runtime: split PCs out of scase

Per-case PCs are only needed for race detector builds, so this allows
skipping allocating stack space for them for non-race builds.

It's possible to arrange the PCs and order arrays consecutively in
memory so that we could just reuse the order0 pointer to identify
both. However, there's more risk of that silently going wrong, so this
commit passes them as separate arguments for now. We can revisit this
in the future.

Updates #40410.

Change-Id: I8468bc25749e559891cb0cb007d1cc4a40fdd0f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/245124
Reviewed-by: Keith Randall <khr@golang.org>

Author: mdempsky

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

@@ -169,8 +169,8 @@ func selectnbsend(hchan chan<- any, elem *any) bool
 func selectnbrecv(elem *any, hchan <-chan any) bool
 func selectnbrecv2(elem *any, received *bool, hchan <-chan any) bool
 
-func selectsetpc(cas *byte)
-func selectgo(cas0 *byte, order0 *byte, ncases int) (int, bool)
+func selectsetpc(pc *uintptr)
+func selectgo(cas0 *byte, order0 *byte, pc0 *uintptr, ncases int) (int, bool)
 func block()
 
 func makeslice(typ *byte, len int, cap int) unsafe.Pointer

src/cmd/compile/internal/gc/builtin/runtime.go

@@ -271,6 +271,14 @@ func walkselectcases(cases *Nodes) []*Node {
 	r = typecheck(r, ctxStmt)
 	init = append(init, r)
 
+	var pc0, pcs *Node
+	if flag_race {
+		pcs = temp(types.NewArray(types.Types[TUINTPTR], int64(n)))
+		pc0 = typecheck(nod(OADDR, nod(OINDEX, pcs, nodintconst(0)), nil), ctxExpr)
+	} else {
+		pc0 = nodnil()
+	}
+
 	// register cases
 	for i, cas := range cases.Slice() {
 		setlineno(cas)
@@ -324,8 +332,8 @@ func walkselectcases(cases *Nodes) []*Node {
 
 		// TODO(mdempsky): There should be a cleaner way to
 		// handle this.
-		if instrumenting {
-			r = mkcall("selectsetpc", nil, nil, bytePtrToIndex(selv, int64(i)))
+		if flag_race {
+			r = mkcall("selectsetpc", nil, nil, nod(OADDR, nod(OINDEX, pcs, nodintconst(int64(i))), nil))
 			init = append(init, r)
 		}
 	}
@@ -337,13 +345,16 @@ func walkselectcases(cases *Nodes) []*Node {
 	r = nod(OAS2, nil, nil)
 	r.List.Set2(chosen, recvOK)
 	fn := syslook("selectgo")
-	r.Rlist.Set1(mkcall1(fn, fn.Type.Results(), nil, bytePtrToIndex(selv, 0), bytePtrToIndex(order, 0), nodintconst(int64(n))))
+	r.Rlist.Set1(mkcall1(fn, fn.Type.Results(), nil, bytePtrToIndex(selv, 0), bytePtrToIndex(order, 0), pc0, nodintconst(int64(n))))
 	r = typecheck(r, ctxStmt)
 	init = append(init, r)
 
 	// selv and order are no longer alive after selectgo.
 	init = append(init, nod(OVARKILL, selv, nil))
 	init = append(init, nod(OVARKILL, order, nil))
+	if flag_race {
+		init = append(init, nod(OVARKILL, pcs, nil))
+	}
 
 	// dispatch cases
 	for i, cas := range cases.Slice() {
@@ -385,7 +396,6 @@ func scasetype() *types.Type {
 			namedfield("c", types.Types[TUNSAFEPTR]),
 			namedfield("elem", types.Types[TUNSAFEPTR]),
 			namedfield("kind", types.Types[TUINT16]),
-			namedfield("pc", types.Types[TUINTPTR]),
 		})
 		scase.SetNoalg(true)
 	}