cmd/gofmt: only resolve go/ast objects when needed

go/parser will by default resolve objects as per the go/ast.Object type,
which is then used by gofmt's rewrite and simplify flags.
However, none of that is needed if neither of the flags is set,
so we can avoid the work entirely for a nice speed-up.

	benchcmd -n 8 GofmtSrcCmd gofmt -l ~/tip/src/cmd

	name         old time/op         new time/op         delta
	GofmtSrcCmd          957ms ± 7%          908ms ± 7%  -5.12%  (p=0.028 n=8+8)

	name         old user-time/op    new user-time/op    delta
	GofmtSrcCmd          11.2s ± 1%          10.4s ± 1%  -7.23%  (p=0.001 n=7+7)

	name         old sys-time/op     new sys-time/op     delta
	GofmtSrcCmd          325ms ±29%          286ms ±22%    ~     (p=0.065 n=8+8)

	name         old peak-RSS-bytes  new peak-RSS-bytes  delta
	GofmtSrcCmd          295MB ±17%          276MB ±15%    ~     (p=0.328 n=8+8)

See #46485.

Change-Id: Iad1ae294953710c233f7837d7eb02e23d11c6185
Reviewed-on: https://go-review.googlesource.com/c/go/+/401454
Run-TryBot: Daniel Martí <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

src/cmd/gofmt/gofmt.go

@@ -76,6 +76,11 @@ func initParserMode() {
 	if *allErrors {
 		parserMode |= parser.AllErrors
 	}
+	// Both -r and -s make use of go/ast's object resolution.
+	// If neither is being used, avoid that unnecessary work.
+	if *rewriteRule == "" && !*simplifyAST {
+		parserMode |= parser.SkipObjectResolution
+	}
 }
 
 func isGoFile(f fs.DirEntry) bool {