cmd/gc: transform closure calls to function calls

Currently we always create context objects for closures that capture variables.
However, it is completely unnecessary for direct calls of closures
(whether it is func()(), defer func()() or go func()()).
This change transforms any OCALLFUNC(OCLOSURE) to normal function call.
Closed variables become function arguments.
This transformation is especially beneficial for go func(),
because we do not need to allocate context object on heap.
But it makes direct closure calls a bit faster as well (see BenchmarkClosureCall).

On implementation level it required to introduce yet another compiler pass.
However, the pass iterates only over xtop, so it should not be an issue.
Transformation consists of two parts: closure transformation and call site
transformation. We can't run these parts on different sides of escape analysis,
because tree state is inconsistent. We can do both parts during typecheck,
we don't know how to capture variables and don't have call site.
We can't do both parts during walk of OCALLFUNC, because we can walk
OCLOSURE body earlier.
So now capturevars pass only decides how to capture variables
(this info is required for escape analysis). New transformclosure
pass, that runs just before order/walk, does all transformations
of a closure. And later walk of OCALLFUNC(OCLOSURE) transforms call site.

benchmark                            old ns/op     new ns/op     delta
BenchmarkClosureCall                 4.89          3.09          -36.81%
BenchmarkCreateGoroutinesCapture     1634          1294          -20.81%

benchmark                            old allocs     new allocs     delta
BenchmarkCreateGoroutinesCapture     6              2              -66.67%

benchmark                            old bytes     new bytes     delta
BenchmarkCreateGoroutinesCapture     176           48            -72.73%

Change-Id: Ic85e1706e18c3235cc45b3c0c031a9c1cdb7a40e
Reviewed-on: https://go-review.googlesource.com/4050
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/closure.c

@@ -105,6 +105,7 @@ typecheckclosure(Node *func, int top)
 	oldfn = curfn;
 	typecheck(&func->ntype, Etype);
 	func->type = func->ntype->type;
+	func->top = top;
 
 	// Type check the body now, but only if we're inside a function.
 	// At top level (in a variable initialization: curfn==nil) we're not
@@ -119,9 +120,6 @@ typecheckclosure(Node *func, int top)
 		curfn = oldfn;
 	}
 
-	// Remember closure context for capturevars.
-	func->etype = (top & (Ecall|Eproc)) == Ecall;
-
 	// Create top-level function 
 	xtop = list(xtop, makeclosure(func));
 }
@@ -172,22 +170,17 @@ makeclosure(Node *func)
 // It decides whether each variable captured by a closure should be captured
 // by value or by reference.
 // We use value capturing for values <= 128 bytes that are never reassigned
-// after declaration.
+// after capturing (effectively constant).
 void
 capturevars(Node *xfunc)
 {
-	Node *func, *v, *addr, *cv, *outer;
-	NodeList *l, *body;
-	char *p;
-	vlong offset;
-	int nvar, lno;
+	Node *func, *v, *outer;
+	NodeList *l;
+	int lno;
 
 	lno = lineno;
 	lineno = xfunc->lineno;
 
-	nvar = 0;
-	body = nil;
-	offset = widthptr;
 	func = xfunc->closure;
 	func->enter = nil;
 	for(l=func->cvars; l; l=l->next) {
@@ -202,7 +195,6 @@ capturevars(Node *xfunc)
 			v->op = OXXX;
 			continue;
 		}
-		nvar++;
 
