Index: llvm/lib/DebugInfo/MSF/MSFBuilder.cpp
===================================================================
--- llvm/lib/DebugInfo/MSF/MSFBuilder.cpp
+++ llvm/lib/DebugInfo/MSF/MSFBuilder.cpp
@@ -341,7 +341,17 @@
 
   Layout = std::move(*L);
 
-  uint64_t FileSize = Layout.SB->BlockSize * Layout.SB->NumBlocks;
+  uint64_t FileSize = uint64_t(Layout.SB->BlockSize) * Layout.SB->NumBlocks;
+  if (FileSize > UINT32_MAX) {
+    // FIXME: Changing the BinaryStream classes to use 64-bit numbers lets
+    // us create PDBs larger than 4 GiB successfully. The file format is
+    // block-based and as long as each stream is small enough, PDBs larger than
+    // 4 GiB might work. Check if tools can handle these large PDBs, and if so
+    // add support for writing them.
+    return make_error<MSFError>(msf_error_code::invalid_format,
+                                "Output larger than 4 GiB");
+  }
+
   auto OutFileOrError = FileOutputBuffer::create(Path, FileSize);
   if (auto EC = OutFileOrError.takeError())
     return std::move(EC);