cmd/compile: remove legacy debug flags

-M, -P, and -R were for debugging backend passes that no longer
exists.

-g is used for debugging instructions generated with Gins, but the SSA
backend mostly generates instructions directly. The handful of
instructions still generated with Gins are pretty useless for
debugging.

-x was used to debug the old lexer, but now it only causes us to print
file names as they're parsed, and only if we manually hack the
compiler to enable tracing.

Change-Id: Ia58d4bc9c1312693466171a3fcefc1221e9a2381
Reviewed-on: https://go-review.googlesource.com/32428
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -30,10 +30,7 @@
 
 package gc
 
-import (
-	"cmd/internal/obj"
-	"fmt"
-)
+import "cmd/internal/obj"
 
 func Prog(as obj.As) *obj.Prog {
 	var p *obj.Prog
@@ -314,9 +311,5 @@ func Gins(as obj.As, f, t *Node) *obj.Prog {
 	p := Prog(as)
 	Naddr(&p.From, f)
 	Naddr(&p.To, t)
-
-	if Debug['g'] != 0 {
-		fmt.Printf("%v\n", p)
-	}
 	return p
 }

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

@@ -155,10 +155,7 @@ func Main() {
 	obj.Flagcount("E", "debug symbol export", &Debug['E'])
 	obj.Flagfn1("I", "add `directory` to import search path", addidir)
 	obj.Flagcount("K", "debug missing line numbers", &Debug['K'])
-	obj.Flagcount("M", "debug move generation", &Debug['M'])
 	obj.Flagcount("N", "disable optimizations", &Debug['N'])
-	obj.Flagcount("P", "debug peephole optimizer", &Debug['P'])
-	obj.Flagcount("R", "debug register optimizer", &Debug['R'])
 	obj.Flagcount("S", "print assembly listing", &Debug['S'])
 	obj.Flagfn0("V", "print compiler version", doversion)
 	obj.Flagcount("W", "debug parse tree after type checking", &Debug['W'])
@@ -168,7 +165,6 @@ func Main() {
 	flag.StringVar(&debugstr, "d", "", "print debug information about items in `list`")
 	obj.Flagcount("e", "no limit on number of errors reported", &Debug['e'])
 	obj.Flagcount("f", "debug stack frames", &Debug['f'])
-	obj.Flagcount("g", "debug code generation", &Debug['g'])
 	obj.Flagcount("h", "halt on error", &Debug['h'])
 	obj.Flagcount("i", "debug line number stack", &Debug['i'])
 	obj.Flagfn1("importmap", "add `definition` of the form source=actual to import map", addImportMap)
@@ -191,7 +187,6 @@ func Main() {
 	obj.Flagcount("v", "increase debug verbosity", &Debug['v'])
 	obj.Flagcount("w", "debug type checking", &Debug['w'])
 	flag.BoolVar(&use_writebarrier, "wb", true, "enable write barrier")
-	obj.Flagcount("x", "debug lexer", &Debug['x'])
 	var flag_shared bool
 	var flag_dynlink bool
 	if supportsDynlink(Thearch.LinkArch.Arch) {
@@ -309,10 +304,6 @@ func Main() {
 	timings.Start("fe", "parse")
 	lexlineno0 := lexlineno
 	for _, infile = range flag.Args() {
-		if trace && Debug['x'] != 0 {
-			fmt.Printf("--- %s ---\n", infile)
-		}
-
 		linehistpush(infile)
 		block = 1
 		iota_ = -1000000

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

@@ -12,8 +12,6 @@ package gc
 // Semicolons are inserted by the lexer. The parser uses one-token look-ahead
 // to handle optional commas and semicolons before a closing ) or } .
 
-const trace = false // if set, parse tracing can be enabled with -x
-
 func mkname(sym *Sym) *Node {
 	n := oldname(sym)
 	if n.Name != nil && n.Name.Pack != nil {