cmd/compile: compile len(ch) as call to runtime.chanlen

An upcoming CL will give this call more to do.
For now, separate out the compiler change that
stops inlining the computation.

Change-Id: I4c5cbd84a0694b306191bff38cc6ea2d69458d7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/564556
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: rsc

src/cmd/compile/internal/ssagen/ssa.go

@@ -6349,6 +6349,9 @@ func (s *state) referenceTypeBuiltin(n *ir.UnaryExpr, x *ssa.Value) *ssa.Value {
 	if !n.X.Type().IsMap() && !n.X.Type().IsChan() {
 		s.Fatalf("node must be a map or a channel")
 	}
+	if n.X.Type().IsChan() && n.Op() == ir.OLEN {
+		s.Fatalf("cannot inline len(chan)") // must use runtime.chanlen now
+	}
 	// if n == nil {
 	//   return 0
 	// } else {

src/cmd/compile/internal/typecheck/_builtin/coverage.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // NOTE: If you change this file you must run "go generate"
+// in cmd/compile/internal/typecheck
 // to update builtin.go. This is not done automatically
 // to avoid depending on having a working compiler binary.
 

src/cmd/compile/internal/typecheck/_builtin/runtime.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // NOTE: If you change this file you must run "go generate"
+// in cmd/compile/internal/typecheck
 // to update builtin.go. This is not done automatically
 // to avoid depending on having a working compiler binary.
 
@@ -158,7 +159,8 @@ func makechan(chanType *byte, size int) (hchan chan any)
 func chanrecv1(hchan <-chan any, elem *any)
 func chanrecv2(hchan <-chan any, elem *any) bool
 func chansend1(hchan chan<- any, elem *any)
-func closechan(hchan any)
+func closechan(hchan chan<- any)
+func chanlen(hchan any) int
 
 var writeBarrier struct {
 	enabled bool

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

@@ -153,9 +153,7 @@ func walkClear(n *ir.UnaryExpr) ir.Node {
 
 // walkClose walks an OCLOSE node.
 func walkClose(n *ir.UnaryExpr, init *ir.Nodes) ir.Node {
-	// cannot use chanfn - closechan takes any, not chan any
-	fn := typecheck.LookupRuntime("closechan", n.X.Type())
-	return mkcall1(fn, nil, init, n.X)
+	return mkcall1(chanfn("closechan", 1, n.X.Type()), nil, init, n.X)
 }
 
 // Lower copy(a, b) to a memmove call or a runtime call.
@@ -263,6 +261,12 @@ func walkLenCap(n *ir.UnaryExpr, init *ir.Nodes) ir.Node {
 		_, len := backingArrayPtrLen(cheapExpr(conv.X, init))
 		return len
 	}
+	if isChanLen(n) {
+		// cannot use chanfn - closechan takes any, not chan any,
+		// because it accepts both send-only and recv-only channels.
+		fn := typecheck.LookupRuntime("chanlen", n.X.Type())
+		return mkcall1(fn, n.Type(), init, n.X)
+	}
 
 	n.X = walkExpr(n.X, init)
 
@@ -887,3 +891,10 @@ func isByteCount(n ir.Node) bool {
 	return base.Flag.N == 0 && !base.Flag.Cfg.Instrumenting && n.Op() == ir.OLEN &&
 		(n.(*ir.UnaryExpr).X.Op() == ir.OBYTES2STR || n.(*ir.UnaryExpr).X.Op() == ir.OBYTES2STRTMP)
 }
+
+// isChanLen reports whether n is of the form len(c) for a channel c.
+// Note that this does not check for -n or instrumenting because this
+// is a correctness rewrite, not an optimization.
+func isChanLen(n ir.Node) bool {
+	return n.Op() == ir.OLEN && n.(*ir.UnaryExpr).X.Type().IsChan()
+}

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

@@ -724,20 +724,21 @@ func reflect_chanrecv(c *hchan, nb bool, elem unsafe.Pointer) (selected bool, re
 	return chanrecv(c, elem, !nb)
 }
 
-//go:linkname reflect_chanlen reflect.chanlen
-func reflect_chanlen(c *hchan) int {
+func chanlen(c *hchan) int {
 	if c == nil {
 		return 0
 	}
 	return int(c.qcount)
 }
 
+//go:linkname reflect_chanlen reflect.chanlen
+func reflect_chanlen(c *hchan) int {
+	return chanlen(c)
+}
+
 //go:linkname reflectlite_chanlen internal/reflectlite.chanlen
 func reflectlite_chanlen(c *hchan) int {
-	if c == nil {
-		return 0
-	}
-	return int(c.qcount)
+	return chanlen(c)
 }
 
 //go:linkname reflect_chancap reflect.chancap