runtime: implement runtime.SysUnused on FreeBSD

madvise was missing so implement it in assembler. This change
needs to be extended to the other BSD variantes (Net and Open)

Without this change the scavenger will attempt to pass memory back
to the operating system when it has become idle, but the memory is
not returned and for long running Go processes the total memory used
can grow until OOM occurs.

I have only been able to test the code on FreeBSD AMD64. The ARM
platforms needs testing.

R=golang-dev, mikioh.mikioh, dave, jgc, minux.ma
CC=golang-dev
https://golang.org/cl/6850081

src/pkg/runtime/defs_freebsd.go

@@ -7,8 +7,9 @@
 /*
 Input to cgo.
 
-GOARCH=amd64 cgo -cdefs defs.go >amd64/defs.h
-GOARCH=386 cgo -cdefs defs.go >386/defs.h
+GOARCH=amd64 go tool cgo -cdefs defs_freebsd.go >defs_freebsd_amd64.h
+GOARCH=386 go tool cgo -cdefs defs_freebsd.go >defs_freebsd_386.h
+GOARCH=arm go tool cgo -cdefs defs_freebsd.go >defs_freebsd_arm.h
 */
 
 package runtime
@@ -38,6 +39,8 @@ const (
 	MAP_PRIVATE = C.MAP_PRIVATE
 	MAP_FIXED   = C.MAP_FIXED
 
+	MADV_FREE = C.MADV_FREE
+
 	SA_SIGINFO = C.SA_SIGINFO
 	SA_RESTART = C.SA_RESTART
 	SA_ONSTACK = C.SA_ONSTACK

src/pkg/runtime/defs_freebsd_386.h

@@ -11,6 +11,7 @@ enum {
 	MAP_ANON = 0x1000,
 	MAP_PRIVATE = 0x2,
 	MAP_FIXED = 0x10,
+	MADV_FREE = 0x5,
 	SA_SIGINFO = 0x40,
 	SA_RESTART = 0x2,
 	SA_ONSTACK = 0x1,

src/pkg/runtime/defs_freebsd_amd64.h

@@ -11,6 +11,7 @@ enum {
 	MAP_ANON = 0x1000,
 	MAP_PRIVATE = 0x2,
 	MAP_FIXED = 0x10,
+	MADV_FREE = 0x5,
 	SA_SIGINFO = 0x40,
 	SA_RESTART = 0x2,
 	SA_ONSTACK = 0x1,

src/pkg/runtime/defs_freebsd_arm.h

@@ -11,6 +11,7 @@ enum {
 	MAP_ANON = 0x1000,
 	MAP_PRIVATE = 0x2,
 	MAP_FIXED = 0x10,
+	MADV_FREE = 0x5,
 	SA_SIGINFO = 0x40,
 	SA_RESTART = 0x2,
 	SA_ONSTACK = 0x1,

src/pkg/runtime/mem_freebsd.c

@@ -23,9 +23,7 @@ runtime·SysAlloc(uintptr n)
 void
 runtime·SysUnused(void *v, uintptr n)
 {
-	USED(v);
-	USED(n);
-	// TODO(rsc): call madvise MADV_DONTNEED
+	runtime·madvise(v, n, MADV_FREE);
 }
 
 void

src/pkg/runtime/sys_freebsd_386.s

@@ -102,6 +102,13 @@ TEXT runtime·munmap(SB),7,$-4
 	MOVL	$0xf1, 0xf1  // crash
 	RET
 
+TEXT runtime·madvise(SB),7,$-4
+	MOVL	$75, AX	// madvise
+	INT	$0x80
+	JAE	2(PC)
+	MOVL	$0xf1, 0xf1  // crash
+	RET
+
 TEXT runtime·setitimer(SB), 7, $-4
 	MOVL	$83, AX
 	INT	$0x80

src/pkg/runtime/sys_freebsd_amd64.s

@@ -184,6 +184,17 @@ TEXT runtime·munmap(SB),7,$0
 	MOVL	$0xf1, 0xf1  // crash
 	RET
 
+TEXT runtime·madvise(SB),7,$0
+	MOVQ	8(SP), DI
+	MOVQ	16(SP), SI
+	MOVQ	24(SP), DX
+	MOVQ	$75, AX	// madvise
+	SYSCALL
+	CMPQ	AX, $0xfffffffffffff001
+	JLS	2(PC)
+	MOVL	$0xf1, 0xf1  // crash
+	RET
+	
 TEXT runtime·sigaltstack(SB),7,$-8
 	MOVQ	new+8(SP), DI
 	MOVQ	old+16(SP), SI

src/pkg/runtime/sys_freebsd_arm.s

@@ -187,6 +187,15 @@ TEXT runtime·munmap(SB),7,$0
 	MOVW.CS R9, (R9)
 	RET
 
+TEXT runtime·madvise(SB),7,$0
+	MOVW 0(FP), R0		// arg 1 addr
+	MOVW 4(FP), R1		// arg 2 len
+	MOVW 8(FP), R2		// arg 3 flags
+	SWI $75
+	MOVW.CS $0, R9 // crash on syscall failure
+	MOVW.CS R9, (R9)
+	RET
+	
 TEXT runtime·sigaltstack(SB),7,$-8
 	MOVW new+0(FP), R0
 	MOVW old+4(FP), R1