lop
+    RET