From 6732e566321b828c83b563d24a10ac31c6d03e28 Mon Sep 17 00:00:00 2001
From: Jon Burgess <jburgess777@gmail.com>
Date: Sun, 28 Oct 2018 14:49:09 +0000
Subject: [PATCH] Avoid assert when label is an empty string

Calling back() on an empty string is not allowed and triggers
an assert with recent gcc:

$ cd manual/PRESENTATION_Intro
$ ../../yosys counter.ys
...
/usr/include/c++/8/bits/basic_string.h:1136: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::back() [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference = char&]: Assertion '!empty()' failed.

802             if (label.back() == ':' && GetSize(label) > 1)
(gdb) p label
$1 = ""
---
 kernel/yosys.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/yosys.cc b/kernel/yosys.cc
index 21eeadaa..f002955a 100644
--- a/kernel/yosys.cc
+++ b/kernel/yosys.cc
@@ -799,7 +799,7 @@ static void handle_label(std::string &command, bool &from_to_active, const std::
 	while (pos < GetSize(command) && command[pos] != ' ' && command[pos] != '\t' && command[pos] != '\r' && command[pos] != '\n')
 		label += command[pos++];
 
-	if (label.back() == ':' && GetSize(label) > 1)
+	if (GetSize(label) > 1 && label.back() == ':')
 	{
 		label = label.substr(0, GetSize(label)-1);
 		command = command.substr(pos);
-- 
2.17.2