runtime: speed up cgo calls

Allocate Defer on stack during cgo calls, as suggested
by dvyukov. Also includes some comment corrections.

benchmark                   old,ns/op   new,ns/op
BenchmarkCgoCall                  669         330
(Intel Xeon CPU 1.80GHz * 4, Linux 386)

R=dvyukov, rsc
CC=golang-dev
https://golang.org/cl/4910041

misc/cgo/test/basic.go

@@ -48,6 +48,10 @@ struct ibv_async_event {
 struct ibv_context {
 	xxpthread_mutex_t mutex;
 };
+
+int add(int x, int y) {
+	return x+y;
+};
 */
 import "C"
 import (
@@ -132,3 +136,11 @@ var (
 type Context struct {
 	ctx *C.struct_ibv_context
 }
+
+func benchCgoCall(b *testing.B) {
+	const x = C.int(2)
+	const y = C.int(3)
+	for i := 0; i < b.N; i++ {
+		C.add(x, y)
+	}
+}

misc/cgo/test/cgo_test.go

@@ -26,3 +26,5 @@ func TestBlocking(t *testing.T)            { testBlocking(t) }
 func Test1328(t *testing.T)                { test1328(t) }
 func TestParallelSleep(t *testing.T)       { testParallelSleep(t) }
 func TestSetEnv(t *testing.T)              { testSetEnv(t) }
+
+func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

src/pkg/runtime/386/asm.s

@@ -432,17 +432,17 @@ TEXT runtime·cgocallback(SB),7,$12
 	PUSHL	(g_sched+gobuf_sp)(SI)
 	MOVL	SP, (g_sched+gobuf_sp)(SI)
 
-	// Switch to m->curg stack and call runtime.cgocallback
+	// Switch to m->curg stack and call runtime.cgocallbackg
 	// with the three arguments.  Because we are taking over
 	// the execution of m->curg but *not* resuming what had
 	// been running, we need to save that information (m->curg->gobuf)
 	// so that we can restore it when we're done. 
 	// We can restore m->curg->gobuf.sp easily, because calling
-	// runtime.cgocallback leaves SP unchanged upon return.
+	// runtime.cgocallbackg leaves SP unchanged upon return.
 	// To save m->curg->gobuf.pc, we push it onto the stack.
 	// This has the added benefit that it looks to the traceback
-	// routine like cgocallback is going to return to that
-	// PC (because we defined cgocallback to have
+	// routine like cgocallbackg is going to return to that
+	// PC (because we defined cgocallbackg to have
 	// a frame size of 12, the same amount that we use below),
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.

src/pkg/runtime/amd64/asm.s

@@ -477,17 +477,17 @@ TEXT runtime·cgocallback(SB),7,$24
 	PUSHQ	(g_sched+gobuf_sp)(SI)
 	MOVQ	SP, (g_sched+gobuf_sp)(SI)
 
-	// Switch to m->curg stack and call runtime.cgocallback
+	// Switch to m->curg stack and call runtime.cgocallbackg
 	// with the three arguments.  Because we are taking over
 	// the execution of m->curg but *not* resuming what had
 	// been running, we need to save that information (m->curg->gobuf)
 	// so that we can restore it when we're done. 
 	// We can restore m->curg->gobuf.sp easily, because calling
-	// runtime.cgocallback leaves SP unchanged upon return.
+	// runtime.cgocallbackg leaves SP unchanged upon return.
 	// To save m->curg->gobuf.pc, we push it onto the stack.
 	// This has the added benefit that it looks to the traceback
-	// routine like cgocallback is going to return to that
-	// PC (because we defined cgocallback to have
+	// routine like cgocallbackg is going to return to that
+	// PC (because we defined cgocallbackg to have
 	// a frame size of 24, the same amount that we use below),
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.

src/pkg/runtime/cgocall.c

@@ -68,7 +68,7 @@
 // stack (not an m->g0 stack).  First it calls runtime.exitsyscall, which will
 // block until the $GOMAXPROCS limit allows running this goroutine.
 // Once exitsyscall has returned, it is safe to do things like call the memory
-// allocator or invoke the Go callback function p.GoF.  runtime.cgocallback
+// allocator or invoke the Go callback function p.GoF.  runtime.cgocallbackg
 // first defers a function to unwind m->g0.sched.sp, so that if p.GoF
 // panics, m->g0.sched.sp will be restored to its old value: the m->g0 stack
 // and the m->curg stack will be unwound in lock step.
@@ -92,7 +92,7 @@ static void unwindm(void);
 void
 runtime·cgocall(void (*fn)(void*), void *arg)
 {
-	Defer *d;
+	Defer d;
 
 	if(!runtime·iscgo)
 		runtime·throw("cgocall unavailable");
@@ -106,18 +106,18 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 	 * Lock g to m to ensure we stay on the same stack if we do a
 	 * cgo callback.
 	 */
-	d = nil;
+	d.nofree = false;
 	if(m->lockedg == nil) {
 		m->lockedg = g;
 		g->lockedm = m;
 
