diff --git a/clang/lib/Frontend/PrecompiledPreamble.cpp b/clang/lib/Frontend/PrecompiledPreamble.cpp
--- a/clang/lib/Frontend/PrecompiledPreamble.cpp
+++ b/clang/lib/Frontend/PrecompiledPreamble.cpp
@@ -765,6 +765,10 @@
   PreprocessorOpts.DisablePCHOrModuleValidation =
       DisableValidationForModuleKind::PCH;
 
+  // So don't bother generating the long version of the predefines buffer.
+  // The is going to overwrite it anyway (to almost nothing).
+  PreprocessorOpts.UsePredefines = false;
+
   setupPreambleStorage(*Storage, PreprocessorOpts, VFS);
 }