cmd/compile: avoid giant init functions due to many user inits

We generate code that calls each user init function one at a time.
When there are lots of user init functions,
usually due to generated code, like test/rotate* or
github.com/juju/govmomi/vim25/types,
we can end up with a giant function,
which can be slow to compile.

This CL puts in an escape valve.
When there are more than 500 functions, instead of doing:

init.0()
init.1()
// ...

we construct a static array of functions:

var fns = [...]func(){init.0, init.1, ... }

and call them in a loop.

This generates marginally bigger, marginally worse code,
so we restrict it to cases in which it might start to matter.

500 was selected as a mostly arbitrary threshold for "lots".
Each call uses two Progs, one for PCDATA and one for the call,
so at 500 calls we use ~1000 Progs.
At concurrency==8, we get a Prog cache of about
1000 Progs per worker.
So a threshold of 500 should more or less avoid
exhausting the Prog cache in most cases.

Change-Id: I276b887173ddbf65b2164ec9f9b5eb04d8c753c2
Reviewed-on: https://go-review.googlesource.com/41500
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -4,7 +4,9 @@
 
 package gc
 
-import "cmd/compile/internal/types"
+import (
+	"cmd/compile/internal/types"
+)
 
 // A function named init is a special case.
 // It is called by the initialization before main is run.
@@ -114,8 +116,9 @@ func fninit(n []*Node) {
 	// (6)
 	for _, s := range types.InitSyms {
 		if s.Def != nil && s != initsym {
-			// could check that it is fn of no args/returns
-			a = nod(OCALL, asNode(s.Def), nil)
+			n := asNode(s.Def)
+			n.checkInitFuncSignature()
+			a = nod(OCALL, n, nil)
 			r = append(r, a)
 		}
 	}
@@ -124,11 +127,63 @@ func fninit(n []*Node) {
 	r = append(r, nf...)
 
 	// (8)
-	// could check that it is fn of no args/returns
-	for i := 0; i < renameinitgen; i++ {
-		s := lookupN("init.", i)
-		a = nod(OCALL, asNode(s.Def), nil)
-		r = append(r, a)
+
+	// maxInlineInitCalls is the threshold at which we switch
+	// from generating calls inline to generating a static array
+	// of functions and calling them in a loop.
+	// See CL 41500 for more discussion.
+	const maxInlineInitCalls = 500
+
+	if renameinitgen < maxInlineInitCalls {
+		// Not many init functions. Just call them all directly.
+		for i := 0; i < renameinitgen; i++ {
+			s := lookupN("init.", i)
+			n := asNode(s.Def)
+			n.checkInitFuncSignature()
+			a = nod(OCALL, n, nil)
+			r = append(r, a)
+		}
+	} else {
+		// Lots of init functions.
+		// Set up an array of functions and loop to call them.
+		// This is faster to compile and similar at runtime.
+
+		// Build type [renameinitgen]func().
+		typ := types.NewArray(functype(nil, nil, nil), int64(renameinitgen))
+
+		// Make and fill array.
+		fnarr := staticname(typ)
+		fnarr.Name.SetReadonly(true)
+		for i := 0; i < renameinitgen; i++ {
+			s := lookupN("init.", i)
+			lhs := nod(OINDEX, fnarr, nodintconst(int64(i)))
+			rhs := asNode(s.Def)
+			rhs.checkInitFuncSignature()
+			as := nod(OAS, lhs, rhs)
+			as = typecheck(as, Etop)
+			genAsStatic(as)
+		}
+
+		// Generate a loop that calls each function in turn.
+		// for i := 0; i < renameinitgen; i++ {
+		//   fnarr[i]()
+		// }
+		i := temp(types.Types[TINT])
+		fnidx := nod(OINDEX, fnarr, i)
+		fnidx.SetBounded(true)
+
+		zero := nod(OAS, i, nodintconst(0))
+		cond := nod(OLT, i, nodintconst(int64(renameinitgen)))
+		incr := nod(OAS, i, nod(OADD, i, nodintconst(1)))
+		body := nod(OCALL, fnidx, nil)
+
+		loop := nod(OFOR, cond, incr)
+		loop.Nbody.Set1(body)
+		loop.Ninit.Set1(zero)
+
+		loop = typecheck(loop, Etop)
+		loop = walkstmt(loop)
+		r = append(r, loop)
 	}
 
 	// (9)
@@ -151,3 +206,10 @@ func fninit(n []*Node) {
 	Curfn = nil
 	funccompile(fn)
 }
+
+func (n *Node) checkInitFuncSignature() {
+	ft := n.Type.FuncType()
+	if ft.Receiver.Fields().Len()+ft.Params.Fields().Len()+ft.Results.Fields().Len() > 0 {
+		Fatalf("init function cannot have receiver, params, or results: %v (%v)", n, n.Type)
+	}
+}

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

@@ -1359,7 +1359,10 @@ func genAsStatic(as *Node) {
 		Fatalf("genAsStatic: lhs %v", as.Left)
 	}
 
-	if as.Right.Op != OLITERAL {
+	switch {
+	case as.Right.Op == OLITERAL:
+	case as.Right.Op == ONAME && as.Right.Class() == PFUNC:
+	default:
 		Fatalf("genAsStatic: rhs %v", as.Right)
 	}