runtime: symbolize morestack caller in throwsplit panic

This attempts to symbolize the PC of morestack's caller when there's a
stack split at a bad time. The stack trace starts at the *caller* of
the function that attempted to grow the stack, so this is useful if it
isn't obvious what's being called at that point, such as in #21431.

Change-Id: I5dee305d87c8069611de2d14e7a3083d76264f8f
Reviewed-on: https://go-review.googlesource.com/84115
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: aclements

src/runtime/stack.go

@@ -938,7 +938,14 @@ func newstack() {
 		morebuf := thisg.m.morebuf
 		gp.syscallsp = morebuf.sp
 		gp.syscallpc = morebuf.pc
-		print("runtime: newstack sp=", hex(gp.sched.sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n",
+		pcname, pcoff := "(unknown)", uintptr(0)
+		f := findfunc(gp.sched.pc)
+		if f.valid() {
+			pcname = funcname(f)
+			pcoff = gp.sched.pc - f.entry
+		}
+		print("runtime: newstack at ", pcname, "+", hex(pcoff),
+			" sp=", hex(gp.sched.sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n",
 			"\tmorebuf={pc:", hex(morebuf.pc), " sp:", hex(morebuf.sp), " lr:", hex(morebuf.lr), "}\n",
 			"\tsched={pc:", hex(gp.sched.pc), " sp:", hex(gp.sched.sp), " lr:", hex(gp.sched.lr), " ctxt:", gp.sched.ctxt, "}\n")