Make sys::ThreadLocal<> zero-initialized on non-thread builds (PR18205)

According to the docs, ThreadLocal<>::get() should return NULL
if no object has been set. This patch makes that the case also for non-thread
builds and adds a very basic unit test to check it.

(This was causing PR18205 because PrettyStackTraceHead didn't get zero-
initialized and we'd crash trying to read past the end of that list. We didn't
notice this so much on Linux since we'd crash after printing all the entries,
but on Mac we print into a SmallString, and would crash before printing that.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197718 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 39 insertions, 0 deletions

diff --git a/unittests/Support/CMakeLists.txt b/unittests/Support/CMakeLists.txt
index c2753629cb..0abc2ffe1d 100644
--- a/unittests/Support/CMakeLists.txt
+++ b/unittests/Support/CMakeLists.txt
@@ -30,6 +30,7 @@ add_llvm_unittest(SupportTests
   RegexTest.cpp
   SourceMgrTest.cpp
   SwapByteOrderTest.cpp
+  ThreadLocalTest.cpp
   TimeValueTest.cpp
   UnicodeTest.cpp
   ValueHandleTest.cpp
diff --git a/unittests/Support/ThreadLocalTest.cpp b/unittests/Support/ThreadLocalTest.cpp
new file mode 100644
index 0000000000..dd4d706f5c
--- /dev/null
+++ b/unittests/Support/ThreadLocalTest.cpp
@@ -0,0 +1,38 @@
+//===- llvm/unittest/Support/ThreadLocalTest.cpp - Therad Local tests   ---===//
+//
+//		       The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/ThreadLocal.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+using namespace sys;
+
+namespace {
+
+class ThreadLocalTest : public ::testing::Test {
+};
+
+struct S {
+  int i;
+};
+
+TEST_F(ThreadLocalTest, Basics) {
+  ThreadLocal<const S> x;
+
+  EXPECT_EQ(0, x.get());
+
+  S s;
+  x.set(&s);
+  EXPECT_EQ(&s, x.get());
+
+  x.erase();
+  EXPECT_EQ(0, x.get());
+}
+
+}