Add -fkeep-system-includes modifier for -E

clang/docs/CommandGuide/clang.rst

@@ -684,6 +684,21 @@ Preprocessor Options
 
   Do not search clang's builtin directory for include files.
 
+.. option:: -fkeep-system-includes
+
+  Usable only with :option:`-E`. Do not copy the preprocessed content of
+  "system" headers to the output; instead, preserve the #include directive.
+  This can greatly reduce the volume of text produced by :option:`-E` which
+  can be helpful when trying to produce a "small" reproduceable test case.
+
+  This option does not guarantee reproduceability, however. If the including
+  source defines preprocessor symbols that influence the behavior of system
+  headers (for example, ``_XOPEN_SOURCE``) the operation of :option:`-E` will
+  remove that definition and thus can change the semantics of the included
+  header. Also, using a different version of the system headers (especially a
+  different version of the STL) may result in different behavior. Always verify
+  the preprocessed file by compiling it separately.
+
 
 ENVIRONMENT
 -----------

clang/docs/ReleaseNotes.rst

@@ -159,6 +159,10 @@ New Compiler Flags
   Since enabling the verifier adds a non-trivial cost of a few percent impact on
   build times, it's disabled by default, unless your LLVM distribution itself is
   compiled with runtime checks enabled.
+* ``-fkeep-system-includes`` modifies the behavior of the ``-E`` option,
+  preserving ``#include`` directives for "system" headers instead of copying
+  the preprocessed text to the output. This can greatly reduce the size of the
+  preprocessed output, which can be helpful when trying to reduce a test case.
 
 Deprecated Compiler Flags
 -------------------------

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -182,8 +182,8 @@ def err_drv_invalid_Xopenmp_target_with_args : Error<
   "invalid -Xopenmp-target argument: '%0', options requiring arguments are unsupported">;
 def err_drv_argument_only_allowed_with : Error<
   "invalid argument '%0' only allowed with '%1'">;
-def err_drv_minws_unsupported_input_type : Error<
-  "'-fminimize-whitespace' invalid for input of type %0">;
+def err_drv_opt_unsupported_input_type : Error<
+  "'%0' invalid for input of type %1">;
 def err_drv_amdgpu_ieee_without_no_honor_nans : Error<
   "invalid argument '-mno-amdgpu-ieee' only allowed with relaxed NaN handling">;
 def err_drv_argument_not_allowed_with : Error<

clang/include/clang/Driver/Options.td

@@ -2519,6 +2519,17 @@ defm minimize_whitespace : BoolFOption<"minimize-whitespace",
           "whitespace such that two files with only formatting changes are "
           "equal.\n\nOnly valid with -E on C-like inputs and incompatible "
           "with -traditional-cpp.">, NegFlag<SetFalse>>;
+defm keep_system_includes : BoolFOption<"keep-system-includes",
+  PreprocessorOutputOpts<"KeepSystemIncludes">, DefaultFalse,
+  PosFlag<SetTrue, [], [ClangOption, CC1Option],
+          "Instead of expanding system headers when emitting preprocessor "
+          "output, preserve the #include directive. Useful when producing "
+          "preprocessed output for test case reduction. May produce incorrect "
+          "output if preprocessor symbols that control the included content "
+          "(e.g. _XOPEN_SOURCE) are defined in the including source file. The "
+          "portability of the resulting source to other compilation environments "
+          "is not guaranteed.\n\nOnly valid with -E.">,
+  NegFlag<SetFalse>>;
 
 def ffreestanding : Flag<["-"], "ffreestanding">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option]>,

clang/include/clang/Frontend/PreprocessorOutputOptions.h

@@ -26,6 +26,7 @@ class PreprocessorOutputOptions {
   unsigned RewriteImports  : 1;    ///< Include contents of transitively-imported modules.
   unsigned MinimizeWhitespace : 1; ///< Ignore whitespace from input.
   unsigned DirectivesOnly : 1; ///< Process directives but do not expand macros.
+  unsigned KeepSystemIncludes : 1; ///< Do not expand system headers.
 
 public:
   PreprocessorOutputOptions() {
@@ -40,6 +41,7 @@ class PreprocessorOutputOptions {
     RewriteImports = 0;
     MinimizeWhitespace = 0;
     DirectivesOnly = 0;
+    KeepSystemIncludes = 0;
   }
 };
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -68,7 +68,9 @@ using namespace llvm::opt;
 static void CheckPreprocessingOptions(const Driver &D, const ArgList &Args) {
   if (Arg *A = Args.getLastArg(clang::driver::options::OPT_C, options::OPT_CC,
                                options::OPT_fminimize_whitespace,
-                               options::OPT_fno_minimize_whitespace)) {
+                               options::OPT_fno_minimize_whitespace,
+                               options::OPT_fkeep_system_includes,
+                               options::OPT_fno_keep_system_includes)) {
     if (!Args.hasArg(options::OPT_E) && !Args.hasArg(options::OPT__SLASH_P) &&
         !Args.hasArg(options::OPT__SLASH_EP) && !D.CCCIsCPP()) {
       D.Diag(clang::diag::err_drv_argument_only_allowed_with)
@@ -6717,11 +6719,21 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                    options::OPT_fno_minimize_whitespace, false)) {
     types::ID InputType = Inputs[0].getType();
     if (!isDerivedFromC(InputType))
-      D.Diag(diag::err_drv_minws_unsupported_input_type)
-          << types::getTypeName(InputType);
+      D.Diag(diag::err_drv_opt_unsupported_input_type)
+          << "-fminimize-whitespace" << types::getTypeName(InputType);
     CmdArgs.push_back("-fminimize-whitespace");
   }
 
+  // -fno-keep-system-includes is default.
+  if (Args.hasFlag(options::OPT_fkeep_system_includes,
+                   options::OPT_fno_keep_system_includes, false)) {
+    types::ID InputType = Inputs[0].getType();
+    if (!isDerivedFromC(InputType))
+      D.Diag(diag::err_drv_opt_unsupported_input_type)
+          << "-fkeep-system-includes" << types::getTypeName(InputType);
+    CmdArgs.push_back("-fkeep-system-includes");
+  }
+
   // -fms-extensions=0 is default.
   if (Args.hasFlag(options::OPT_fms_extensions, options::OPT_fno_ms_extensions,
                    IsWindowsMSVC))