[clang] Add warning when `-include-pch` is passed multiple times

[keith]

Summary

Previously this argument passed multiple times would result in the first
being silently ignored.

[keith]

Add group to warning

[MaskRay]

There are many options whether the latter one overrides the previous ones. How is this special?

[keith]

Fair question! I think this case is a bit different since it's quite subtle. The strange thing here is that the header you're intending to provide a pch for can still be read successfully, but not getting the benefits of the pch that you were expecting without knowing about it.

[kastiglione]

Is there any reason that -include-pch shouldn't follow the precedent of -include, which can be used multiple times? If not, then the end goal should be to support multiple uses, but in the mean time a warning is helpful.

[keith]

Yea seems like a reasonable request, looking through the uses nothing immediately pops out as being not supporting multiple PCHs but I guess before I went down that path I'd want someone who knew the code better to agree on that

clang/include/clang/Basic/DiagnosticDriverKinds.td

 def warn_drv_overriding_flag_option : Warning<
   "overriding '%0' option with '%1'">,
   InGroup<DiagGroup<"overriding-t-option">>;
 def warn_drv_treating_input_as_cxx : Warning<
   "treating '%0' input as '%1' when in C++ mode, this behavior is deprecated">,
   InGroup<Deprecated>;
 def warn_drv_pch_not_first_include : Warning<
   "precompiled header '%0' was ignored because '%1' is not first '-include'">;
+def warn_drv_multiple_include_pch : Warning<
+  "-include-pch passed multiple times but only the last '%0' is being used">,
+   InGroup<UnusedCommandLineArgument>;
 def warn_missing_sysroot : Warning<"no such sysroot directory: '%0'">,
   InGroup<DiagGroup<"missing-sysroot">>;
 def warn_incompatible_sysroot : Warning<"using sysroot for '%0' but targeting '%1'">,
   InGroup<DiagGroup<"incompatible-sysroot">>;
 def warn_debug_compression_unavailable : Warning<"cannot compress debug sections (zlib not installed)">,
   InGroup<DiagGroup<"debug-compression-unavailable">>;
 def warn_drv_disabling_vptr_no_rtti_default : Warning<
   "implicitly disabling vptr sanitizer because rtti wasn't enabled">,

clang/lib/Frontend/CompilerInvocation.cpp

case frontend::PrintDependencyDirectivesSourceMinimizerOutput:
     return true;
   }
   llvm_unreachable("invalid frontend action");
 }
 
 static void ParsePreprocessorArgs(PreprocessorOptions &Opts, ArgList &Args,
                                   DiagnosticsEngine &Diags,
                                   frontend::ActionKind Action) {
+  auto allPchFiles = Args.getAllArgValues(OPT_include_pch);
+  if (!allPchFiles.empty()) {
+    Opts.ImplicitPCHInclude = std::string(allPchFiles.back());
+    if (allPchFiles.size() > 1)
+      Diags.Report(diag::warn_drv_multiple_include_pch)
+          << Opts.ImplicitPCHInclude;
+  }
+
   Opts.PCHWithHdrStop = Args.hasArg(OPT_pch_through_hdrstop_create) ||
                         Args.hasArg(OPT_pch_through_hdrstop_use);
 
   for (const auto *A : Args.filtered(OPT_error_on_deserialized_pch_decl))
     Opts.DeserializedPCHDeclsToErrorOn.insert(A->getValue());
 
   for (const auto &A : Args.getAllArgValues(OPT_fmacro_prefix_map_EQ)) {
     auto Split = StringRef(A).split('=');

clang/test/PCH/multiple-include-pch-warning.c

+// RUN: %clang_cc1 -emit-pch -o %t1.pch %s
+// RUN: %clang_cc1 -emit-pch -o %t2.pch %s
+// RUN: %clang_cc1 %s -include-pch %t1.pch -include-pch %t2.pch 2>&1 | FileCheck %s
+
+// CHECK: warning: -include-pch passed multiple times but only the last '{{.*\.pch}}' is being used