[ELF] Handle references to garbage collected common symbols

https://reviews.llvm.org/rL312796 meant that references to garbage collected common symbols would cause a segfault.

This change fixes the behaviour for references to stripped common symbols.

Differential Revision: https://reviews.llvm.org/D37718

llvm-svn: 313086

Author: bd1976bris

lld/ELF/Symbols.cpp

@@ -103,6 +103,8 @@ static uint64_t getSymVA(const SymbolBody &Body, int64_t &Addend) {
     if (!Config->DefineCommon)
       return 0;
     auto DC = cast<DefinedCommon>(Body);
+    if (!DC.Live)
+      return 0;
     return DC.Section->getParent()->Addr + DC.Section->OutSecOff;
   }
   case SymbolBody::SharedKind: {

lld/test/ELF/common-gc3.s

@@ -0,0 +1,18 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o -o %t1 --gc-sections
+# RUN: llvm-objdump -s %t1 | FileCheck %s
+
+# CHECK:      Contents of section .noalloc:
+# 0000 00000000 00000000                    ........
+
+	.section	.text._start,"ax",@progbits
+	.globl	_start
+_start:
+	retq
+
+	.type	unused,@object
+	.comm	unused,4,4
+
+	.section	.noalloc,"",@progbits
+	.quad	unused