small changes from go's 6l. there's a further change to optab.c, but that one is wrong
(unless my amd64 manual is wrong)

---
it turns out that intel deprecates the use of /63 without rex.w, so i'll make a further change once i've checked it.

Reference: /n/sources/patch/applied/6l-20111130
Date: Thu Dec  1 18:30:19 CET 2011
Signed-off-by: forsyth@terzarima.net

--- /sys/src/cmd/6l/span.c	Wed Nov 30 11:37:30 2011
+++ /sys/src/cmd/6l/span.c	Wed Nov 30 11:37:24 2011
@@ -1574,7 +1574,7 @@
 		/*
 		 * as befits the whole approach of the architecture,
 		 * the rex prefix must appear before the first opcode byte
-		 * (and thus after any 66/67/f2/f3 prefix bytes, but
+		 * (and thus after any 66/67/f2/f3/26/2e/3e prefix bytes, but
 		 * before the 0f opcode escape!), or it might be ignored.
 		 * note that the handbook often misleadingly shows 66/f2/f3 in `opcode'.
 		 */
@@ -1583,7 +1583,7 @@
 		n = andptr - and;
 		for(np = 0; np < n; np++) {
 			c = and[np];
-			if(c != 0x66 && c != 0xf2 && c != 0xf3 && c != 0x67)
+			if(c != 0xf2 && c != 0xf3 && (c < 0x64 || c > 0x67) && c != 0x2e && c != 0x3e && c != 0x26)
 				break;
 		}
 		memmove(and+np+1, and+np, n-np);
--- /sys/src/cmd/6l/optab.c	Wed Nov 30 11:37:38 2011
+++ /sys/src/cmd/6l/optab.c	Wed Nov 30 11:37:33 2011
@@ -753,7 +753,7 @@
 	{ AMOVBWSX,	ymb_rl,	Pq, 0xbe },
 	{ AMOVBWZX,	ymb_rl,	Pq, 0xb6 },
 	{ AMOVO,	yxmov,	Pe, 0x6f,0x7f },
-	{ AMOVOU,	yxmov,	Pf2, 0x6f,0x7f },
+	{ AMOVOU,	yxmov,	Pf3, 0x6f,0x7f },
 	{ AMOVHLPS,	yxr,	Pm, 0x12 },
 	{ AMOVHPD,	yxmov,	Pe, 0x16,0x17 },
 	{ AMOVHPS,	yxmov,	Pm, 0x16,0x17 },
@@ -877,9 +877,9 @@
 	{ APOPQ,	ypopl,	Py, 0x58,0x8f,(00) },
 	{ APOPW,	ypopl,	Pe, 0x58,0x8f,(00) },
 	{ APOR,		ymm,	Py, 0xeb,Pe,0xeb },
-	{ APSADBW,	yxm,	Pw, Pe,0xf6 },
+	{ APSADBW,	yxm,	Pq, 0xf6 },
 	{ APSHUFHW,	yxshuf,	Pf3, 0x70 },
-	{ APSHUFL,	yxm,	Pw, Pe,0x70 },
+	{ APSHUFL,	yxshuf,	Pq, 0x70 },
 	{ APSHUFLW,	yxshuf,	Pf2, 0x70 },
 	{ APSHUFW,	ymshuf,	Pm, 0x70 },
 	{ APSLLO,	ypsdq,	Pq, 0x73,(07) },