cmd/compile: avoid extra mapaccess in "m[k] op= r"

Currently, order desugars map assignment operations like

    m[k] op= r

into

    m[k] = m[k] op r

which in turn is transformed during walk into:

    tmp := *mapaccess(m, k)
    tmp = tmp op r
    *mapassign(m, k) = tmp

However, this is suboptimal, as we could instead produce just:

    *mapassign(m, k) op= r

One complication though is if "r == 0", then "m[k] /= r" and "m[k] %=
r" will panic, and they need to do so *before* calling mapassign,
otherwise we may insert a new zero-value element into the map.

It would be spec compliant to just emit the "r != 0" check before
calling mapassign (see #23735), but currently these checks aren't
generated until SSA construction. For now, it's simpler to continue
desugaring /= and %= into two map indexing operations.

Fixes #23661.

Change-Id: I46e3739d9adef10e92b46fdd78b88d5aabe68952
Reviewed-on: https://go-review.googlesource.com/91557
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

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

@@ -14,7 +14,7 @@ import (
 // order of evaluation. Makes walk easier, because it
 // can (after this runs) reorder at will within an expression.
 //
-// Rewrite x op= y into x = x op y.
+// Rewrite m[k] op= r into m[k] = m[k] op r if op is / or %.
 //
 // Introduce temporaries as needed by runtime routines.
 // For example, the map runtime routines take the map key
@@ -434,7 +434,7 @@ func (o *Order) mapAssign(n *Node) {
 	default:
 		Fatalf("ordermapassign %v", n.Op)
 
-	case OAS:
+	case OAS, OASOP:
 		if n.Left.Op == OINDEXMAP {
 			// Make sure we evaluate the RHS before starting the map insert.
 			// We need to make sure the RHS won't panic.  See issue 22881.
@@ -514,26 +514,31 @@ func (o *Order) stmt(n *Node) {
 		o.cleanTemp(t)
 
 	case OASOP:
-		// Special: rewrite l op= r into l = l op r.
-		// This simplifies quite a few operations;
-		// most important is that it lets us separate
-		// out map read from map write when l is
-		// a map index expression.
 		t := o.markTemp()
 		n.Left = o.expr(n.Left, nil)
 		n.Right = o.expr(n.Right, nil)
 
+		if instrumenting || n.Left.Op == OINDEXMAP && (n.SubOp() == ODIV || n.SubOp() == OMOD) {
+			// Rewrite m[k] op= r into m[k] = m[k] op r so
+			// that we can ensure that if op panics
+			// because r is zero, the panic happens before
+			// the map assignment.
+
 			n.Left = o.safeExpr(n.Left)
-		tmp1 := treecopy(n.Left, src.NoXPos)
-		if tmp1.Op == OINDEXMAP {
-			tmp1.SetIndexMapLValue(false)
+
+			l := treecopy(n.Left, src.NoXPos)
+			if l.Op == OINDEXMAP {
+				l.SetIndexMapLValue(false)
 			}
-		tmp1 = o.copyExpr(tmp1, n.Left.Type, false)
-		n.Right = nod(n.SubOp(), tmp1, n.Right)
+			l = o.copyExpr(l, n.Left.Type, false)
+			n.Right = nod(n.SubOp(), l, n.Right)
 			n.Right = typecheck(n.Right, Erv)
 			n.Right = o.expr(n.Right, nil)
+
 			n.Op = OAS
 			n.ResetAux()
+		}
+
 		o.mapAssign(n)
 		o.cleanTemp(t)
 

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

@@ -667,12 +667,21 @@ opswitch:
 		updateHasCall(n.Left)
 		n.List.Set(reorder1(ll))
 
-	case OAS:
+	case OAS, OASOP:
 		init.AppendNodes(&n.Ninit)
 
 		n.Left = walkexpr(n.Left, init)
 		n.Left = safeexpr(n.Left, init)
 