 		// type check the & of closed variables outside the closure,
 		// so that the outer frame also grabs them and knows they escape.
@@ -213,10 +205,8 @@ capturevars(Node *xfunc)
 		if(outer->class != PPARAMOUT && !v->closure->addrtaken && !v->closure->assigned && v->type->width <= 128)
 			v->byval = 1;
 		else {
+			v->closure->addrtaken = 1;
 			outer = nod(OADDR, outer, N);
-			// For a closure that is called in place, but not
-			// inside a go statement, avoid moving variables to the heap.
-			outer->etype = func->etype;
 		}
 		if(debug['m'] > 1)
 			warnl(v->lineno, "%S capturing by %s: %S (addr=%d assign=%d width=%d)",
@@ -224,42 +214,132 @@ capturevars(Node *xfunc)
 				v->sym, v->closure->addrtaken, v->closure->assigned, (int32)v->type->width);
 		typecheck(&outer, Erv);
 		func->enter = list(func->enter, outer);
-
-		// declare variables holding addresses taken from closure
-		// and initialize in entry prologue.
-		addr = nod(ONAME, N, N);
-		p = smprint("&%s", v->sym->name);
-		addr->sym = lookup(p);
-		free(p);
-		addr->ntype = nod(OIND, typenod(v->type), N);
-		addr->class = PAUTO;
-		addr->addable = 1;
-		addr->ullman = 1;
-		addr->used = 1;
-		addr->curfn = xfunc;
-		xfunc->dcl = list(xfunc->dcl, addr);
-		v->heapaddr = addr;
-		cv = nod(OCLOSUREVAR, N, N);
-		if(v->byval) {
-			cv->type = v->type;
-			offset = rnd(offset, v->type->align);
-			cv->xoffset = offset;
-			offset += v->type->width;
-			body = list(body, nod(OAS, addr, nod(OADDR, cv, N)));
-		} else {
-			v->closure->addrtaken = 1;
-			cv->type = ptrto(v->type);
-			offset = rnd(offset, widthptr);
-			cv->xoffset = offset;
-			offset += widthptr;
-			body = list(body, nod(OAS, addr, cv));
-		}
 	}
-	typechecklist(body, Etop);
-	walkstmtlist(body);
-	xfunc->enter = body;
-	xfunc->needctxt = nvar > 0;
-	func->etype = 0;
+
+	lineno = lno;
+}
+
+// transformclosure is called in a separate phase after escape analysis.
+// It transform closure bodies to properly reference captured variables.
+void
+transformclosure(Node *xfunc)
+{
+	Node *func, *cv, *addr, *v, *f;
+	NodeList *l, *body;
+	Type **param, *fld;
+	vlong offset;
+	int lno, nvar;
+
+	lno = lineno;
+	lineno = xfunc->lineno;
+	func = xfunc->closure;
+
+	if(func->top&Ecall) {
+		// If the closure is directly called, we transform it to a plain function call
+		// with variables passed as args. This avoids allocation of a closure object.
+		// Here we do only a part of the transformation. Walk of OCALLFUNC(OCLOSURE)
+		// will complete the transformation later.
+		// For illustration, the following closure:
+		//	func(a int) {
+		//		println(byval)
+		//		byref++
+		//	}(42)
+		// becomes:
+		//	func(a int, byval int, &byref *int) {
+		//		println(byval)
+		//		(*&byref)++
+		//	}(42, byval, &byref)
+
+		// f is ONAME of the actual function.
+		f = xfunc->nname;
+		// Get pointer to input arguments and rewind to the end.
+		// We are going to append captured variables to input args.
+		param = &getinargx(f->type)->type;
+		for(; *param; param = &(*param)->down) {
+		}
+		for(l=func->cvars; l; l=l->next) {
+			v = l->n;
+			if(v->op == OXXX)
+				continue;
+			fld = typ(TFIELD);
+			fld->funarg = 1;
+			if(v->byval) {
+				// If v is captured by value, we merely downgrade it to PPARAM.
+				v->class = PPARAM;
+				v->ullman = 1;
+				fld->nname = v;
+			} else {
+				// If v of type T is captured by reference,
+				// we introduce function param &v *T
+				// and v remains PPARAMREF with &v heapaddr
+				// (accesses will implicitly deref &v).
+				snprint(namebuf, sizeof namebuf, "&%s", v->sym->name);
+				addr = newname(lookup(namebuf));
+				addr->type = ptrto(v->type);
+				addr->class = PPARAM;
+				v->heapaddr = addr;
+				fld->nname = addr;
+			}
+			fld->type = fld->nname->type;
+			fld->sym = fld->nname->sym;
+			// Declare the new param and append it to input arguments.
+			xfunc->dcl = list(xfunc->dcl, fld->nname);
+			*param = fld;
+			param = &fld->down;
+		}
+		// Recalculate param offsets.
+		if(f->type->width > 0)
+			fatal("transformclosure: width is already calculated");
+		dowidth(f->type);
+		xfunc->type = f->type; // update type of ODCLFUNC
+	} else {
+		// The closure is not called, so it is going to stay as closure.
+		nvar = 0;
+		body = nil;
+		offset = widthptr;
+		for(l=func->cvars; l; l=l->next) {
+			v = l->n;
+			if(v->op == OXXX)
+				continue;
+			nvar++;
+			// cv refers to the field inside of closure OSTRUCTLIT.
+			cv = nod(OCLOSUREVAR, N, N);
+			cv->type = v->type;
+			if(!v->byval)
+				cv->type = ptrto(v->type);
+			offset = rnd(offset, cv->type->align);
+			cv->xoffset = offset;
+			offset += cv->type->width;
+
+			if(v->byval && v->type->width <= 2*widthptr && arch.thechar == '6') {
+				//  If it is a small variable captured by value, downgrade it to PAUTO.
+				// This optimization is currently enabled only for amd64, see:
+				// https://github.com/golang/go/issues/9865
+				v->class = PAUTO;
+				v->ullman = 1;
+				xfunc->dcl = list(xfunc->dcl, v);
+				body = list(body, nod(OAS, v, cv));
+			} else {
+				// Declare variable holding addresses taken from closure
+				// and initialize in entry prologue.
+				snprint(namebuf, sizeof namebuf, "&%s", v->sym->name);
+				addr = newname(lookup(namebuf));
+				addr->ntype = nod(OIND, typenod(v->type), N);
+				addr->class = PAUTO;
+				addr->used = 1;
+				addr->curfn = xfunc;
+				xfunc->dcl = list(xfunc->dcl, addr);
+				v->heapaddr = addr;
+				if(v->byval)
+					cv = nod(OADDR, cv, N);
+				body = list(body, nod(OAS, addr, cv));
+			}
+		}
+		typechecklist(body, Etop);
+		walkstmtlist(body);
+		xfunc->enter = body;
+		xfunc->needctxt = nvar > 0;
+	}
 
