cmd/6g: faster memmove/memset-like code using unaligned load/stores.

This changes makes sgen and clearfat use unaligned instructions for
the trailing bytes, like the runtime memmove does, resulting in faster
code when manipulating types whose size is not a multiple of 8.

LGTM=khr
R=khr, iant, rsc
CC=golang-codereviews
https://golang.org/cl/51740044

src/cmd/6g/cgen.c

@@ -1436,15 +1436,34 @@ sgen(Node *n, Node *ns, int64 w)
 			gins(AMOVSQ, N, N);	// MOVQ *(SI)+,*(DI)+
 			q--;
 		}
-
-		if(c >= 4) {
-			gins(AMOVSL, N, N);	// MOVL *(SI)+,*(DI)+
-			c -= 4;
-		}
+		// copy the remaining c bytes
+		if(w < 4 || c <= 1 || (odst < osrc && osrc < odst+w)) {
 			while(c > 0) {
 				gins(AMOVSB, N, N);	// MOVB *(SI)+,*(DI)+
 				c--;
 			}
+		} else if(w < 8 || c <= 4) {
+			nodsi.op = OINDREG;
+			noddi.op = OINDREG;
+			nodsi.type = types[TINT32];
+			noddi.type = types[TINT32];
+			if(c > 4) {
+				nodsi.xoffset = 0;
+				noddi.xoffset = 0;
+				gmove(&nodsi, &noddi);
+			}
+			nodsi.xoffset = c-4;
+			noddi.xoffset = c-4;
+			gmove(&nodsi, &noddi);
+		} else {
+			nodsi.op = OINDREG;
+			noddi.op = OINDREG;
+			nodsi.type = types[TINT64];
+			noddi.type = types[TINT64];
+			nodsi.xoffset = c-8;
+			noddi.xoffset = c-8;
+			gmove(&nodsi, &noddi);
+		}
 	}
 
 	restx(&cx, &oldcx);

src/cmd/6g/ggen.c

@@ -1016,7 +1016,8 @@ void
 clearfat(Node *nl)
 {
 	int64 w, c, q;
-	Node n1, oldn1, ax, oldax;
+	Node n1, oldn1, ax, oldax, di, z;
+	Prog *p;
 
 	/* clear a fat object */
 	if(debug['g'])
@@ -1048,10 +1049,23 @@ clearfat(Node *nl)
 		q--;
 	}
 
-	if(c >= 4) {
-		gconreg(AMOVQ, c, D_CX);
-		gins(AREP, N, N);	// repeat
-		gins(ASTOSB, N, N);	// STOB AL,*(DI)+
+	z = ax;
+	di = n1;
+	if(w >= 8 && c >= 4) {
+		di.op = OINDREG;
+		di.type = z.type = types[TINT64];
+		p = gins(AMOVQ, &z, &di);
+		p->to.scale = 1;
+		p->to.offset = c-8;
+	} else if(c >= 4) {
+		di.op = OINDREG;
+		di.type = z.type = types[TINT32];
+		p = gins(AMOVL, &z, &di);
+		if(c > 4) {
+			p = gins(AMOVL, &z, &di);
+			p->to.scale = 1;
+			p->to.offset = c-4;
+		}
 	} else
 	while(c > 0) {
 		gins(ASTOSB, N, N);	// STOB AL,*(DI)+