cmd/compile: hide testdclstack behind debug flag

This reduces compilation time for the program
in #18602 from 7 hours to 30 min.

Updates #14781
Updates #18602

Change-Id: I3c4af878a08920e6373d3b3b0c4453ee002e32eb
Reviewed-on: https://go-review.googlesource.com/35113
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -303,7 +303,9 @@ func genhash(sym *Sym, t *Type) {
 	typecheckslice(fn.Nbody.Slice(), Etop)
 	Curfn = nil
 	popdcl()
-	testdclstack()
+	if debug_dclstack != 0 {
+		testdclstack()
+	}
 
 	// Disable safemode while compiling this code: the code we
 	// generate internally can refer to unsafe.Pointer.
@@ -493,7 +495,9 @@ func geneq(sym *Sym, t *Type) {
 	typecheckslice(fn.Nbody.Slice(), Etop)
 	Curfn = nil
 	popdcl()
-	testdclstack()
+	if debug_dclstack != 0 {
+		testdclstack()
+	}
 
 	// Disable safemode while compiling this code: the code we
 	// generate internally can refer to unsafe.Pointer.

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

@@ -217,7 +217,9 @@ func Import(in *bufio.Reader) {
 	typecheckok = tcok
 	resumecheckwidth()
 
-	testdclstack() // debugging only
+	if debug_dclstack != 0 {
+		testdclstack()
+	}
 }
 
 func formatErrorf(format string, args ...interface{}) {

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

@@ -30,11 +30,12 @@ var (
 )
 
 var (
-	Debug_append  int
-	Debug_closure int
-	Debug_panic   int
-	Debug_slice   int
-	Debug_wb      int
+	Debug_append   int
+	Debug_closure  int
+	debug_dclstack int
+	Debug_panic    int
+	Debug_slice    int
+	Debug_wb       int
 )
 
 // Debug arguments.
@@ -48,6 +49,7 @@ var debugtab = []struct {
 	{"append", &Debug_append},         // print information about append compilation
 	{"closure", &Debug_closure},       // print information about closure compilation
 	{"disablenil", &disable_checknil}, // disable nil checks
+	{"dclstack", &debug_dclstack},     // run internal dclstack checks
 	{"gcprog", &Debug_gcprog},         // print dump of GC programs
 	{"nil", &Debug_checknil},          // print information about nil checks
 	{"panic", &Debug_panic},           // do not hide any compiler panic
@@ -325,7 +327,6 @@ func Main() {
 	timings.Stop()
 	timings.AddEvent(int64(lexlineno-lexlineno0), "lines")
 
-	testdclstack()
 	mkpackage(localpkg.Name) // final import not used checks
 	finishUniverse()
 

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

@@ -34,6 +34,7 @@ func parseFile(filename string) {
 	}
 
 	if nsyntaxerrors == 0 {
+		// Always run testdclstack here, even when debug_dclstack is not set, as a sanity measure.
 		testdclstack()
 	}
 }

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

@@ -1833,7 +1833,9 @@ func genwrapper(rcvr *Type, method *Field, newnam *Sym, iface int) {
 	funcbody(fn)
 	Curfn = fn
 	popdcl()
-	testdclstack()
+	if debug_dclstack != 0 {
+		testdclstack()
+	}
 
 	// wrappers where T is anonymous (struct or interface) can be duplicated.
 	if rcvr.IsStruct() || rcvr.IsInterface() || rcvr.IsPtr() && rcvr.Elem().IsStruct() {