4 files changed, 44 insertions, 2 deletions
diff --git a/src/cmd/compile/internal/ir/expr.go b/src/cmd/compile/internal/ir/expr.go
index 68303c0581..156fe96493 100644
--- a/src/cmd/compile/internal/ir/expr.go
+++ b/src/cmd/compile/internal/ir/expr.go
@@ -202,7 +202,10 @@ type CompLitExpr struct {
 	Ntype    Ntype
 	List     Nodes // initialized values
 	Prealloc *Name
-	Len      int64 // backing array length for OSLICELIT
+	// For OSLICELIT, Len is the backing array length.
+	// For OMAPLIT, Len is the number of entries that we've removed from List and
+	// generated explicit mapassign calls for. This is used to inform the map alloc hint.
+	Len int64
 }
 
 func NewCompLitExpr(pos src.XPos, op Op, typ Ntype, list []Node) *CompLitExpr {
diff --git a/src/cmd/compile/internal/walk/complit.go b/src/cmd/compile/internal/walk/complit.go
index b985b4caeb..e46f828d65 100644
--- a/src/cmd/compile/internal/walk/complit.go
+++ b/src/cmd/compile/internal/walk/complit.go
@@ -419,7 +419,7 @@ func maplit(n *ir.CompLitExpr, m ir.Node, init *ir.Nodes) {
 	// make the map var
 	a := ir.NewCallExpr(base.Pos, ir.OMAKE, nil, nil)
 	a.SetEsc(n.Esc())
-	a.Args = []ir.Node{ir.TypeNode(n.Type()), ir.NewInt(int64(len(n.List)))}
+	a.Args = []ir.Node{ir.TypeNode(n.Type()), ir.NewInt(n.Len + int64(len(n.List)))}
 	litas(m, a, init)
 
 	entries := n.List
diff --git a/src/cmd/compile/internal/walk/order.go b/src/cmd/compile/internal/walk/order.go
index 861c122456..cc37f95764 100644
--- a/src/cmd/compile/internal/walk/order.go
+++ b/src/cmd/compile/internal/walk/order.go
@@ -1433,6 +1433,15 @@ func (o *orderState) expr1(n, lhs ir.Node) ir.Node {
 			typecheck.Stmt(as) // Note: this converts the OINDEX to an OINDEXMAP
 			o.stmt(as)
 		}
+
+		// Remember that we issued these assignments so we can include that count
+		// in the map alloc hint.
+		// We're assuming here that all the keys in the map literal are distinct.
+		// If any are equal, this will be an overcount. Probably not worth accounting
+		// for that, as equal keys in map literals are rare, and at worst we waste
+		// a bit of space.
+		n.Len += int64(len(dynamics))
+
 		return m
 	}
 
diff --git a/test/codegen/maps.go b/test/codegen/maps.go
index dcb4a9381f..ea3a70d1f0 100644
--- a/test/codegen/maps.go
+++ b/test/codegen/maps.go
@@ -122,3 +122,33 @@ func MapClearSideEffect(m map[int]int) int {
 	}
 	return k
 }
+
+func MapLiteralSizing(x int) (map[int]int, map[int]int) {
+	// amd64:"MOVL\t[$]10,"
+	m := map[int]int{
+		0: 0,
+		1: 1,
+		2: 2,
+		3: 3,
+		4: 4,
+		5: 5,
+		6: 6,
+		7: 7,
+		8: 8,
+		9: 9,
+	}
+	// amd64:"MOVL\t[$]10,"
+	n := map[int]int{
+		0: x,
+		1: x,
+		2: x,
+		3: x,
+		4: x,
+		5: x,
+		6: x,
+		7: x,
+		8: x,
+		9: x,
+	}
+	return m, n
+}