cmd/compile: better dclcontext handling in func{hdr,body}

funchdr and funcbody currently assume that either (1) Curfn == nil &&
dclcontext == PEXTERN, or (2) Curfn != nil && dclcontext == PAUTO.
This is a reasonable assumption during parsing. However, these
functions end up getting used in other contexts, and not all callers
are so disciplined about Curfn/dclcontext handling.

This CL changes them to save/restore arbitrary Curfn/dclcontext pairs
instead. This is necessary for the followup CL, which pushes fninit
earlier. Otherwise, Curfn/dclcontext fall out of sync, and funchdr
panics.

Passes toolstash-check.

Updates golang#33485.

Change-Id: I19b1be23db1bad6475345ae5c81bbdc66291a3a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/254838
Run-TryBot: Matthew Dempsky <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Trust: Matthew Dempsky <[email protected]>

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

@@ -382,14 +382,11 @@ func ifacedcl(n *Node) {
 // returns in auto-declaration context.
 func funchdr(n *Node) {
 	// change the declaration context from extern to auto
-	if Curfn == nil && dclcontext != PEXTERN {
-		Fatalf("funchdr: dclcontext = %d", dclcontext)
-	}
-
+	funcStack = append(funcStack, funcStackEnt{Curfn, dclcontext})
+	Curfn = n
 	dclcontext = PAUTO
+
 	types.Markdcl()
-	funcstack = append(funcstack, Curfn)
-	Curfn = n
 
 	if n.Func.Nname != nil {
 		funcargs(n.Func.Nname.Name.Param.Ntype)
@@ -497,21 +494,22 @@ func funcarg2(f *types.Field, ctxt Class) {
 	declare(n, ctxt)
 }
 
-var funcstack []*Node // stack of previous values of Curfn
+var funcStack []funcStackEnt // stack of previous values of Curfn/dclcontext
+
+type funcStackEnt struct {
+	curfn      *Node
+	dclcontext Class
+}
 
 // finish the body.
 // called in auto-declaration context.
 // returns in extern-declaration context.
 func funcbody() {
-	// change the declaration context from auto to extern
-	if dclcontext != PAUTO {
-		Fatalf("funcbody: unexpected dclcontext %d", dclcontext)
-	}
+	// change the declaration context from auto to previous context
 	types.Popdcl()
-	funcstack, Curfn = funcstack[:len(funcstack)-1], funcstack[len(funcstack)-1]
-	if Curfn == nil {
-		dclcontext = PEXTERN
-	}
+	var e funcStackEnt
+	funcStack, e = funcStack[:len(funcStack)-1], funcStack[len(funcStack)-1]
+	Curfn, dclcontext = e.curfn, e.dclcontext
 }
 
 // structs, functions, and methods.

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

@@ -809,6 +809,9 @@ func Main(archInit func(*Arch)) {
 		}
 	}
 
+	if len(funcStack) != 0 {
+		Fatalf("funcStack is non-empty: %v", len(funcStack))
+	}
 	if len(compilequeue) != 0 {
 		Fatalf("%d uncompiled functions", len(compilequeue))
 	}