cmd/compile: eliminate stores to unread auto variables

This is a crude compiler pass to eliminate stores to auto variables
that are only ever written to.

Eliminates an unnecessary store to x from the following code:

func f() int {
	var x := 1
	return *(&x)
}

Fixes #19765.

Change-Id: If2c63a8ae67b8c590b6e0cc98a9610939a3eeffa
Reviewed-on: https://go-review.googlesource.com/38746
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -973,13 +973,23 @@ var linuxAMD64Tests = []*asmTest{
 		// make sure assembly output has matching offset and base register.
 		`
 		func f72(a, b int) int {
-			var x [16]byte // use some frame
+			//go:noinline
-			_ = x
+			func() {_, _ = a, b} () // use some frame
 			return b
 		}
 		`,
 		[]string{"b\\+40\\(SP\\)"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]0-8"},
+	},
 }
 
 var linux386Tests = []*asmTest{
@@ -1015,6 +1025,16 @@ var linux386Tests = []*asmTest{
 		}`,
 		[]string{"\tADDL\t[$]19", "\tIMULL"}, // (n+19)*a
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]0-4"},
+	},
 }
 
 var linuxS390XTests = []*asmTest{
@@ -1293,6 +1313,16 @@ var linuxS390XTests = []*asmTest{
 		`,
 		[]string{"\tFLOGR\t"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]0-8"},
+	},
 }
 
 var linuxARMTests = []*asmTest{
@@ -1404,13 +1434,23 @@ var linuxARMTests = []*asmTest{
 		// make sure assembly output has matching offset and base register.
 		`
 		func f13(a, b int) int {
-			var x [16]byte // use some frame
+			//go:noinline
-			_ = x
+			func() {_, _ = a, b} () // use some frame
 			return b
 		}
 		`,
 		[]string{"b\\+4\\(FP\\)"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]-4-4"},
+	},
 }
 
 var linuxARM64Tests = []*asmTest{
@@ -1584,6 +1624,16 @@ var linuxARM64Tests = []*asmTest{
 		`,
 		[]string{"\tMOVD\t\"\"\\.a\\+[0-9]+\\(FP\\), R[0-9]+", "\tMOVD\tR[0-9]+, \"\"\\.b\\+[0-9]+\\(FP\\)"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]-8-8"},
+	},
 }
 
 var linuxMIPSTests = []*asmTest{
@@ -1667,6 +1717,16 @@ var linuxMIPSTests = []*asmTest{
 		`,
 		[]string{"\tCLZ\t"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]-4-4"},
+	},
 }
 
 var linuxPPC64LETests = []*asmTest{
@@ -1751,6 +1811,16 @@ var linuxPPC64LETests = []*asmTest{
 		`,
 		[]string{"\tROTL\t"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]0-8"},
+	},
 }
 
 // TestLineNumber checks to make sure the generated assembly has line numbers

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

@@ -291,7 +291,14 @@ func affectedNode(v *ssa.Value) (*Node, ssa.SymEffect) {
 		return n, ssa.SymWrite
 
 	case ssa.OpVarLive:
-		return v.Aux.(*Node), ssa.SymRead
+		switch a := v.Aux.(type) {
+		case *ssa.ArgSymbol:
+			return a.Node.(*Node), ssa.SymRead
+		case *ssa.AutoSymbol:
+			return a.Node.(*Node), ssa.SymRead
+		default:
+			Fatalf("unknown VarLive aux type: %s", v.LongString())
+		}
 	case ssa.OpVarDef, ssa.OpVarKill:
 		return v.Aux.(*Node), ssa.SymWrite
 	case ssa.OpKeepAlive:

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

@@ -936,7 +936,16 @@ func (s *state) stmt(n *Node) {
 		if !n.Left.Addrtaken() {
 			s.Fatalf("VARLIVE variable %v must have Addrtaken set", n.Left)
 		}
-		s.vars[&memVar] = s.newValue1A(ssa.OpVarLive, types.TypeMem, n.Left, s.mem())
+		var aux interface{}
+		switch n.Left.Class() {
+		case PAUTO:
+			aux = s.lookupSymbol(n.Left, &ssa.AutoSymbol{Node: n.Left})
+		case PPARAM, PPARAMOUT:
+			aux = s.lookupSymbol(n.Left, &ssa.ArgSymbol{Node: n.Left})
+		default:
+			s.Fatalf("VARLIVE variable %v must be Auto or Arg", n.Left)
+		}
+		s.vars[&memVar] = s.newValue1A(ssa.OpVarLive, types.TypeMem, aux, s.mem())
 
 	case OCHECKNIL:
 		p := s.expr(n.Left)

