[Driver] Use -fuse-line-directives by default in MSVC mode

[mstorsjo]

Summary

Don't use the GNU extension form of line markers in MSVC mode.

[hans]

Seems reasonable to me (but please update the comment before landing).

lib/Driver/ToolChains/Clang.cpp

4218

The comment needs an update I think.

[mstorsjo]

lib/Driver/ToolChains/Clang.cpp

4218

Thanks, will change into "-fno-use-line-directives is default, except for MSVC targets." before pushing.

[rnk]

Please don't do this, this is actually really problematic, since #line directives lose information about what's a system header. That means the result of -E usually won't compile, since Windows headers are typically full of warnings and default-error warnings.

[mstorsjo]

In D46520#1092233, @rnk wrote:

Please don't do this, this is actually really problematic, since #line directives lose information about what's a system header. That means the result of -E usually won't compile, since Windows headers are typically full of warnings and default-error warnings.

Oh, ok - I can revert it then; I didn't have any specific need for this, I just noticed that clang in msvc mode and cl.exe differed in this aspect.

[hans]

In D46520#1092233, @rnk wrote:

Please don't do this, this is actually really problematic, since #line directives lose information about what's a system header. That means the result of -E usually won't compile, since Windows headers are typically full of warnings and default-error warnings.

Sorry, I didn't realize this was the case :-/

[mstorsjo]

Reverted in SVN r331858.

[rnk]

In D46520#1092681, @mstorsjo wrote:

Reverted in SVN r331858.

Thanks! When one attempts to generate pre-processed source with clang and compile it with MSVC, you usually have to remove clang's internal intrinsic headers from the include search path, otherwise you'll end up with references to builtins that don't exist in MSVC. Adding an extra -fuse-line-directives flag is just one more thing that you have to remember for that corner-case usage.

lib/Driver/ToolChains/Clang.cpp

if (Args.hasFlag(options::OPT_fregister_global_dtors_with_atexit,
                    RawTriple.isOSDarwin() && !KernelOrKext))
     CmdArgs.push_back("-fregister-global-dtors-with-atexit");
 
   // -fms-extensions=0 is default.
   if (Args.hasFlag(options::OPT_fms_extensions, options::OPT_fno_ms_extensions,
                    IsWindowsMSVC))
     CmdArgs.push_back("-fms-extensions");
 
-  // -fno-use-line-directives is default.
+  // -fno-use-line-directives is default, except for MSVC targets.
   if (Args.hasFlag(options::OPT_fuse_line_directives,
-                   options::OPT_fno_use_line_directives, false))
+                   options::OPT_fno_use_line_directives, IsWindowsMSVC))
     CmdArgs.push_back("-fuse-line-directives");
 
   // -fms-compatibility=0 is default.
   if (Args.hasFlag(options::OPT_fms_compatibility,
                    options::OPT_fno_ms_compatibility,
                    (IsWindowsMSVC &&
                     Args.hasFlag(options::OPT_fms_extensions,
                                  options::OPT_fno_ms_extensions, true))))

test/Driver/cl-options.c

 // D: "-D" "bar=baz"
 // D: "-D" "MYDEF=value"
 // D: "-D" "MYDEF2=foo#bar"
 // D: "-D" "MYDEF3=a=b"
 // D: "-D" "MYDEF4="
 
 // RUN: %clang_cl /E -### -- %s 2>&1 | FileCheck -check-prefix=E %s
 // E: "-E"
+// E: "-fuse-line-directives"
 // E: "-o" "-"
 
 // RUN: %clang_cl /EP -### -- %s 2>&1 | FileCheck -check-prefix=EP %s
 // EP: "-E"
 // EP: "-P"
 // EP: "-o" "-"
 
 // RUN: %clang_cl /fp:fast /fp:except -### -- %s 2>&1 | FileCheck -check-prefix=fpexcept %s