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>

1 files changed, 2 insertions, 3 deletions

diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index 122fc30df2..ff9c64113f 100644
--- 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")
 	})