+		if n.Op == OASOP {
+			// Rewrite x op= y into x = x op y.
+			n.Right = nod(n.SubOp(), n.Left, n.Right)
+			n.Right = typecheck(n.Right, Erv)
+
+			n.Op = OAS
+			n.ResetAux()
+		}
+
 		if oaslit(n, init) {
 			break
 		}

src/runtime/map_test.go

@@ -52,14 +52,7 @@ func TestNegativeZero(t *testing.T) {
 	}
 }
 
-// nan is a good test because nan != nan, and nan has
-// a randomized hash value.
-func TestNan(t *testing.T) {
-	m := make(map[float64]int, 0)
-	nan := math.NaN()
-	m[nan] = 1
-	m[nan] = 2
-	m[nan] = 4
+func testMapNan(t *testing.T, m map[float64]int) {
 	if len(m) != 3 {
 		t.Error("length wrong")
 	}
@@ -78,6 +71,49 @@ func TestNan(t *testing.T) {
 	}
 }
 
+// nan is a good test because nan != nan, and nan has
+// a randomized hash value.
+func TestMapAssignmentNan(t *testing.T) {
+	m := make(map[float64]int, 0)
+	nan := math.NaN()
+
+	// Test assignment.
+	m[nan] = 1
+	m[nan] = 2
+	m[nan] = 4
+	testMapNan(t, m)
+}
+
+// nan is a good test because nan != nan, and nan has
+// a randomized hash value.
+func TestMapOperatorAssignmentNan(t *testing.T) {
+	m := make(map[float64]int, 0)
+	nan := math.NaN()
+
+	// Test assignment operations.
+	m[nan] += 1
+	m[nan] += 2
+	m[nan] += 4
+	testMapNan(t, m)
+}
+
+func TestMapOperatorAssignment(t *testing.T) {
+	m := make(map[int]int, 0)
+
+	// "m[k] op= x" is rewritten into "m[k] = m[k] op x"
+	// differently when op is / or % than when it isn't.
+	// Simple test to make sure they all work as expected.
+	m[0] = 12345
+	m[0] += 67890
+	m[0] /= 123
+	m[0] %= 456
+
+	const want = (12345 + 67890) / 123 % 456
+	if got := m[0]; got != want {
+		t.Errorf("got %d, want %d", got, want)
+	}
+}
+
 // Maps aren't actually copied on assignment.
 func TestAlias(t *testing.T) {
 	m := make(map[int]int, 0)
@@ -92,18 +128,25 @@ func TestAlias(t *testing.T) {
 func TestGrowWithNaN(t *testing.T) {
 	m := make(map[float64]int, 4)
 	nan := math.NaN()
+
+	// Use both assignment and assignment operations as they may
+	// behave differently.
 	m[nan] = 1
 	m[nan] = 2
-	m[nan] = 4
+	m[nan] += 4
+
 	cnt := 0
 	s := 0
 	growflag := true
 	for k, v := range m {
 		if growflag {
 			// force a hashtable resize
-			for i := 0; i < 100; i++ {
+			for i := 0; i < 50; i++ {
 				m[float64(i)] = i
 			}
+			for i := 50; i < 100; i++ {
+				m[float64(i)] += i
+			}
 			growflag = false
 		}
 		if k != k {
@@ -128,8 +171,8 @@ func TestGrowWithNegativeZero(t *testing.T) {
 	negzero := math.Copysign(0.0, -1.0)
 	m := make(map[FloatInt]int, 4)
 	m[FloatInt{0.0, 0}] = 1
-	m[FloatInt{0.0, 1}] = 2
-	m[FloatInt{0.0, 2}] = 4
+	m[FloatInt{0.0, 1}] += 2
+	m[FloatInt{0.0, 2}] += 4
 	m[FloatInt{0.0, 3}] = 8
 	growflag := true
 	s := 0
@@ -211,9 +254,12 @@ func TestIterGrowAndDelete(t *testing.T) {
 // an iterator is still using them.
 func TestIterGrowWithGC(t *testing.T) {
 	m := make(map[int]int, 4)
-	for i := 0; i < 16; i++ {
+	for i := 0; i < 8; i++ {
 		m[i] = i
 	}
+	for i := 8; i < 16; i++ {
+		m[i] += i
+	}
 	growflag := true
 	bitmask := 0
 	for k := range m {
@@ -786,6 +832,13 @@ func benchmarkMapAssignInt32(b *testing.B, n int) {
 	}
 }
 