 	lineno = lno;
 }

src/cmd/gc/go.h

@@ -331,6 +331,7 @@ struct	Node
 	// ONAME closure param with PPARAMREF
 	Node*	outer;	// outer PPARAMREF in nested closure
 	Node*	closure;	// ONAME/PHEAP <-> ONAME/PPARAMREF
+	int	top;	// top context (Ecall, Eproc, etc)
 
 	// ONAME substitute while inlining
 	Node* inlvar;
@@ -1075,6 +1076,7 @@ Node*	closurebody(NodeList *body);
 void	closurehdr(Node *ntype);
 void	typecheckclosure(Node *func, int top);
 void	capturevars(Node *func);
+void	transformclosure(Node *func);
 Node*	walkclosure(Node *func, NodeList **init);
 void	typecheckpartialcall(Node*, Node*);
 Node*	walkpartialcall(Node*, NodeList**);

src/cmd/gc/lex.c

@@ -405,8 +405,9 @@ gcmain(int argc, char *argv[])
 		}
 	}
 
-	// Phase 4: Decide how to capture variables
-	// and transform closure bodies accordingly.
+	// Phase 4: Decide how to capture closed variables.
+	// This needs to run before escape analysis,
+	// because variables captured by value do not escape.
 	for(l=xtop; l; l=l->next) {
 		if(l->n->op == ODCLFUNC && l->n->closure) {
 			curfn = l->n;
@@ -457,7 +458,18 @@ gcmain(int argc, char *argv[])
 	// Move large values off stack too.
 	movelarge(xtop);
 
