cmd/compile: set stricter inlining threshold in large functions

If we're compiling a large function, be more picky about how big
the function we're inlining is.  If the function is >5000 nodes,
we lower the inlining threshold from a cost of 80 to 20.

Turns out reflect.Value's cost is exactly 80.  That's the function
at issue in golang#26546.

20 was chosen as a proxy for "inlined body is smaller than the call would be".
Simple functions still get inlined, like this one at cost 7:

func ifaceIndir(t *rtype) bool {
	return t.kind&kindDirectIface == 0
}

5000 nodes was chosen as the big function size.  Here are all the
5000+ node (~~1000+ lines) functions in the stdlib:

5187 cmd/internal/obj/arm (*ctxt5).asmout
6879 cmd/internal/obj/s390x (*ctxtz).asmout
6567 cmd/internal/obj/ppc64 (*ctxt9).asmout
9643 cmd/internal/obj/arm64 (*ctxt7).asmout
5042 cmd/internal/obj/x86 (*AsmBuf).doasm
8768 cmd/compile/internal/ssa rewriteBlockAMD64
8878 cmd/compile/internal/ssa rewriteBlockARM
8344 cmd/compile/internal/ssa rewriteValueARM64_OpARM64OR_20
7916 cmd/compile/internal/ssa rewriteValueARM64_OpARM64OR_30
5427 cmd/compile/internal/ssa rewriteBlockARM64
5126 cmd/compile/internal/ssa rewriteValuePPC64_OpPPC64OR_50
6152 cmd/compile/internal/ssa rewriteValuePPC64_OpPPC64OR_60
6412 cmd/compile/internal/ssa rewriteValuePPC64_OpPPC64OR_70
6486 cmd/compile/internal/ssa rewriteValuePPC64_OpPPC64OR_80
6534 cmd/compile/internal/ssa rewriteValuePPC64_OpPPC64OR_90
6534 cmd/compile/internal/ssa rewriteValuePPC64_OpPPC64OR_100
6534 cmd/compile/internal/ssa rewriteValuePPC64_OpPPC64OR_110
6675 cmd/compile/internal/gc typecheck1
5433 cmd/compile/internal/gc walkexpr
14070 cmd/vendor/golang.org/x/arch/arm64/arm64asm decodeArg

There are a lot more smaller (~1000 node) functions in the stdlib.
The function in golang#26546 has 12477 nodes.

At some point it might be nice to have a better heuristic for "inlined
body is smaller than the call", a non-cliff way to scale down the cost
as the function gets bigger, doing cheaper inlined calls first, etc.
All that can wait for another release. I'd like to do this CL for
1.11.

Fixes golang#26546
Update golang#17566

Change-Id: Idda13020e46ec2b28d79a17217f44b189f8139ac
Reviewed-on: https://go-review.googlesource.com/125516
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: randall77

src/cmd/compile/internal/gc/inl.go

@@ -41,6 +41,9 @@ const (
 	inlineExtraCallCost   = inlineMaxBudget // default is do not inline, -l=4 enables by using 1 instead.
 	inlineExtraPanicCost  = 1               // do not penalize inlining panics.
 	inlineExtraThrowCost  = inlineMaxBudget // with current (2018-05/1.11) code, inlining runtime.throw does not help.
+
+	inlineBigFunctionNodes   = 5000 // Functions with this many nodes are considered "big".
+	inlineBigFunctionMaxCost = 20   // Max cost of inlinee when inlining into a "big" function.
 )
 
 // Get the function's package. For ordinary functions it's on the ->sym, but for imported methods
@@ -459,12 +462,38 @@ func inlcopy(n *Node) *Node {
 	return m
 }
 
+func countNodes(n *Node) int {
+	if n == nil {
+		return 0
+	}
+	cnt := 1
+	cnt += countNodes(n.Left)
+	cnt += countNodes(n.Right)
+	for _, n1 := range n.Ninit.Slice() {
+		cnt += countNodes(n1)
+	}
+	for _, n1 := range n.Nbody.Slice() {
+		cnt += countNodes(n1)
+	}
+	for _, n1 := range n.List.Slice() {
+		cnt += countNodes(n1)
+	}
+	for _, n1 := range n.Rlist.Slice() {
+		cnt += countNodes(n1)
+	}
+	return cnt
+}
+
 // Inlcalls/nodelist/node walks fn's statements and expressions and substitutes any
 // calls made to inlineable functions. This is the external entry point.
 func inlcalls(fn *Node) {
 	savefn := Curfn
 	Curfn = fn
-	fn = inlnode(fn)
+	maxCost := int32(inlineMaxBudget)
+	if countNodes(fn) >= inlineBigFunctionNodes {
+		maxCost = inlineBigFunctionMaxCost
+	}
+	fn = inlnode(fn, maxCost)
 	if fn != Curfn {
 		Fatalf("inlnode replaced curfn")
 	}
@@ -505,10 +534,10 @@ func inlconv2list(n *Node) []*Node {
 	return s
 }
 
-func inlnodelist(l Nodes) {
+func inlnodelist(l Nodes, maxCost int32) {
 	s := l.Slice()
 	for i := range s {
-		s[i] = inlnode(s[i])
+		s[i] = inlnode(s[i], maxCost)
 	}
 }
 
@@ -525,7 +554,7 @@ func inlnodelist(l Nodes) {
 // shorter and less complicated.
 // The result of inlnode MUST be assigned back to n, e.g.
 // 	n.Left = inlnode(n.Left)
-func inlnode(n *Node) *Node {
+func inlnode(n *Node, maxCost int32) *Node {
 	if n == nil {
 		return n
 	}
@@ -547,19 +576,19 @@ func inlnode(n *Node) *Node {
 
 	lno := setlineno(n)
 
