cmd/gc: eliminate dead code in switch statements
Ordinary switch statements are rewritten
into a sequence of if statements.
Staticly dead cases were not being eliminated
because the rewrite introduced a temporary,
which hid the fact that the case was a constant.
Stop doing that.
This eliminates dead code in the standard library at:
runtime/cgocall.go:219
runtime/cgocall.go:269
debug/gosym/pclntab.go:175
debug/macho/file.go:208
math/big/nat.go:635
math/big/nat.go:850
math/big/nat.go:1058
cmd/pprof/internal/commands/commands.go:86
net/sock_bsd.go:19
cmd/go/build.go:2657
cmd/go/env.go:90
Fixes #9608.
Change-Id: Ic23a05dfbb1ad91d5f62a6506b35a13e51b33e38
Reviewed-on: https://go-review.googlesource.com/3980
Reviewed-by: Keith Randall <khr@golang.org>
diff --git a/src/cmd/gc/swt.c b/src/cmd/gc/swt.c
index ca5455d..e75971d 100644
--- a/src/cmd/gc/swt.c
+++ b/src/cmd/gc/swt.c
@@ -503,7 +503,7 @@
/*
* normal (expression) switch.
- * rebulid case statements into if .. goto
+ * rebuild case statements into if .. goto
*/
static void
exprswitch(Node *sw)
@@ -533,12 +533,15 @@
*/
exprname = N;
cas = nil;
- if(arg != Strue && arg != Sfalse) {
+ if(arg == Strue || arg == Sfalse)
+ exprname = nodbool(arg == Strue);
+ else if(consttype(sw->ntest) >= 0)
+ // leave constants to enable dead code elimination (issue 9608)
+ exprname = sw->ntest;
+ else {
exprname = temp(sw->ntest->type);
cas = list1(nod(OAS, exprname, sw->ntest));
typechecklist(cas, Etop);
- } else {
- exprname = nodbool(arg == Strue);
}
c0 = mkcaselist(sw, arg);
diff --git a/test/fixedbugs/issue9608.dir/issue9608.go b/test/fixedbugs/issue9608.dir/issue9608.go
new file mode 100644
index 0000000..56b52cc
--- /dev/null
+++ b/test/fixedbugs/issue9608.dir/issue9608.go
@@ -0,0 +1,73 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func fail() // unimplemented, to test dead code elimination
+
+// Test dead code elimination in if statements
+func init() {
+ if false {
+ fail()
+ }
+ if 0 == 1 {
+ fail()
+ }
+}
+
+// Test dead code elimination in ordinary switch statements
+func init() {
+ const x = 0
+ switch x {
+ case 1:
+ fail()
+ }
+
+ switch 1 {
+ case x:
+ fail()
+ }
+
+ switch {
+ case false:
+ fail()
+ }
+
+ const a = "a"
+ switch a {
+ case "b":
+ fail()
+ }
+
+ const snowman = '☃'
+ switch snowman {
+ case '☀':
+ fail()
+ }
+
+ const zero = float64(0.0)
+ const one = float64(1.0)
+ switch one {
+ case -1.0:
+ fail()
+ case zero:
+ fail()
+ }
+
+ switch 1.0i {
+ case 1:
+ fail()
+ case -1i:
+ fail()
+ }
+
+ const no = false
+ switch no {
+ case true:
+ fail()
+ }
+}
+
+func main() {
+}
diff --git a/test/fixedbugs/issue9608.go b/test/fixedbugs/issue9608.go
new file mode 100644
index 0000000..92592d7
--- /dev/null
+++ b/test/fixedbugs/issue9608.go
@@ -0,0 +1,14 @@
+// rundir
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 9608: dead code elimination in switch statements.
+
+// This has to be done as a package rather than as a file,
+// because run.go runs files with 'go run', which passes the
+// -complete flag to compiler, causing it to complain about
+// the intentionally unimplemented function fail.
+
+package ignored
