diff --git a/llvm/cmake/config-ix.cmake b/llvm/cmake/config-ix.cmake
--- a/llvm/cmake/config-ix.cmake
+++ b/llvm/cmake/config-ix.cmake
@@ -134,6 +134,8 @@
     endif()
   endif()
   set(LLVM_ENABLE_ZLIB "${HAVE_ZLIB}")
+else()
+  set(LLVM_ENABLE_ZLIB 0)
 endif()
 
 if(LLVM_ENABLE_LIBXML2)
diff --git a/llvm/lib/Support/Compression.cpp b/llvm/lib/Support/Compression.cpp
--- a/llvm/lib/Support/Compression.cpp
+++ b/llvm/lib/Support/Compression.cpp
@@ -57,7 +57,8 @@
   // Tell MemorySanitizer that zlib output buffer is fully initialized.
   // This avoids a false report when running LLVM with uninstrumented ZLib.
   __msan_unpoison(CompressedBuffer.data(), CompressedSize);
-  CompressedBuffer.truncate(CompressedSize);
+  if (CompressedSize < CompressedBuffer.size())
+    CompressedBuffer.truncate(CompressedSize);
 }
 
 Error zlib::uncompress(StringRef InputBuffer, char *UncompressedBuffer,
@@ -79,7 +80,8 @@
   UncompressedBuffer.resize_for_overwrite(UncompressedSize);
   Error E = zlib::uncompress(InputBuffer, UncompressedBuffer.data(),
                              UncompressedSize);
-  UncompressedBuffer.truncate(UncompressedSize);
+  if (UncompressedSize < UncompressedBuffer.size())
+    UncompressedBuffer.truncate(UncompressedSize);
   return E;
 }
 
diff --git a/llvm/unittests/Support/CompressionTest.cpp b/llvm/unittests/Support/CompressionTest.cpp
--- a/llvm/unittests/Support/CompressionTest.cpp
+++ b/llvm/unittests/Support/CompressionTest.cpp
@@ -23,11 +23,9 @@
 namespace {
 
 #if LLVM_ENABLE_ZLIB
-
-void TestZlibCompression(StringRef Input, int Level) {
+static void testZlibCompression(StringRef Input, int Level) {
   SmallString<32> Compressed;
   SmallString<32> Uncompressed;
-
   zlib::compress(Input, Compressed, Level);
 
   // Check that uncompressed buffer is the same as original.
@@ -43,26 +41,24 @@
 }
 
 TEST(CompressionTest, Zlib) {
-  TestZlibCompression("", zlib::DefaultCompression);
+  testZlibCompression("", zlib::DefaultCompression);
 
-  TestZlibCompression("hello, world!", zlib::NoCompression);
-  TestZlibCompression("hello, world!", zlib::BestSizeCompression);
-  TestZlibCompression("hello, world!", zlib::BestSpeedCompression);
-  TestZlibCompression("hello, world!", zlib::DefaultCompression);
+  testZlibCompression("hello, world!", zlib::NoCompression);
+  testZlibCompression("hello, world!", zlib::BestSizeCompression);
+  testZlibCompression("hello, world!", zlib::BestSpeedCompression);
+  testZlibCompression("hello, world!", zlib::DefaultCompression);
 
   const size_t kSize = 1024;
   char BinaryData[kSize];
-  for (size_t i = 0; i < kSize; ++i) {
+  for (size_t i = 0; i < kSize; ++i)
     BinaryData[i] = i & 255;
-  }
   StringRef BinaryDataStr(BinaryData, kSize);
 
-  TestZlibCompression(BinaryDataStr, zlib::NoCompression);
-  TestZlibCompression(BinaryDataStr, zlib::BestSizeCompression);
-  TestZlibCompression(BinaryDataStr, zlib::BestSpeedCompression);
-  TestZlibCompression(BinaryDataStr, zlib::DefaultCompression);
+  testZlibCompression(BinaryDataStr, zlib::NoCompression);
+  testZlibCompression(BinaryDataStr, zlib::BestSizeCompression);
+  testZlibCompression(BinaryDataStr, zlib::BestSpeedCompression);
+  testZlibCompression(BinaryDataStr, zlib::DefaultCompression);
 }
-
 #endif
 
 }