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

@@ -356,6 +356,7 @@ var passes = [...]pass{
 	{name: "tighten", fn: tighten}, // move values closer to their uses
 	{name: "lower", fn: lower, required: true},
 	{name: "lowered cse", fn: cse},
+	{name: "elim unread autos", fn: elimUnreadAutos},
 	{name: "lowered deadcode", fn: deadcode, required: true},
 	{name: "checkLower", fn: checkLower, required: true},
 	{name: "late phielim", fn: phielim},

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

@@ -131,3 +131,55 @@ func dse(f *Func) {
 		}
 	}
 }
+
+// elimUnreadAutos deletes stores to autos that are never read from.
+func elimUnreadAutos(f *Func) {
+	// Loop over all ops that affect autos taking note of which
+	// autos we need and also stores that we might be able to
+	// eliminate.
+	seen := make(map[GCNode]bool)
+	var stores []*Value
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			var sym *AutoSymbol
+			sym, ok := v.Aux.(*AutoSymbol)
+			if !ok {
+				continue
+			}
+
+			effect := v.Op.SymEffect()
+			switch effect {
+			case SymWrite:
+				// If we haven't seen the auto yet
+				// then this might be a store we can
+				// eliminate.
+				if !seen[sym.Node] {
+					stores = append(stores, v)
+				}
+			default:
+				// Assume the auto is needed (loaded,
+				// has its address taken, etc.).
+				// Note we have to check the uses
+				// because dead loads haven't been
+				// eliminated yet.
+				if v.Uses > 0 {
+					seen[sym.Node] = true
+				}
+			}
+		}
+	}
+
+	// Eliminate stores to unread autos.
+	for _, store := range stores {
+		sym, _ := store.Aux.(*AutoSymbol)
+		if seen[sym.Node] {
+			continue
+		}
+
+		// replace store with OpCopy
+		store.SetArgs1(store.MemoryArg())
+		store.Aux = nil
+		store.AuxInt = 0
+		store.Op = OpCopy
+	}
+}

src/runtime/runtime-gdb_test.go

@@ -84,7 +84,7 @@ func main() {
 	slicevar := make([]string, 0, 16)
 	slicevar = append(slicevar, mapvar["abc"])
 	fmt.Println("hi") // line 13
-	_ = ptrvar
+	runtime.KeepAlive(ptrvar)
 	gslice = slicevar
 	runtime.KeepAlive(mapvar)
 }

test/live.go

@@ -239,15 +239,6 @@ func f14() {
 
 func g14() string
 
-func f15() {
-	var x string
-	_ = &x
-	x = g15()      // ERROR "live at call to g15: x$"
-	printstring(x) // ERROR "live at call to printstring: x$"
-}
-
-func g15() string
-
 // Checking that various temporaries do not persist or cause
 // ambiguously live values that must be zeroed.
 // The exact temporary names are inconsequential but we are
@@ -384,10 +375,9 @@ func f25(b bool) {
 		return
 	}
 	var x string
-	_ = &x
+	x = g14()
-	x = g15()      // ERROR "live at call to g15: x$"
+	printstring(x)
-	printstring(x) // ERROR "live at call to printstring: x$"
+}
-} // ERROR "live at call to deferreturn: x$"
 
 func g25()
 
@@ -641,6 +631,9 @@ type T40 struct {
 	m map[int]int
 }
 
+//go:noescape
+func useT40(*T40)
+
 func newT40() *T40 {
 	ret := T40{}
 	ret.m = make(map[int]int) // ERROR "live at call to makemap: &ret$"
@@ -658,7 +651,7 @@ func good40() {
 	ret.m = make(map[int]int) // ERROR "live at call to makemap: .autotmp_[0-9]+ ret$"
 	t := &ret
 	printnl() // ERROR "live at call to printnl: .autotmp_[0-9]+ ret$"
-	_ = t
+	useT40(t) // ERROR "live at call to useT40: .autotmp_[0-9]+ ret$"
 }
 
 func ddd1(x, y *int) { // ERROR "live at entry to ddd1: x y$"

test/live2.go

@@ -14,6 +14,9 @@ package main
 
 func printnl()
 
+//go:noescape
+func useT40(*T40)
+
 type T40 struct {
 	m map[int]int
 }
@@ -27,7 +30,7 @@ func newT40() *T40 {
 func bad40() {
 	t := newT40() // ERROR "live at call to makemap: .autotmp_[0-9]+ ret$"
 	printnl()     // ERROR "live at call to printnl: .autotmp_[0-9]+ ret$"
-	_ = t
+	useT40(t)     // ERROR "live at call to useT40: .autotmp_[0-9]+ ret$"
 }
 
 func good40() {
@@ -35,5 +38,5 @@ func good40() {
 	ret.m = make(map[int]int) // ERROR "live at call to makemap: .autotmp_[0-9]+ ret$"
 	t := &ret
 	printnl() // ERROR "live at call to printnl: .autotmp_[0-9]+ ret$"
-	_ = t
+	useT40(t) // ERROR "live at call to useT40: .autotmp_[0-9]+ ret$"
 }