runtime: make convTstring write barrier unreachable from throw

CL 581215 changed 'throw' so that instead of print(s) it called
a more complicated function, printpanicval, that statically
appeared to have convTstring in its call graph, even though this
isn't dynamically reachable when called with a string argument.

However, this caused the link-time static callgraph test to point
out that throw (which is called in nowritebarrierrec contexts
such as markgc) reaches a write barrier.

The solution is to inline and specialize the printpanicval
function for strings; it reduces to printindented.

Thanks to mpratt for pointing out that the reachability
check is on the fully lowered code, and is thus sensitive
to optimizations such as inlining.
I added an explanatory comment on the line that generates
the error message to help future users confused as I was.

Fixes golang/go#67274

Change-Id: Ief110d554de365ce4c09509dceee000cbee30ad9
Reviewed-on: https://go-review.googlesource.com/c/go/+/584617
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

diff --git a/src/cmd/compile/internal/ssagen/nowb.go b/src/cmd/compile/internal/ssagen/nowb.go
index b8756eea61092ba436b3ef56509ad38393818946..8e776695e3cef5db8e623ee44af51af33e2ff34a 100644 (file)
--- a/src/cmd/compile/internal/ssagen/nowb.go
+++ b/src/cmd/compile/internal/ssagen/nowb.go
@@ -174,6 +174,14 @@ func (c *nowritebarrierrecChecker) check() {
                                fmt.Fprintf(&err, "\n\t%v: called by %v", base.FmtPos(call.lineno), call.target.Nname)
                                call = funcs[call.target]
                        }
+                       // Seeing this error in a failed CI run? It indicates that
+                       // a function in the runtime package marked nowritebarrierrec
+                       // (the outermost stack element) was found, by a static
+                       // reachability analysis over the fully lowered optimized code,
+                       // to call a function (fn) that involves a write barrier.
+                       //
+                       // Even if the call path is infeasable,
+                       // you will need to reorganize the code to avoid it.
                        base.ErrorfAt(fn.WBPos, 0, "write barrier prohibited by caller; %v%s", fn.Nname, err.String())
                        continue
                }

diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index 122fc30df26fa94f16ec2c91adef3a9548d2e0bb..ff9c64113fafeac5fac772f8a631359b97b19def 100644 (file)
--- a/src/runtime/panic.go
+++ b/src/runtime/panic.go
@@ -1014,13 +1014,12 @@ func sync_fatal(s string) {
 // issue #67274, so as to fix longtest builders.
 //
 //go:nosplit
-//go:noinline
 func throw(s string) {
        // Everything throw does should be recursively nosplit so it
        // can be called even when it's unsafe to grow the stack.
        systemstack(func() {
                print("fatal error: ")
-               printpanicval(s)
+               printindented(s) // logically printpanicval(s), but avoids convTstring write barrier
                print("\n")
        })
 
@@ -1041,7 +1040,7 @@ func fatal(s string) {
        // can be called even when it's unsafe to grow the stack.
        systemstack(func() {
                print("fatal error: ")
-               printpanicval(s)
+               printindented(s) // logically printpanicval(s), but avoids convTstring write barrier
                print("\n")
        })