cmd/compile: refactor maplit

Instead of walking the list of nodes twice,
once to find static entries to add to an array
and once to find dynamic entries to generate code for,
do the split once up front, into two slices.
Then process each slice individually.
This makes the code easier to read
and more importantly, easier to modify.

While we're here, add a TODO to avoid
using temporaries for mapassign_fast calls.
It's not an important TODO;
the generated code would be basically identical.
It would just avoid a minor amount of
pointless SSA optimization work.

Passes toolstash-check.
No measureable compiler performance impact.

Updates #19751

Change-Id: I84a8f2c22f9025c718ef34639059d7bd02a3c406
Reviewed-on: https://go-review.googlesource.com/39351
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -924,30 +924,28 @@ func slicelit(ctxt initContext, n *Node, var_ *Node, init *Nodes) {
 
 func maplit(n *Node, m *Node, init *Nodes) {
 	// make the map var
-	nerr := nerrors
-
 	a := nod(OMAKE, nil, nil)
 	a.List.Set2(typenod(n.Type), nodintconst(int64(n.List.Len())))
 	litas(m, a, init)
 
-	// count the initializers
-	b := 0
+	// Split the initializers into static and dynamic.
+	var stat, dyn []*Node
 	for _, r := range n.List.Slice() {
 		if r.Op != OKEY {
 			Fatalf("maplit: rhs not OKEY: %v", r)
 		}
-		index := r.Left
-		value := r.Right
-
-		if isliteral(index) && isliteral(value) {
-			b++
+		if isliteral(r.Left) && isliteral(r.Right) {
+			stat = append(stat, r)
+		} else {
+			dyn = append(dyn, r)
 		}
 	}
 
-	if b != 0 {
+	// Add static entries.
+	if len(stat) > 0 {
 		// build types [count]Tindex and [count]Tvalue
-		tk := typArray(n.Type.Key(), int64(b))
-		tv := typArray(n.Type.Val(), int64(b))
+		tk := typArray(n.Type.Key(), int64(len(stat)))
+		tv := typArray(n.Type.Val(), int64(len(stat)))
 
 		// TODO(josharian): suppress alg generation for these types?
 		dowidth(tk)
@@ -959,31 +957,23 @@ func maplit(n *Node, m *Node, init *Nodes) {
 		vstatv := staticname(tv)
 		vstatv.Name.SetReadonly(true)
 
-		b := int64(0)
-		for _, r := range n.List.Slice() {
-			if r.Op != OKEY {
-				Fatalf("maplit: rhs not OKEY: %v", r)
-			}
+		for i, r := range stat {
 			index := r.Left
 			value := r.Right
 
-			if isliteral(index) && isliteral(value) {
-				// build vstatk[b] = index
-				setlineno(index)
-				lhs := nod(OINDEX, vstatk, nodintconst(b))
-				as := nod(OAS, lhs, index)
-				as = typecheck(as, Etop)
-				genAsStatic(as)
-
-				// build vstatv[b] = value
-				setlineno(value)
-				lhs = nod(OINDEX, vstatv, nodintconst(b))
-				as = nod(OAS, lhs, value)
-				as = typecheck(as, Etop)
-				genAsStatic(as)
-
-				b++
-			}
+			// build vstatk[b] = index
+			setlineno(index)
+			lhs := nod(OINDEX, vstatk, nodintconst(int64(i)))
+			as := nod(OAS, lhs, index)
+			as = typecheck(as, Etop)
+			genAsStatic(as)
+
+			// build vstatv[b] = value
+			setlineno(value)
+			lhs = nod(OINDEX, vstatv, nodintconst(int64(i)))
+			as = nod(OAS, lhs, value)
+			as = typecheck(as, Etop)
+			genAsStatic(as)
 		}
 
 		// loop adding structure elements to map
@@ -1012,28 +1002,28 @@ func maplit(n *Node, m *Node, init *Nodes) {
 		init.Append(loop)
 	}
 
-	// put in dynamic entries one-at-a-time
-	var key, val *Node
-	for _, r := range n.List.Slice() {
-		if r.Op != OKEY {
-			Fatalf("maplit: rhs not OKEY: %v", r)
-		}
-		index := r.Left
-		value := r.Right
+	// Add dynamic entries.
+	addMapEntries(m, dyn, init)
+}
 
-		if isliteral(index) && isliteral(value) {
-			continue
-		}
+func addMapEntries(m *Node, dyn []*Node, init *Nodes) {
+	if len(dyn) == 0 {
+		return
+	}
 
-		// build list of var[c] = expr.
-		// use temporary so that mapassign1 can have addressable key, val.
-		if key == nil {
-			key = temp(m.Type.Key())
-			val = temp(m.Type.Val())
-		}
+	nerr := nerrors
+
+	// Build list of var[c] = expr.
+	// Use temporaries so that mapassign1 can have addressable key, val.
+	// TODO(josharian): avoid map key temporaries for mapfast_* assignments with literal keys.
+	key := temp(m.Type.Key())
+	val := temp(m.Type.Val())
+
+	for _, r := range dyn {
+		index, value := r.Left, r.Right
 
 		setlineno(index)
-		a = nod(OAS, key, index)
+		a := nod(OAS, key, index)
 		a = typecheck(a, Etop)
 		a = walkstmt(a)
 		init.Append(a)
@@ -1055,14 +1045,12 @@ func maplit(n *Node, m *Node, init *Nodes) {
 		}
 	}
 
-	if key != nil {
-		a = nod(OVARKILL, key, nil)
-		a = typecheck(a, Etop)
-		init.Append(a)
-		a = nod(OVARKILL, val, nil)
-		a = typecheck(a, Etop)
-		init.Append(a)
-	}
+	a := nod(OVARKILL, key, nil)
+	a = typecheck(a, Etop)
+	init.Append(a)
+	a = nod(OVARKILL, val, nil)
+	a = typecheck(a, Etop)
+	init.Append(a)
 }
 
 func anylit(n *Node, var_ *Node, init *Nodes) {