diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -2639,12 +2639,11 @@
         // so we verify all input files.  Otherwise, verify only user input
         // files.
 
-        unsigned N = NumUserInputs;
-        if (ValidateSystemInputs ||
-            (HSOpts.ModulesValidateOncePerBuildSession &&
-             F.InputFilesValidationTimestamp <= HSOpts.BuildSessionTimestamp &&
-             F.Kind == MK_ImplicitModule))
-          N = NumInputs;
+        unsigned N = ValidateSystemInputs ? NumInputs : NumUserInputs;
+        if (HSOpts.ModulesValidateOncePerBuildSession &&
+            F.InputFilesValidationTimestamp > HSOpts.BuildSessionTimestamp &&
+            F.Kind == MK_ImplicitModule)
+          N = NumUserInputs;
 
         for (unsigned I = 0; I < N; ++I) {
           InputFile IF = getInputFile(F, I+1, Complain);
diff --git a/clang/test/Modules/fmodules-validate-once-per-build-session.c b/clang/test/Modules/fmodules-validate-once-per-build-session.c
--- a/clang/test/Modules/fmodules-validate-once-per-build-session.c
+++ b/clang/test/Modules/fmodules-validate-once-per-build-session.c
@@ -17,8 +17,8 @@
 
 // ===
 // Compile the module.
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
+// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
+// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
 // RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
 // RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
 // RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
@@ -30,8 +30,8 @@
 
 // ===
 // Use it, and make sure that we did not recompile it.
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
+// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
+// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
 // RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
 // RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
 // RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
@@ -54,8 +54,8 @@
 // ===
 // Use the module, and make sure that we did not recompile it if foo.h or
 // module.map are system files, even though the sources changed.
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
+// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
+// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
 // RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
 // RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
 // RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
@@ -73,7 +73,7 @@
 
 // ===
 // Recompile the module if the today's date is before 01 January 2100.
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fbuild-session-timestamp=4102441200 -fmodules-validate-once-per-build-session %s
+// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=4102441200 -fmodules-validate-once-per-build-session %s
 // RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
 // RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
 // RUN: cp %t/modules-cache/Foo.pcm %t/modules-to-compare/Foo-after.pcm
diff --git a/clang/test/Modules/validate-system-headers.m b/clang/test/Modules/validate-system-headers.m
--- a/clang/test/Modules/validate-system-headers.m
+++ b/clang/test/Modules/validate-system-headers.m
@@ -34,8 +34,8 @@
 // RUN: %clang_cc1 -isystem %t/Inputs/usr/include -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/ModuleCache -fdisable-module-hash -x objective-c-header -fsyntax-only %s -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session
 // RUN: diff %t/ModuleCache/Foo.pcm %t/Foo.pcm.saved
 
-// Now add -fmodules-validate-system-headers and rebuild
+// Now add -fmodules-validate-system-headers and rebuild. No recompilation due to -fmodules-validate-once-per-build-session
 // RUN: %clang_cc1 -isystem %t/Inputs/usr/include -fmodules -fimplicit-module-maps -fmodules-validate-system-headers -fmodules-cache-path=%t/ModuleCache -fdisable-module-hash -x objective-c-header -fsyntax-only %s -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session
-// RUN: not diff %t/ModuleCache/Foo.pcm %t/Foo.pcm.saved
+// RUN: diff %t/ModuleCache/Foo.pcm %t/Foo.pcm.saved
 
 @import Foo;