From 55789b6ba93faf388296233cb97fe19f4852674e Mon Sep 17 00:00:00 2001
From: Rafael Espindola <rafael.espindola@gmail.com>
Date: Mon, 28 Apr 2014 12:40:50 +0000
Subject: [PATCH] Produce an error instead of a crash in an expr we cannot
 represent.

llvm-svn: 207414
---
 lib/MC/ELFObjectWriter.cpp      | 7 ++++++-
 test/MC/ELF/subtraction-error.s | 8 ++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 test/MC/ELF/subtraction-error.s

diff --git a/lib/MC/ELFObjectWriter.cpp b/lib/MC/ELFObjectWriter.cpp
index e8b34d56f5b..12b5f595ca4 100644
--- a/lib/MC/ELFObjectWriter.cpp
+++ b/lib/MC/ELFObjectWriter.cpp
@@ -606,7 +606,12 @@ static const MCSymbol *getBaseSymbol(const MCAsmLayout &Layout,
   MCValue Value;
   if (!Expr->EvaluateAsRelocatable(Value, &Layout))
     llvm_unreachable("Invalid Expression");
-  assert(!Value.getSymB());
+  const MCSymbolRefExpr *RefB = Value.getSymB();
+  if (RefB) {
+    Layout.getAssembler().getContext().FatalError(
+        SMLoc(), Twine("symbol '") + RefB->getSymbol().getName() +
+                     "' could not be evaluated in a subtraction expression");
+  }
   const MCSymbolRefExpr *A = Value.getSymA();
   if (!A)
     return nullptr;
diff --git a/test/MC/ELF/subtraction-error.s b/test/MC/ELF/subtraction-error.s
new file mode 100644
index 00000000000..6b93d3aee5b
--- /dev/null
+++ b/test/MC/ELF/subtraction-error.s
@@ -0,0 +1,8 @@
+// RUN: not llvm-mc -filetype=obj -triple x86_64-pc-linux < %s 2>&1 | FileCheck %s
+
+a:
+    .section foo
+b:
+c = b - a
+
+; CHECK: symbol 'a' could not be evaluated in a subtraction expression