diff --git a/lib/Support/Compression.cpp b/lib/Support/Compression.cpp
index c32eb21343e..17ae2957d12 100644
--- a/lib/Support/Compression.cpp
+++ b/lib/Support/Compression.cpp
@@ -54,6 +54,9 @@ zlib::Status zlib::compress(StringRef InputBuffer,
   Status Res = encodeZlibReturnValue(::compress2(
       (Bytef *)CompressedBuffer.data(), &CompressedSize,
       (const Bytef *)InputBuffer.data(), InputBuffer.size(), CLevel));
+  // 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.resize(CompressedSize);
   return Res;
 }
@@ -65,6 +68,9 @@ zlib::Status zlib::uncompress(StringRef InputBuffer,
   Status Res = encodeZlibReturnValue(::uncompress(
       (Bytef *)UncompressedBuffer.data(), (uLongf *)&UncompressedSize,
       (const Bytef *)InputBuffer.data(), InputBuffer.size()));
+  // Tell MemorySanitizer that zlib output buffer is fully initialized.
+  // This avoids a false report when running LLVM with uninstrumented ZLib.
+  __msan_unpoison(UncompressedBuffer.data(), UncompressedSize);
   UncompressedBuffer.resize(UncompressedSize);
   return Res;
 }