-	inlnodelist(n.Ninit)
+	inlnodelist(n.Ninit, maxCost)
 	for _, n1 := range n.Ninit.Slice() {
 		if n1.Op == OINLCALL {
 			inlconv2stmt(n1)
 		}
 	}
 
-	n.Left = inlnode(n.Left)
+	n.Left = inlnode(n.Left, maxCost)
 	if n.Left != nil && n.Left.Op == OINLCALL {
 		n.Left = inlconv2expr(n.Left)
 	}
 
-	n.Right = inlnode(n.Right)
+	n.Right = inlnode(n.Right, maxCost)
 	if n.Right != nil && n.Right.Op == OINLCALL {
 		if n.Op == OFOR || n.Op == OFORUNTIL {
 			inlconv2stmt(n.Right)
@@ -568,7 +597,7 @@ func inlnode(n *Node) *Node {
 		}
 	}
 
-	inlnodelist(n.List)
+	inlnodelist(n.List, maxCost)
 	switch n.Op {
 	case OBLOCK:
 		for _, n2 := range n.List.Slice() {
@@ -595,7 +624,7 @@ func inlnode(n *Node) *Node {
 		}
 	}
 
-	inlnodelist(n.Rlist)
+	inlnodelist(n.Rlist, maxCost)
 	if n.Op == OAS2FUNC && n.Rlist.First().Op == OINLCALL {
 		n.Rlist.Set(inlconv2list(n.Rlist.First()))
 		n.Op = OAS2
@@ -614,7 +643,7 @@ func inlnode(n *Node) *Node {
 		}
 	}
 
-	inlnodelist(n.Nbody)
+	inlnodelist(n.Nbody, maxCost)
 	for _, n := range n.Nbody.Slice() {
 		if n.Op == OINLCALL {
 			inlconv2stmt(n)
@@ -637,12 +666,12 @@ func inlnode(n *Node) *Node {
 			fmt.Printf("%v:call to func %+v\n", n.Line(), n.Left)
 		}
 		if n.Left.Func != nil && n.Left.Func.Inl != nil && !isIntrinsicCall(n) { // normal case
-			n = mkinlcall(n, n.Left)
+			n = mkinlcall(n, n.Left, maxCost)
 		} else if n.Left.isMethodExpression() && asNode(n.Left.Sym.Def) != nil {
-			n = mkinlcall(n, asNode(n.Left.Sym.Def))
+			n = mkinlcall(n, asNode(n.Left.Sym.Def), maxCost)
 		} else if n.Left.Op == OCLOSURE {
 			if f := inlinableClosure(n.Left); f != nil {
-				n = mkinlcall(n, f)
+				n = mkinlcall(n, f, maxCost)
 			}
 		} else if n.Left.Op == ONAME && n.Left.Name != nil && n.Left.Name.Defn != nil {
 			if d := n.Left.Name.Defn; d.Op == OAS && d.Right.Op == OCLOSURE {
@@ -668,7 +697,7 @@ func inlnode(n *Node) *Node {
 						}
 						break
 					}
-					n = mkinlcall(n, f)
+					n = mkinlcall(n, f, maxCost)
 				}
 			}
 		}
@@ -687,7 +716,7 @@ func inlnode(n *Node) *Node {
 			Fatalf("no function definition for [%p] %+v\n", n.Left.Type, n.Left.Type)
 		}
 
-		n = mkinlcall(n, asNode(n.Left.Type.FuncType().Nname))
+		n = mkinlcall(n, asNode(n.Left.Type.FuncType().Nname), maxCost)
 	}
 
 	lineno = lno
@@ -788,7 +817,7 @@ func (v *reassignVisitor) visitList(l Nodes) *Node {
 
 // The result of mkinlcall MUST be assigned back to n, e.g.
 // 	n.Left = mkinlcall(n.Left, fn, isddd)
-func mkinlcall(n *Node, fn *Node) *Node {
+func mkinlcall(n *Node, fn *Node, maxCost int32) *Node {
 	save_safemode := safemode
 
 	// imported functions may refer to unsafe as long as the
@@ -798,7 +827,7 @@ func mkinlcall(n *Node, fn *Node) *Node {
 	if pkg != localpkg && pkg != nil {
 		safemode = false
 	}
-	n = mkinlcall1(n, fn)
+	n = mkinlcall1(n, fn, maxCost)
 	safemode = save_safemode
 	return n
 }
@@ -824,11 +853,16 @@ var inlgen int
 // parameters.
 // The result of mkinlcall1 MUST be assigned back to n, e.g.
 // 	n.Left = mkinlcall1(n.Left, fn, isddd)
-func mkinlcall1(n, fn *Node) *Node {
+func mkinlcall1(n, fn *Node, maxCost int32) *Node {
 	if fn.Func.Inl == nil {
 		// No inlinable body.
 		return n
 	}
+	if fn.Func.Inl.Cost > maxCost {
+		// The inlined function body is too big. Typically we use this check to restrict
+		// inlining into very big functions.  See issue 26546 and 17566.
+		return n
+	}
 
 	if fn == Curfn || fn.Name.Defn == Curfn {
 		// Can't recursively inline a function into itself.
@@ -1094,7 +1128,7 @@ func mkinlcall1(n, fn *Node) *Node {
 	// instead we emit the things that the body needs
 	// and each use must redo the inlining.
 	// luckily these are small.
-	inlnodelist(call.Nbody)
+	inlnodelist(call.Nbody, maxCost)
 	for _, n := range call.Nbody.Slice() {
 		if n.Op == OINLCALL {
 			inlconv2stmt(n)