[PATCH] ADT: Correct APInt::getActiveWords for zero values

[Meador Inge]

PR15138 was opened because of a segfault in the Bitcode writer.
The actual issue ended up being a bug in APInt where calls to
APInt::getActiveWords returns a bogus value when the APInt value
is 0.  This patch fixes the problem by ensuring that getActiveWords
returns 1 for 0 valued APInts.

OK?

---
 include/llvm/ADT/APInt.h    | 3 ++-
 unittests/ADT/APIntTest.cpp | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/llvm/ADT/APInt.h b/include/llvm/ADT/APInt.h
index e67e032..2e623a1 100644
--- a/include/llvm/ADT/APInt.h
+++ b/include/llvm/ADT/APInt.h
@@ -1191,7 +1191,8 @@ public:
   /// APInt. This is used in conjunction with getActiveData to extract the raw
   /// value of the APInt.
   unsigned getActiveWords() const {
-    return whichWord(getActiveBits()-1) + 1;
+    unsigned numActiveBits = getActiveBits();
+    return numActiveBits ? whichWord(numActiveBits - 1) + 1 : 1;
   }
 
   /// Computes the minimum bit width for this APInt while considering it to be
diff --git a/unittests/ADT/APIntTest.cpp b/unittests/ADT/APIntTest.cpp
index 2a4c5b4..2510bc7 100644
--- a/unittests/ADT/APIntTest.cpp
+++ b/unittests/ADT/APIntTest.cpp
@@ -56,6 +56,14 @@ TEST(APIntTest, i33_Count) {
 #endif
 
 TEST(APIntTest, i65_Count) {
+  APInt i65(65, 0, true);
+  EXPECT_EQ(65u, i65.countLeadingZeros());
+  EXPECT_EQ(0u, i65.countLeadingOnes());
+  EXPECT_EQ(0u, i65.getActiveBits());
+  EXPECT_EQ(1u, i65.getActiveWords());
+  EXPECT_EQ(65u, i65.countTrailingZeros());
+  EXPECT_EQ(0u, i65.countPopulation());
+
   APInt i65minus(65, 0, true);
   i65minus.setBit(64);
   EXPECT_EQ(0u, i65minus.countLeadingZeros());
-- 
1.8.1