cmd/compile: add go:nowritebarrierrec annotation

This introduces a recursive variant of the go:nowritebarrier
annotation that prohibits write barriers not only in the annotated
function, but in all functions it calls, recursively. The error
message gives the shortest call stack from the annotated function to
the function containing the prohibited write barrier, including the
names of the functions and the line numbers of the calls.

To demonstrate the annotation, we apply it to gcmarkwb_m, the write
barrier itself.

This is a new annotation rather than a modification of the existing
go:nowritebarrier annotation because, for better or worse, there are
many go:nowritebarrier functions that do call functions with write
barriers. In most of these cases this is benign because the annotation
was conservative, but it prohibits simply coopting the existing
annotation.

Change-Id: I225ca483c8f699e8436373ed96349e80ca2c2479
Reviewed-on: https://go-review.googlesource.com/16554
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -169,14 +169,17 @@ type Func struct {
 
 	Endlineno int32
 
-	Norace         bool // func must not have race detector annotations
-	Nosplit        bool // func should not execute on separate stack
-	Noinline       bool // func should not be inlined
-	Nowritebarrier bool // emit compiler error instead of write barrier
-	Dupok          bool // duplicate definitions ok
-	Wrapper        bool // is method wrapper
-	Needctxt       bool // function uses context register (has closure variables)
-	Systemstack    bool // must run on system stack
+	Norace            bool // func must not have race detector annotations
+	Nosplit           bool // func should not execute on separate stack
+	Noinline          bool // func should not be inlined
+	Nowritebarrier    bool // emit compiler error instead of write barrier
+	Nowritebarrierrec bool // error on write barrier in this or recursive callees
+	Dupok             bool // duplicate definitions ok
+	Wrapper           bool // is method wrapper
+	Needctxt          bool // function uses context register (has closure variables)
+	Systemstack       bool // must run on system stack
+
+	WBLineno int32 // line number of first write barrier
 }
 
 type Op uint8