cmd/compile: include all entries in map literal hint size

Currently we only include static entries in the hint for sizing
the map when allocating a map for a map literal. Change that to
include all entries.

This will be an overallocation if the dynamic entries in the map have
equal keys, but equal keys in map literals are rare, and at worst we
waste a bit of space.

Fixes #43020

Change-Id: I232f82f15316bdf4ea6d657d25a0b094b77884ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/383634
Run-TryBot: Keith Randall <khr@golang.org>
Trust: Keith Randall <khr@golang.org>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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 {

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

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
 	}
 

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
+}