From 4e86f54fdbcc375e14fdfc61ded8a97e8e030996 Mon Sep 17 00:00:00 2001
From: Rafael Espindola <rafael.espindola@gmail.com>
Date: Tue, 24 Mar 2015 23:48:44 +0000
Subject: [PATCH] Don't be over eager in evaluating a subtraction with a weak
 symbol.

In a subtraction of the form A - B, if B is weak, there is no way to represent
that on ELF since all relocations add the value of a symbol.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233139 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/MC/ELFObjectWriter.cpp | 10 +++++++++-
 test/MC/ELF/weak-diff.s    | 12 ++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 test/MC/ELF/weak-diff.s

diff --git a/lib/MC/ELFObjectWriter.cpp b/lib/MC/ELFObjectWriter.cpp
index c05241b53f7..0bc17ef3543 100644
--- a/lib/MC/ELFObjectWriter.cpp
+++ b/lib/MC/ELFObjectWriter.cpp
@@ -803,6 +803,10 @@ static const MCSymbol *getWeakRef(const MCSymbolRefExpr &Ref) {
   return nullptr;
 }
 
+static bool isWeak(const MCSymbolData &D) {
+  return D.getFlags() & ELF_STB_Weak || MCELF::GetType(D) == ELF::STT_GNU_IFUNC;
+}
+
 void ELFObjectWriter::RecordRelocation(MCAssembler &Asm,
                                        const MCAsmLayout &Layout,
                                        const MCFragment *Fragment,
@@ -843,6 +847,10 @@ void ELFObjectWriter::RecordRelocation(MCAssembler &Asm,
           Fixup.getLoc(), "Cannot represent a difference across sections");
 
     const MCSymbolData &SymBD = Asm.getSymbolData(SymB);
+    if (isWeak(SymBD))
+      Asm.getContext().FatalError(
+          Fixup.getLoc(), "Cannot represent a subtraction with a weak symbol");
+
     uint64_t SymBOffset = Layout.getSymbolOffset(&SymBD);
     uint64_t K = SymBOffset - FixupOffset;
     IsPCRel = true;
@@ -1809,7 +1817,7 @@ ELFObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                                       const MCFragment &FB,
                                                       bool InSet,
                                                       bool IsPCRel) const {
-  if (DataA.getFlags() & ELF_STB_Weak || MCELF::GetType(DataA) == ELF::STT_GNU_IFUNC)
+  if (isWeak(DataA))
     return false;
   return MCObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(
                                                  Asm, DataA, FB,InSet, IsPCRel);
diff --git a/test/MC/ELF/weak-diff.s b/test/MC/ELF/weak-diff.s
new file mode 100644
index 00000000000..d270bbb7334
--- /dev/null
+++ b/test/MC/ELF/weak-diff.s
@@ -0,0 +1,12 @@
+// RUN: not llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o %t 2>&1 | FileCheck %s
+
+// CHECK: error: Cannot represent a subtraction with a weak symbol
+
+.weak f
+.weak g
+f:
+    nop
+g:
+    nop
+
+.quad g - f