 		// Add entry to defer stack in case of panic.
-		d = runtime·malloc(sizeof(*d));
-		d->fn = (byte*)unlockm;
-		d->siz = 0;
-		d->link = g->defer;
-		d->argp = (void*)-1;  // unused because unwindm never recovers
-		g->defer = d;
+		d.fn = (byte*)unlockm;
+		d.siz = 0;
+		d.link = g->defer;
+		d.argp = (void*)-1;  // unused because unlockm never recovers
+		d.nofree = true;
+		g->defer = &d;
 	}
 
 	/*
@@ -135,11 +135,10 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 	runtime·asmcgocall(fn, arg);
 	runtime·exitsyscall();
 
-	if(d != nil) {
-		if(g->defer != d || d->fn != (byte*)unlockm)
+	if(d.nofree) {
+		if(g->defer != &d || d.fn != (byte*)unlockm)
 			runtime·throw("runtime: bad defer entry in cgocallback");
-		g->defer = d->link;
-		runtime·free(d);
+		g->defer = d.link;
 		unlockm();
 	}
 }
@@ -192,7 +191,7 @@ runtime·cfree(void *p)
 void
 runtime·cgocallbackg(void (*fn)(void), void *arg, uintptr argsize)
 {
-	Defer *d;
+	Defer d;
 
 	if(g != m->curg)
 		runtime·throw("runtime: bad g in cgocallback");
@@ -200,12 +199,12 @@ runtime·cgocallbackg(void (*fn)(void), void *arg, uintptr argsize)
 	runtime·exitsyscall();	// coming out of cgo call
 
 	// Add entry to defer stack in case of panic.
-	d = runtime·malloc(sizeof(*d));
-	d->fn = (byte*)unwindm;
-	d->siz = 0;
-	d->link = g->defer;
-	d->argp = (void*)-1;  // unused because unwindm never recovers
-	g->defer = d;
+	d.fn = (byte*)unwindm;
+	d.siz = 0;
+	d.link = g->defer;
+	d.argp = (void*)-1;  // unused because unwindm never recovers
+	d.nofree = true;
+	g->defer = &d;
 
 	// Invoke callback.
 	reflect·call((byte*)fn, arg, argsize);
@@ -213,10 +212,9 @@ runtime·cgocallbackg(void (*fn)(void), void *arg, uintptr argsize)
 	// Pop defer.
 	// Do not unwind m->g0->sched.sp.
 	// Our caller, cgocallback, will do that.
-	if(g->defer != d || d->fn != (byte*)unwindm)
+	if(g->defer != &d || d.fn != (byte*)unwindm)
 		runtime·throw("runtime: bad defer entry in cgocallback");
-	g->defer = d->link;
-	runtime·free(d);
+	g->defer = d.link;
 
 	runtime·entersyscall();	// going back to cgo call
 }

src/pkg/runtime/proc.c

@@ -1153,6 +1153,7 @@ runtime·deferreturn(uintptr arg0)
 	runtime·memmove(argp, d->args, d->siz);
 	g->defer = d->link;
 	fn = d->fn;
+	if(!d->nofree)
 		runtime·free(d);
 	runtime·jmpdefer(fn, argp);
 }
@@ -1165,6 +1166,7 @@ rundefer(void)
 	while((d = g->defer) != nil) {
 		g->defer = d->link;
 		reflect·call(d->fn, d->args, d->siz);
+		if(!d->nofree)
 			runtime·free(d);
 	}
 }
@@ -1245,6 +1247,7 @@ runtime·panic(Eface e)
 			runtime·mcall(recovery);
 			runtime·throw("recovery failed"); // mcall should not return
 		}
+		if(!d->nofree)
 			runtime·free(d);
 	}
 
@@ -1280,6 +1283,7 @@ recovery(G *gp)
 	else
 		gp->sched.sp = (byte*)d->argp - 2*sizeof(uintptr);
 	gp->sched.pc = d->pc;
+	if(!d->nofree)
 		runtime·free(d);
 	runtime·gogo(&gp->sched, 1);
 }

src/pkg/runtime/runtime.h

@@ -359,6 +359,7 @@ enum {
 struct Defer
 {
 	int32	siz;
+	bool	nofree;
 	byte*	argp;  // where args were copied from
 	byte*	pc;
 	byte*	fn;