+func benchmarkMapOperatorAssignInt32(b *testing.B, n int) {
+	a := make(map[int32]int)
+	for i := 0; i < b.N; i++ {
+		a[int32(i&(n-1))] += i
+	}
+}
+
 func benchmarkMapDeleteInt32(b *testing.B, n int) {
 	a := make(map[int32]int, n)
 	b.ResetTimer()
@@ -808,6 +861,13 @@ func benchmarkMapAssignInt64(b *testing.B, n int) {
 	}
 }
 
+func benchmarkMapOperatorAssignInt64(b *testing.B, n int) {
+	a := make(map[int64]int)
+	for i := 0; i < b.N; i++ {
+		a[int64(i&(n-1))] += i
+	}
+}
+
 func benchmarkMapDeleteInt64(b *testing.B, n int) {
 	a := make(map[int64]int, n)
 	b.ResetTimer()
@@ -835,6 +895,19 @@ func benchmarkMapAssignStr(b *testing.B, n int) {
 	}
 }
 
+func benchmarkMapOperatorAssignStr(b *testing.B, n int) {
+	k := make([]string, n)
+	for i := 0; i < len(k); i++ {
+		k[i] = strconv.Itoa(i)
+	}
+	b.ResetTimer()
+	a := make(map[string]string)
+	for i := 0; i < b.N; i++ {
+		key := k[i&(n-1)]
+		a[key] += key
+	}
+}
+
 func benchmarkMapDeleteStr(b *testing.B, n int) {
 	i2s := make([]string, n)
 	for i := 0; i < n; i++ {
@@ -870,6 +943,12 @@ func BenchmarkMapAssign(b *testing.B) {
 	b.Run("Str", runWith(benchmarkMapAssignStr, 1<<8, 1<<16))
 }
 
+func BenchmarkMapOperatorAssign(b *testing.B) {
+	b.Run("Int32", runWith(benchmarkMapOperatorAssignInt32, 1<<8, 1<<16))
+	b.Run("Int64", runWith(benchmarkMapOperatorAssignInt64, 1<<8, 1<<16))
+	b.Run("Str", runWith(benchmarkMapOperatorAssignStr, 1<<8, 1<<16))
+}
+
 func BenchmarkMapDelete(b *testing.B) {
 	b.Run("Int32", runWith(benchmarkMapDeleteInt32, 100, 1000, 10000))
 	b.Run("Int64", runWith(benchmarkMapDeleteInt64, 100, 1000, 10000))

test/fixedbugs/issue19359.go

@@ -28,10 +28,38 @@ func del(m map[interface{}]interface{}, key interface{}) (err error) {
 	return nil
 }
 
+func addInt(m map[interface{}]int, key interface{}) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("addInt failed: %v", r)
+		}
+	}()
+	m[key] += 2018
+	return nil
+}
+
+func addStr(m map[interface{}]string, key interface{}) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("addStr failed: %v", r)
+		}
+	}()
+	m[key] += "hello, go"
+	return nil
+}
+
 func main() {
 	m := make(map[interface{}]interface{})
 	set(m, []int{1, 2, 3})
 	set(m, "abc") // used to throw
 	del(m, []int{1, 2, 3})
 	del(m, "abc") // used to throw
+
+	mi := make(map[interface{}]int)
+	addInt(mi, []int{1, 2, 3})
+	addInt(mi, "abc") // used to throw
+
+	ms := make(map[interface{}]string)
+	addStr(ms, []int{1, 2, 3})
+	addStr(ms, "abc") // used to throw
 }

test/fixedbugs/issue22881.go

@@ -13,7 +13,7 @@ import "fmt"
 
 func main() {
 	for i, f := range []func(map[int]int){
-		f0, f1, f2, f3, f4, f5, f6, f7,
+		f0, f1, f2, f3, f4, f5, f6, f7, f8,
 	} {
 		m := map[int]int{}
 		func() { // wrapper to scope the defer.
@@ -69,4 +69,9 @@ func f7(m map[int]int) {
 	m[0] = a[0]
 }
 
+func f8(m map[int]int) {
+	var z int
+	m[0] %= z
+}
+
 var sink bool