-	// Phase 7: Compile top level functions.
+	// Phase 7: Transform closure bodies to properly reference captured variables.
+	// This needs to happen before walk, because closures must be transformed
+	// before walk reaches a call of a closure.
+	for(l=xtop; l; l=l->next) {
+		if(l->n->op == ODCLFUNC && l->n->closure) {
+			curfn = l->n;
+			transformclosure(l->n);
+		}
+	}
+	curfn = N;
+
+	// Phase 8: Compile top level functions.
 	for(l=xtop; l; l=l->next)
 		if(l->n->op == ODCLFUNC)
 			funccompile(l->n);
@@ -465,7 +477,7 @@ gcmain(int argc, char *argv[])
 	if(nsavederrors+nerrors == 0)
 		fninit(xtop);
 
-	// Phase 8: Check external declarations.
+	// Phase 9: Check external declarations.
 	for(l=externdcl; l; l=l->next)
 		if(l->n->op == ONAME)
 			typecheck(&l->n, Erv);

src/cmd/gc/walk.c

@@ -628,6 +628,26 @@ walkexpr(Node **np, NodeList **init)
 		goto ret;
 
 	case OCALLFUNC:
+		if(n->left->op == OCLOSURE) {
+			// Transform direct call of a closure to call of a normal function.
+			// transformclosure already did all preparation work.
+
+			// Append captured variables to argument list.
+			n->list = concat(n->list, n->left->enter);
+			n->left->enter = NULL;
+			// Replace OCLOSURE with ONAME/PFUNC.
+			n->left = n->left->closure->nname;
+			// Update type of OCALLFUNC node.
+			// Output arguments had not changed, but their offsets could.
+			if(n->left->type->outtuple == 1) {
+				t = getoutargx(n->left->type)->type;
+				if(t->etype == TFIELD)
+					t = t->type;
+				n->type = t;
+			} else
+				n->type = getoutargx(n->left->type);
+		}
+
 		t = n->left->type;
 		if(n->list && n->list->n->op == OAS)
 			goto ret;

src/runtime/proc_test.go

@@ -435,6 +435,18 @@ func BenchmarkCreateGoroutinesCapture(b *testing.B) {
 	}
 }
 
+func BenchmarkClosureCall(b *testing.B) {
+	sum := 0
+	off1 := 1
+	for i := 0; i < b.N; i++ {
+		off2 := 2
+		func() {
+			sum += i + off1 + off2
+		}()
+	}
+	_ = sum
+}
+
 type Matrix [][]float64
 
 func BenchmarkMatmult(b *testing.B) {

test/fixedbugs/bug346.go

@@ -9,11 +9,28 @@ package main
 import "os"
 
 func main() {
-	x := 4
-	a, b, c, d := func(i int) (p int, q int, r int, s int) { return 1, i, 3, x }(2)
+	// Test unclosed closure.
+	{
+		x := 4
+		a, b, c, d := func(i int) (p int, q int, r int, s int) { return 1, i, 3, x }(2)
 
-	if a != 1 || b != 2 || c != 3 || d != 4 {
-		println("abcd: expected 1 2 3 4 got", a, b, c, d)
-		os.Exit(1)
+		if a != 1 || b != 2 || c != 3 || d != 4 {
+			println("1# abcd: expected 1 2 3 4 got", a, b, c, d)
+			os.Exit(1)
+		}
+	}
+	// Test real closure.
+	{
+		x := 4
+		gf = func(i int) (p int, q int, r int, s int) { return 1, i, 3, x }
+
+		a, b, c, d := gf(2)
+
+		if a != 1 || b != 2 || c != 3 || d != 4 {
+			println("2# abcd: expected 1 2 3 4 got", a, b, c, d)
+			os.Exit(1)
+		}
 	}
 }
+
+var gf func(int) (int, int, int, int)