cmd/compile,runtime: update instrumentation comments

The compiler's instrumentation pass has some out-of-date comments
about the write barrier and some confusing comments about
typedslicecopy. Update these comments and add a comment to
typedslicecopy explaining why it's manually instrumented while none of
the other operations are.

Change-Id: I024e5361d53f1c3c122db0c85155368a30cabd6b
Reviewed-on: https://go-review.googlesource.com/74430Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -170,10 +170,14 @@ func instrumentnode(np **Node, init *Nodes, wr int, skip int) {
 	case OCALLINTER:
 		instrumentnode(&n.Left, init, 0, 0)
 
-	// Instrument dst argument of runtime.writebarrier* calls
-	// as we do not instrument runtime code.
-	// typedslicecopy is instrumented in runtime.
 	case OCALLFUNC:
+		// Note that runtime.typedslicecopy is the only
+		// assignment-like function call in the AST at this
+		// point (between walk and SSA); since we don't
+		// instrument it here, typedslicecopy is manually
+		// instrumented in runtime. Calls to the write barrier
+		// and typedmemmove are created later by SSA, so those
+		// still appear as OAS nodes at this point.
 		instrumentnode(&n.Left, init, 0, 0)
 
 	case ONOT,

src/runtime/mbarrier.go

@@ -326,6 +326,10 @@ func typedslicecopy(typ *_type, dst, src slice) int {
 	dstp := dst.array
 	srcp := src.array
 
+	// The compiler emits calls to typedslicecopy before
+	// instrumentation runs, so unlike the other copying and
+	// assignment operations, it's not instrumented in the calling
+	// code and needs its own instrumentation.
 	if raceenabled {
 		callerpc := getcallerpc()
 		pc := funcPC(slicecopy)