[dev.ssa] cmd/compile: enable SSA on ARM by default

As Josh mentioned in CL 24716, there has been requests for using SSA
for ARM. SSA can still be disabled by setting -ssa=0 for cmd/compile,
or partially enabled with GOSSAFUNC, GOSSAPKG, and GOSSAHASH.

Not enable SSA by default on NaCl, which is not supported yet.

Enable SSA-specific tests on ARM: live_ssa.go and nilptr3_ssa.go;
disable non-SSA tests: live.go, nilptr3.go, and slicepot.go.

Updates #15365.

Change-Id: Ic2ca8d166aeca8517b9d262a55e92f2130683a16
Reviewed-on: https://go-review.googlesource.com/23953
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -37,8 +37,13 @@ func shouldssa(fn *Node) bool {
 		if os.Getenv("SSATEST") == "" {
 			return false
 		}
-		// Generally available.
+	case "arm":
+		// nacl/arm doesn't work yet
+		if obj.Getgoos() == "nacl" && os.Getenv("SSATEST") == "" {
+			return false
+		}
 	case "amd64":
+		// Generally available.
 	}
 	if !ssaEnabled {
 		return false

test/live.go

-// +build !amd64
+// +build !amd64,!arm nacl,arm
 // errorcheck -0 -l -live -wb=0
 
 // Copyright 2014 The Go Authors. All rights reserved.

test/live_ssa.go

-// +build amd64
+// +build amd64 arm,!nacl
 // errorcheck -0 -l -live -wb=0
 
 // Copyright 2014 The Go Authors. All rights reserved.

test/nilptr3.go

@@ -2,7 +2,7 @@
 // Fails on ppc64x because of incomplete optimization.
 // See issues 9058.
 // Same reason for mips64x and s390x.
-// +build !ppc64,!ppc64le,!mips64,!mips64le,!amd64,!s390x
+// +build !ppc64,!ppc64le,!mips64,!mips64le,!amd64,!s390x,!arm nacl,arm
 
 // Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style

test/nilptr3_ssa.go

 // errorcheck -0 -d=nil
-// Fails on ppc64x because of incomplete optimization.
-// See issues 9058.
-// +build !ppc64,!ppc64le,amd64
+// +build amd64 arm,!nacl
 
 // Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style

test/sliceopt.go

-// +build !amd64
+// +build !amd64,!arm nacl,arm
 // errorcheck -0 -d=append,slice
 
 // Copyright 2015 The Go Authors. All rights reserved.