StringRef::copy shouldn't allocate anything for length 0 strings.

The BumpPtrAllocator currently doesn't handle zero length allocations well.
The discussion for how to fix that is ongoing.  However, there's no need
for StringRef::copy to actually allocate anything here anyway, so just
return StringRef() when we get a zero length copy.

Reviewed by David Blaikie

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264201 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/StringRef.h

@@ -133,6 +133,9 @@ namespace llvm {
 
     // copy - Allocate copy in Allocator and return StringRef to it.
     template <typename Allocator> StringRef copy(Allocator &A) const {
+      // Don't request a length 0 copy from the allocator.
+      if (empty())
+        return StringRef();
       char *S = A.template Allocate<char>(Length);
       std::copy(begin(), end(), S);
       return StringRef(S, Length);

unittests/ADT/StringRefTest.cpp

@@ -589,6 +589,15 @@ TEST(StringRefTest, joinStrings) {
 
 TEST(StringRefTest, AllocatorCopy) {
   BumpPtrAllocator Alloc;
+  // First test empty strings.  We don't want these to allocate anything on the
+  // allocator.
+  StringRef StrEmpty = "";
+  StringRef StrEmptyc = StrEmpty.copy(Alloc);
+  EXPECT_TRUE(StrEmpty.equals(StrEmptyc));
+  EXPECT_EQ(StrEmptyc.data(), nullptr);
+  EXPECT_EQ(StrEmptyc.size(), 0u);
+  EXPECT_EQ(Alloc.getTotalMemory(), 0u);
+
   StringRef Str1 = "hello";
   StringRef Str2 = "bye";
   StringRef Str1c = Str1.copy(Alloc);