cmd/compile: use constants directly for fast map access calls

CL 35554 taught order.go to use static variables
for constants that needed to be addressable for runtime routines.
However, there is one class of runtime routines that
do not actually need an addressable value: fast map access routines.
This CL teaches order.go to avoid using static variables
for addressability in those cases.
Instead, it avoids introducing a temp at all,
which the backend would just have to optimize away.

Fixes #19015.

Change-Id: I5ef780c604fac3fb48dabb23a344435e283cb832
Reviewed-on: https://go-review.googlesource.com/36693
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -440,6 +440,38 @@ func f(t *T) {
 `,
 		[]string{"\tROLL\t[$]7,"},
 	},
+
+	// Direct use of constants in fast map access calls. Issue 19015.
+	{"amd64", "linux", `
+	func f(m map[int]int) int {
+		return m[5]
+	}
+`,
+		[]string{"\tMOVQ\t[$]5,"},
+	},
+	{"amd64", "linux", `
+	func f(m map[int]int) bool {
+		_, ok := m[5]
+		return ok
+	}
+`,
+		[]string{"\tMOVQ\t[$]5,"},
+	},
+	{"amd64", "linux", `
+	func f(m map[string]int) int {
+		return m["abc"]
+	}
+`,
+		[]string{"\"abc\""},
+	},
+	{"amd64", "linux", `
+	func f(m map[string]int) bool {
+		_, ok := m["abc"]
+		return ok
+	}
+`,
+		[]string{"\"abc\""},
+	},
 }
 
 // mergeEnvLists merges the two environment lists such that

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

@@ -206,6 +206,18 @@ func orderaddrtemp(n *Node, order *Order) *Node {
 	return ordercopyexpr(n, n.Type, order, 0)
 }
 
+// ordermapkeytemp prepares n.Right to be a key in a map lookup.
+func ordermapkeytemp(n *Node, order *Order) {
+	// Most map calls need to take the address of the key.
+	// Exception: mapaccessN_fast* calls. See golang.org/issue/19015.
+	p, _ := mapaccessfast(n.Left.Type)
+	fastaccess := p != "" && n.Etype == 0 // Etype == 0 iff n is an rvalue
+	if fastaccess {
+		return
+	}
+	n.Right = orderaddrtemp(n.Right, order)
+}
+
 type ordermarker int
 
 // Marktemp returns the top of the temporary variable stack.
@@ -527,7 +539,7 @@ func orderstmt(n *Node, order *Order) {
 		ordermapassign(n, order)
 		cleantemp(t, order)
 
-	// Special: make sure key is addressable,
+	// Special: make sure key is addressable if needed,
 	// and make sure OINDEXMAP is not copied out.
 	case OAS2MAPR:
 		t := marktemp(order)
@@ -541,7 +553,7 @@ func orderstmt(n *Node, order *Order) {
 		if r.Right.Op == OARRAYBYTESTR {
 			r.Right.Op = OARRAYBYTESTRTMP
 		}
-		r.Right = orderaddrtemp(r.Right, order)
+		ordermapkeytemp(r, order)
 		ordermapassign(n, order)
 		cleantemp(t, order)
 
@@ -1074,9 +1086,7 @@ func orderexpr(n *Node, order *Order, lhs *Node) *Node {
 			needCopy = true
 		}
 
-		// Map calls need to take the address of the key.
-		n.Right = orderaddrtemp(n.Right, order)
-
+		ordermapkeytemp(n, order)
 		if needCopy {
 			n = ordercopyexpr(n, n.Type, order, 0)
 		}

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

@@ -791,18 +791,8 @@ opswitch:
 		r.Left = walkexpr(r.Left, init)
 		r.Right = walkexpr(r.Right, init)
 		t := r.Left.Type
-		p := ""
-		if t.Val().Width <= 128 { // Check ../../runtime/hashmap.go:maxValueSize before changing.
-			switch algtype(t.Key()) {
-			case AMEM32:
-				p = "mapaccess2_fast32"
-			case AMEM64:
-				p = "mapaccess2_fast64"
-			case ASTRING:
-				p = "mapaccess2_faststr"
-			}
-		}
 
+		_, p := mapaccessfast(t)
 		var key *Node
 		if p != "" {
 			// fast versions take key by value
@@ -811,7 +801,6 @@ opswitch:
 			// standard version takes key by reference
 			// orderexpr made sure key is addressable.
 			key = nod(OADDR, r.Right, nil)
-
 			p = "mapaccess2"
 		}
 
@@ -1173,18 +1162,7 @@ opswitch:
 			n = mkcall1(mapfn("mapassign", t), nil, init, typename(t), map_, key)
 		} else {
 			// m[k] is not the target of an assignment.
-			p := ""
-			if t.Val().Width <= 128 { // Check ../../runtime/hashmap.go:maxValueSize before changing.
-				switch algtype(t.Key()) {
-				case AMEM32:
-					p = "mapaccess1_fast32"
-				case AMEM64:
-					p = "mapaccess1_fast64"
-				case ASTRING:
-					p = "mapaccess1_faststr"
-				}
-			}
-
+			p, _ := mapaccessfast(t)
 			if p == "" {
 				// standard version takes key by reference.
 				// orderexpr made sure key is addressable.
@@ -2700,6 +2678,23 @@ func mapfndel(name string, t *Type) *Node {
 	return fn
 }
 
+// mapaccessfast returns the names of the fast map access runtime routines for t.
+func mapaccessfast(t *Type) (access1, access2 string) {
+	// Check ../../runtime/hashmap.go:maxValueSize before changing.
+	if t.Val().Width > 128 {
+		return "", ""
+	}
+	switch algtype(t.Key()) {
+	case AMEM32:
+		return "mapaccess1_fast32", "mapaccess2_fast32"
+	case AMEM64:
+		return "mapaccess1_fast64", "mapaccess2_fast64"
+	case ASTRING:
+		return "mapaccess1_faststr", "mapaccess2_faststr"
+	}
+	return "", ""
+}
+
 func writebarrierfn(name string, l *Type, r *Type) *Node {
 	fn := syslook(name)
 	fn = substArgTypes(fn, l, r)