[clang-cl][MSVC] Map /external:Wn n=1-4 to -Wsystem-headers

[steplong]

Summary

https://docs.microsoft.com/en-us/cpp/build/reference/external-external-headers-diagnostics?view=msvc-170

[steplong]

I don't think /external:Wn behaves exactly like -Wsystem-headers. /external:Wn applies /Wn to the system headers, while -Wsystem-headers applies whatever is on the command line to the system headers. I think /external:Wn behaves more like:

// foo.c
#pragma clang diagnostic push
#pragma clang diagnostic warning "-Wall"
#include <foo.h>
#pragma clang diagnostic pop


void foo() {
  int i = 10;
}

// foo.h
void bar() {
   int i = 11;
}

Passing -Wno-unused-variables only suppresses the warnings for the rest of foo.c. It looks like the pragma is calling setSeverityForGroup(diag::Flavor::WarningOrError, "-Wall", diag::Serverity::Warning, Loc), but I'm not sure where we can detect that we are including a system header.

[hans]

Right, this is not the same, but I suppose it's better than ignoring the flag.

clang/include/clang/Driver/Options.td

6480

should we map it to -Wno-system-headers?

clang/test/Driver/cl-zc.cpp

106 ↗

(On Diff #435710)

This is not the right test file, cl-zc.cpp is for testing /Zc flags.
I don't think we have a special file for clang-cl warning flags, so clang/test/Driver/cl-options.c would be the file to use.

[steplong]

clang/test/Driver/cl-zc.cpp

106 ↗

(On Diff #435710)

Oh whoops, I think I added some /kernel tests here too for D126719

[steplong]

• Move tests to cl-options.c
• Map /external:W0 to -Wno-system-headers

[hans]

lgtm

clang/include/clang/Driver/Options.td

6480

For the help text, the "with -Wno-system-headers" part seems redundant.
How about:

Ignore warnings from system headers (default)

[steplong]

• Change helptext for /external:W0

[steplong]

Hmm do you know how I can restart the pre-merge checks? It looks like the x64 debian openmp tests failed. I don't think this patch is related, but I want to make sure

[hans]

In D127452#3577729, @steplong wrote:

Hmm do you know how I can restart the pre-merge checks? It looks like the x64 debian openmp tests failed. I don't think this patch is related, but I want to make sure

You can rebase and upload the patch again. But those bots are pretty flaky, I don't think it's related to this patch either.

[steplong]

• Rebase patch

clang/include/clang/Driver/Options.td

 def U : JoinedOrSeparate<["-"], "U">, Group<Preprocessor_Group>,
   Flags<[CC1Option, FlangOption, FC1Option]>, MetaVarName<"<macro>">, HelpText<"Undefine macro <macro>">;
 def V : JoinedOrSeparate<["-"], "V">, Flags<[NoXarchOption, Unsupported]>;
 def Wa_COMMA : CommaJoined<["-"], "Wa,">,
   HelpText<"Pass the comma separated arguments in <arg> to the assembler">,
   MetaVarName<"<arg>">;
 def Wall : Flag<["-"], "Wall">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
 def WCL4 : Flag<["-"], "WCL4">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
+def Wsystem_headers : Flag<["-"], "Wsystem-headers">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
+def Wno_system_headers : Flag<["-"], "Wno-system-headers">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
 def Wdeprecated : Flag<["-"], "Wdeprecated">, Group<W_Group>, Flags<[CC1Option]>,
   HelpText<"Enable warnings for deprecated constructs and define __DEPRECATED">;
 def Wno_deprecated : Flag<["-"], "Wno-deprecated">, Group<W_Group>, Flags<[CC1Option]>;
 def Wl_COMMA : CommaJoined<["-"], "Wl,">, Flags<[LinkerInput, RenderAsInput]>,
   HelpText<"Pass the comma separated arguments in <arg> to the linker">,
   MetaVarName<"<arg>">, Group<Link_Group>;
 // FIXME: This is broken; these should not be Joined arguments.
 def Wno_nonportable_cfstrings : Joined<["-"], "Wno-nonportable-cfstrings">, Group<W_Group>,

 def _SLASH_std : CLCompileJoined<"std:">,
   HelpText<"Set language version (c++14,c++17,c++20,c++latest,c11,c17)">;
 def _SLASH_U : CLJoinedOrSeparate<"U">, HelpText<"Undefine macro">,
   MetaVarName<"<macro>">, Alias<U>;
 def _SLASH_validate_charset : CLFlag<"validate-charset">,
   Alias<W_Joined>, AliasArgs<["invalid-source-encoding"]>;
 def _SLASH_validate_charset_ : CLFlag<"validate-charset-">,
   Alias<W_Joined>, AliasArgs<["no-invalid-source-encoding"]>;
+def _SLASH_external_W0 : CLFlag<"external:W0">, HelpText<"Ignore warnings from system headers (default)">, Alias<Wno_system_headers>;
+def _SLASH_external_W1 : CLFlag<"external:W1">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
+def _SLASH_external_W2 : CLFlag<"external:W2">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
+def _SLASH_external_W3 : CLFlag<"external:W3">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
+def _SLASH_external_W4 : CLFlag<"external:W4">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
 def _SLASH_W0 : CLFlag<"W0">, HelpText<"Disable all warnings">, Alias<w>;
 def _SLASH_W1 : CLFlag<"W1">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W2 : CLFlag<"W2">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W3 : CLFlag<"W3">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W4 : CLFlag<"W4">, HelpText<"Enable -Wall and -Wextra">, Alias<WCL4>;
 def _SLASH_Wall : CLFlag<"Wall">, HelpText<"Enable -Weverything">,
   Alias<W_Joined>, AliasArgs<["everything"]>;
 def _SLASH_WX : CLFlag<"WX">, HelpText<"Treat warnings as errors">,

clang/test/Driver/cl-options.c

 
 // RUN: %clang_cl /JMC /c -### -- %s 2>&1 | FileCheck %s --check-prefix NOJMC
 // RUN: %clang_cl /JMC /Z7 /JMC- /c -### -- %s 2>&1 | FileCheck %s --check-prefix NOJMC
 // NOJMC-NOT: -fjmc
 
 // RUN: %clang_cl /JMC /Z7 /c -### -- %s 2>&1 | FileCheck %s --check-prefix JMC
 // JMC: -fjmc
 
+// RUN: %clang_cl /external:W0 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_W0 %s
+// RUN: %clang_cl /external:W1 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// RUN: %clang_cl /external:W2 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// RUN: %clang_cl /external:W3 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// RUN: %clang_cl /external:W4 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// EXTERNAL_W0: "-Wno-system-headers"
+// EXTERNAL_Wn: "-Wsystem-headers"
+
 void f(void) { }