From: Russ Cox <rsc@golang.org>
Date: Sat, 22 Dec 2012 21:42:22 +0000 (-0500)
Subject: runtime: coalesce 0-size allocations
X-Git-Tag: go1.1rc2~1545
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=ec59b840f93fff4631b60323e28a44967c704e7d;p=gostls13.git

runtime: coalesce 0-size allocations

Fixes #3996.

R=ken2
CC=golang-dev
https://golang.org/cl/7001052
---

diff --git a/src/pkg/runtime/malloc.goc b/src/pkg/runtime/malloc.goc
index f58045e63e..e37f8927ba 100644
--- a/src/pkg/runtime/malloc.goc
+++ b/src/pkg/runtime/malloc.goc
@@ -697,14 +697,22 @@ runtime·new(Type *typ, uint8 *ret)
 
 	if(raceenabled)
 		m->racepc = runtime·getcallerpc(&typ);
-	flag = typ->kind&KindNoPointers ? FlagNoPointers : 0;
-	ret = runtime·mallocgc(typ->size, flag, 1, 1);
 
-	if(UseSpanType && !flag) {
-		if(false) {
-			runtime·printf("new %S: %p\n", *typ->string, ret);
+	if(typ->size == 0) {
+		// All 0-length allocations use this pointer.
+		// The language does not require the allocations to
+		// have distinct values.
+		ret = (uint8*)&runtime·zerobase;
+	} else {
+		flag = typ->kind&KindNoPointers ? FlagNoPointers : 0;
+		ret = runtime·mallocgc(typ->size, flag, 1, 1);
+
+		if(UseSpanType && !flag) {
+			if(false) {
+				runtime·printf("new %S: %p\n", *typ->string, ret);
+			}
+			runtime·settype(ret, (uintptr)typ | TypeInfo_SingleObject);
 		}
-		runtime·settype(ret, (uintptr)typ | TypeInfo_SingleObject);
 	}
 
 	FLUSH(&ret);
@@ -719,15 +727,24 @@ runtime·cnew(Type *typ)
 
 	if(raceenabled)
 		m->racepc = runtime·getcallerpc(&typ);
-	flag = typ->kind&KindNoPointers ? FlagNoPointers : 0;
-	ret = runtime·mallocgc(typ->size, flag, 1, 1);
 
-	if(UseSpanType && !flag) {
-		if(false) {
-			runtime·printf("new %S: %p\n", *typ->string, ret);
+	if(typ->size == 0) {
+		// All 0-length allocations use this pointer.
+		// The language does not require the allocations to
+		// have distinct values.
+		ret = (uint8*)&runtime·zerobase;
+	} else {
+		flag = typ->kind&KindNoPointers ? FlagNoPointers : 0;
+		ret = runtime·mallocgc(typ->size, flag, 1, 1);
+
+		if(UseSpanType && !flag) {
+			if(false) {
+				runtime·printf("new %S: %p\n", *typ->string, ret);
+			}
+			runtime·settype(ret, (uintptr)typ | TypeInfo_SingleObject);
 		}
-		runtime·settype(ret, (uintptr)typ | TypeInfo_SingleObject);
 	}
+
 	return ret;
 }