From 8d60ffe314306e5295fb76338c6391e5fe986dea Mon Sep 17 00:00:00 2001
From: Andrew Kelley <superjoe30@gmail.com>
Date: Fri, 15 Jan 2016 17:12:26 -0700
Subject: [PATCH] solve the mystery of undefined reference error

big surprise, C++ is to blame
---
 README.md        | 14 ++++++++++++++
 src/config.h.in  |  2 ++
 src/zig_llvm.cpp |  7 +++++++
 3 files changed, 23 insertions(+)

diff --git a/README.md b/README.md
index 1b7ec0c9bd..b83eeeac5f 100644
--- a/README.md
+++ b/README.md
@@ -90,3 +90,17 @@ cmake .. -DCMAKE_BUILD_TYPE=Release -DZIG_LIBC_DIR=path/to/libc/dir
 make
 sudo make install
 ```
+
+### Troubleshooting
+
+If you get one of these:
+
+ * `undefined reference to `_ZNK4llvm17SubtargetFeatures9getStringB5cxx11Ev'`
+ * `undefined reference to `llvm::SubtargetFeatures::getString() const'`
+
+This is because of [C++'s Dual ABI](https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html).
+Most likely LLVM was compiled with one compiler while Zig was compiled with a
+different one, for example GCC vs clang.
+
+To fix this, compile Zig with the same compiler that LLVM was compiled with, or
+add `-DZIG_LLVM_OLD_CXX_ABI=yes` to the cmake configure line.
diff --git a/src/config.h.in b/src/config.h.in
index c64c2b40f4..8b78e56238 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -10,4 +10,6 @@
 #define ZIG_STD_DIR "@CMAKE_INSTALL_PREFIX@/@ZIG_STD_DEST@"
 #define ZIG_LIBC_DIR "@ZIG_LIBC_DIR@"
 
+#cmakedefine ZIG_LLVM_OLD_CXX_ABI
+
 #endif
diff --git a/src/zig_llvm.cpp b/src/zig_llvm.cpp
index 699963f5d3..be4cf2ac05 100644
--- a/src/zig_llvm.cpp
+++ b/src/zig_llvm.cpp
@@ -5,6 +5,13 @@
  * See http://opensource.org/licenses/MIT
  */
 
+// This must go before all includes.
+#include "config.h"
+#if defined(ZIG_LLVM_OLD_CXX_ABI)
+#define _GLIBCXX_USE_CXX11_ABI 0
+#endif
+
+
 #include "zig_llvm.hpp"
 
 /*