runtime: correct mmap return value checking on netbsd/openbsd

The current SysAlloc implementation suffers from a signed vs unsigned
comparision bug. Since the error code from mmap is negated, the
unsigned comparision of v < 4096 is always false on error. Fix this
by switching to the darwin/freebsd/linux mmap model and leave the mmap
return value unmodified.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7870044

src/pkg/runtime/mem_netbsd.c

@@ -50,10 +50,9 @@ runtime·SysReserve(void *v, uintptr n)
 		return v;
 
 	p = runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
-	if (p == ((void *)-ENOMEM))
+	if(p == (void*)ENOMEM)
 		return nil;
-	else
-		return p;
+	return p;
 }
 
 void
@@ -66,7 +65,7 @@ runtime·SysMap(void *v, uintptr n)
 	// On 64-bit, we don't actually have v reserved, so tread carefully.
 	if(sizeof(void*) == 8) {
 		p = runtime·mmap(v, n, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, -1, 0);
-		if(p == (void*)-ENOMEM)
+		if(p == (void*)ENOMEM)
 			runtime·throw("runtime: out of memory");
 		if(p != v) {
 			runtime·printf("runtime: address space conflict: map(%p) = %p\n", v, p);
@@ -76,7 +75,7 @@ runtime·SysMap(void *v, uintptr n)
 	}
 
 	p = runtime·mmap(v, n, PROT_READ|PROT_WRITE, MAP_ANON|MAP_FIXED|MAP_PRIVATE, -1, 0);
-	if(p == (void*)-ENOMEM)
+	if(p == (void*)ENOMEM)
 		runtime·throw("runtime: out of memory");
 	if(p != v)
 		runtime·throw("runtime: cannot map pages in arena address space");

src/pkg/runtime/mem_openbsd.c

@@ -50,10 +50,9 @@ runtime·SysReserve(void *v, uintptr n)
 		return v;
 
 	p = runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
-	if (p == ((void *)-ENOMEM))
+	if(p == (void*)ENOMEM)
 		return nil;
-	else
-		return p;
+	return p;
 }
 
 void
@@ -66,7 +65,7 @@ runtime·SysMap(void *v, uintptr n)
 	// On 64-bit, we don't actually have v reserved, so tread carefully.
 	if(sizeof(void*) == 8) {
 		p = runtime·mmap(v, n, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, -1, 0);
-		if(p == (void*)-ENOMEM)
+		if(p == (void*)ENOMEM)
 			runtime·throw("runtime: out of memory");
 		if(p != v) {
 			runtime·printf("runtime: address space conflict: map(%p) = %p\n", v, p);
@@ -76,7 +75,7 @@ runtime·SysMap(void *v, uintptr n)
 	}
 
 	p = runtime·mmap(v, n, PROT_READ|PROT_WRITE, MAP_ANON|MAP_FIXED|MAP_PRIVATE, -1, 0);
-	if(p == (void*)-ENOMEM)
+	if(p == (void*)ENOMEM)
 		runtime·throw("runtime: out of memory");
 	if(p != v)
 		runtime·throw("runtime: cannot map pages in arena address space");

src/pkg/runtime/sys_netbsd_386.s

@@ -88,8 +88,6 @@ TEXT runtime·mmap(SB),7,$36
 	STOSL
 	MOVL	$197, AX		// sys_mmap
 	INT	$0x80
-	JCC	2(PC)
-	NEGL	AX
 	RET
 
 TEXT runtime·munmap(SB),7,$-4

src/pkg/runtime/sys_netbsd_amd64.s

@@ -253,8 +253,6 @@ TEXT runtime·mmap(SB),7,$0
 	MOVQ	$0, R9			// arg 6 - pad
 	MOVL	$197, AX		// sys_mmap
 	SYSCALL
-	JCC	2(PC)
-	NEGQ	AX
 	ADDQ	$16, SP
 	RET
 

src/pkg/runtime/sys_openbsd_386.s

@@ -89,8 +89,6 @@ TEXT runtime·mmap(SB),7,$36
 	STOSL
 	MOVL	$197, AX		// sys_mmap
 	INT	$0x80
-	JCC	2(PC)
-	NEGL	AX
 	RET
 
 TEXT runtime·munmap(SB),7,$-4

src/pkg/runtime/sys_openbsd_amd64.s

@@ -242,8 +242,6 @@ TEXT runtime·mmap(SB),7,$0
 	MOVQ	$0, R9			// arg 6 - pad
 	MOVL	$197, AX
 	SYSCALL
-	JCC	2(PC)
-	NEGQ	AX
 	ADDQ	$16, SP
 	RET