From 2b93c4dd06932ee9a5770353c75956910ace1c9b Mon Sep 17 00:00:00 2001
From: =?utf8?q?R=C3=A9my=20Oudompheng?= <oudomphe@phare.normalesup.org>
Date: Fri, 7 Feb 2014 23:58:21 +0100
Subject: [PATCH] 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 | 35 +++++++++++++++++++++++++++--------
 src/cmd/6g/ggen.c | 24 +++++++++++++++++++-----
 2 files changed, 46 insertions(+), 13 deletions(-)

diff --git a/src/cmd/6g/cgen.c b/src/cmd/6g/cgen.c
index da16071a8d..76ece93b04 100644
--- a/src/cmd/6g/cgen.c
+++ b/src/cmd/6g/cgen.c
@@ -1436,14 +1436,33 @@ 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;
-		}
-		while(c > 0) {
-			gins(AMOVSB, N, N);	// MOVB *(SI)+,*(DI)+
-			c--;
+		// 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);
 		}
 	}
 
diff --git a/src/cmd/6g/ggen.c b/src/cmd/6g/ggen.c
index 2bdb12bdd0..1b8bf7e400 100644
--- a/src/cmd/6g/ggen.c
+++ b/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)+
-- 
2.48.1