cmd/compile: strongly favor closure inlining

This tweaks the inlining cost knob for closures
specifically, they receive a doubled budget.  The
rationale for this is that closures have a lot of
"crud" in their IR that will disappear after inlining,
so the standard budget penalizes them unnecessarily.

This is also the cause of these bugs -- looking at the
code involved, these closures "should" be inlineable,
therefore tweak the parameters until behavior matches
expectations.  It's not costly in binary size, because
the only-called-from-one-site case is common (especially
for rangefunc iterators).

I can imagine better fixes and I am going to try to
get that done, but this one is small and makes things
better.

Fixes #69411, #69539.

Change-Id: I8a892c40323173a723799e0ddad69dcc2724a8f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/629195
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/cgo/internal/test/callback_windows.go

@@ -69,11 +69,11 @@ func testCallbackCallersSEH(t *testing.T) {
 	want := []string{
 		"test._Cfunc_backtrace",
 		"test.testCallbackCallersSEH.func1.1",
-		"test.testCallbackCallersSEH.func1",
+		// "test.testCallbackCallersSEH.func1", // hidden by inlining
 		"test.goCallback",
 		"test._Cfunc_callback",
 		"test.nestedCall.func1",
-		"test.nestedCall",
+		// "test.nestedCall", // hidden by inlining
 		"test.testCallbackCallersSEH",
 		"test.TestCallbackCallersSEH",
 	}
@@ -84,6 +84,7 @@ func testCallbackCallersSEH(t *testing.T) {
 	})
 	got := make([]string, 0, n)
 	for i := 0; i < n; i++ {
+		// This test is brittle in the face of inliner changes
 		f := runtime.FuncForPC(pc[i] - 1)
 		if f == nil {
 			continue

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

@@ -206,6 +206,9 @@ func inlineBudget(fn *ir.Func, profile *pgoir.Profile, relaxed bool, verbose boo
 	if relaxed {
 		budget += inlheur.BudgetExpansion(inlineMaxBudget)
 	}
+	if fn.ClosureParent != nil {
+		budget *= 2
+	}
 	return budget
 }
 
@@ -861,6 +864,10 @@ var InlineCall = func(callerfn *ir.Func, call *ir.CallExpr, fn *ir.Func, inlInde
 //   - whether the inlined function is "hot" according to PGO.
 func inlineCostOK(n *ir.CallExpr, caller, callee *ir.Func, bigCaller bool) (bool, int32, int32, bool) {
 	maxCost := int32(inlineMaxBudget)
+	if callee.ClosureParent != nil {
+		maxCost *= 2 // favor inlining closures
+	}
+
 	if bigCaller {
 		// We use this to restrict inlining into very big functions.
 		// See issue 26546 and 17566.

test/closure3.dir/main.go

@@ -221,14 +221,14 @@ func main() {
 
 	{
 		b := 2
-		func(b int) { // ERROR "func literal does not escape"
+		func(b int) { // ERROR "can inline main.func25"
-			func() { // ERROR "can inline main.func25.1"
+			func() { // ERROR "can inline main.func25.1" "can inline main.main.func25.func33"
 				b = 3
 			}() // ERROR "inlining call to main.func25.1"
 			if b != 3 {
 				ppanic("b != 3")
 			}
-		}(b)
+		}(b) // ERROR "inlining call to main.func25" "inlining call to main.main.func25.func33"
 		if b != 2 {
 			ppanic("b != 2")
 		}
@@ -258,13 +258,13 @@ func main() {
 		// revisit those. E.g., func34 and func36 are constructed by the inliner.
 		if r := func(x int) int { // ERROR "can inline main.func27"
 			b := 3
-			return func(y int) int { // ERROR "can inline main.func27.1" "can inline main.main.func27.func34"
+			return func(y int) int { // ERROR "can inline main.func27.1" "can inline main.main.func27.func35"
 				c := 5
-				return func(z int) int { // ERROR "can inline main.func27.1.1" "can inline main.main.func27.func34.1" "can inline main.func27.main.func27.1.2" "can inline main.main.func27.main.main.func27.func34.func36"
+				return func(z int) int { // ERROR "can inline main.func27.1.1" "can inline main.main.func27.func35.1" "can inline main.func27.main.func27.1.2" "can inline main.main.func27.main.main.func27.func35.func37"
 					return a*x + b*y + c*z
 				}(10) // ERROR "inlining call to main.func27.1.1"
 			}(100) // ERROR "inlining call to main.func27.1" "inlining call to main.func27.main.func27.1.2"
-		}(1000); r != 2350 { // ERROR "inlining call to main.func27" "inlining call to main.main.func27.func34" "inlining call to main.main.func27.main.main.func27.func34.func36"
+		}(1000); r != 2350 { // ERROR "inlining call to main.func27" "inlining call to main.main.func27.func35" "inlining call to main.main.func27.main.main.func27.func35.func37"
 			ppanic("r != 2350")
 		}
 	}
@@ -273,16 +273,16 @@ func main() {
 		a := 2
 		if r := func(x int) int { // ERROR "can inline main.func28"
 			b := 3
-			return func(y int) int { // ERROR "can inline main.func28.1" "can inline main.main.func28.func35"
+			return func(y int) int { // ERROR "can inline main.func28.1" "can inline main.main.func28.func36"
 				c := 5
-				func(z int) { // ERROR "can inline main.func28.1.1" "can inline main.func28.main.func28.1.2" "can inline main.main.func28.func35.1" "can inline main.main.func28.main.main.func28.func35.func37"
+				func(z int) { // ERROR "can inline main.func28.1.1" "can inline main.func28.main.func28.1.2" "can inline main.main.func28.func36.1" "can inline main.main.func28.main.main.func28.func36.func38"
 					a = a * x
 					b = b * y
 					c = c * z
 				}(10) // ERROR "inlining call to main.func28.1.1"
 				return a + c
 			}(100) + b // ERROR "inlining call to main.func28.1" "inlining call to main.func28.main.func28.1.2"
-		}(1000); r != 2350 { // ERROR "inlining call to main.func28" "inlining call to main.main.func28.func35" "inlining call to main.main.func28.main.main.func28.func35.func37"
+		}(1000); r != 2350 { // ERROR "inlining call to main.func28" "inlining call to main.main.func28.func36" "inlining call to main.main.func28.main.main.func28.func36.func38"
 			ppanic("r != 2350")
 		}
 		if a != 2000 {