cmd/compile: add "deadlocals" pass to remove unused locals

This CL adds a "deadlocals" pass, which runs after inlining and before
escape analysis, to prune any unneeded local variables and
assignments. In particular, this helps avoid unnecessary Addrtaken
markings from unreachable closures.

Deadlocals is sensitive to "_ = ..." as a signal of explicit
use for testing.  This signal occurs only if the entire
left-hand-side is "_" targets; if it is
  `_, ok := someInlinedFunc(args)`
then the first return value is eligible for dead code elimination.

Use this (`_ = x`) to fix tests broken by deadlocals elimination.

Includes a test, based on one of the tests that required modification.

Matthew Dempsky wrote this, changing ownership to allow rebases, commits, tweaks.

Fixes #65158.

Old-Change-Id: I723fb69ccd7baadaae04d415702ce6c8901eaf4e
Change-Id: I1f25f4293b19527f305c18c3680b214237a7714c
Reviewed-on: https://go-review.googlesource.com/c/go/+/600498
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: David Chase <drchase@google.com>
Commit-Queue: David Chase <drchase@google.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/src/runtime/race/testdata/mop_test.go b/src/runtime/race/testdata/mop_test.go
index 6b1069fcca..0d7d879d83 100644
--- a/src/runtime/race/testdata/mop_test.go
+++ b/src/runtime/race/testdata/mop_test.go
@@ -612,6 +612,8 @@ func TestNoRaceEnoughRegisters(t *testing.T) {
 }
 
 // emptyFunc should not be inlined.
+//
+//go:noinline
 func emptyFunc(x int) {
 	if false {
 		fmt.Println(x)
@@ -1176,7 +1178,7 @@ func TestNoRaceHeapReallocation(t *testing.T) {
 	// others.
 	const n = 2
 	done := make(chan bool, n)
-	empty := func(p *int) {}
+	empty := func(p *int) { _ = p }
 	for i := 0; i < n; i++ {
 		ms := i
 		go func() {
@@ -1417,7 +1419,7 @@ func TestRaceInterCall2(t *testing.T) {
 
 func TestRaceFuncCall(t *testing.T) {
 	c := make(chan bool, 1)
-	f := func(x, y int) {}
+	f := func(x, y int) { _ = y }
 	x, y := 0, 0
 	go func() {
 		y = 42
@@ -1804,6 +1806,7 @@ func TestRaceAsFunc2(t *testing.T) {
 	x := 0
 	go func() {
 		func(x int) {
+			_ = x
 		}(x)
 		c <- true
 	}()
@@ -1817,6 +1820,7 @@ func TestRaceAsFunc3(t *testing.T) {
 	x := 0
 	go func() {
 		func(x int) {
+			_ = x
 			mu.Lock()
 		}(x) // Read of x must be outside of the mutex.
 		mu.Unlock()