[clang] Enable -mms-bitfields by default for mingw targets

[mstorsjo]

Summary

This matches GCC, which enabled -mms-bitfields by default for mingw targets in 4.7 [1].

[1] https://www.gnu.org/software/gcc/gcc-4.7/changes.html

[rjmccall]

Seems reasonable; GCC is the "system compiler" for this platform. Does isWindowsGNUEnvironment exactly track the condition that GCC uses? It's just MinGW, not Cygwin?

[mstorsjo]

In D81795#2092878, @rjmccall wrote:

Seems reasonable; GCC is the "system compiler" for this platform. Does isWindowsGNUEnvironment exactly track the condition that GCC uses? It's just MinGW, not Cygwin?

Yes: https://github.com/llvm/llvm-project/blob/d700237f1aa1bc05d584a0f50fdad89370e17987/llvm/include/llvm/ADT/Triple.h#L563-L574

And the corresponding change in GCC, https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=99e7ad2ec6a78a6a55cefaa952b57870594c08ae, only changed it for MinGW, not Cygwin (and the corresponding files for cygwin don't have that bit set).

[rjmccall]

Okay, thanks. LGTM, then.

clang/lib/Driver/ToolChains/Clang.cpp

if (!RawTriple.isOSDarwin() && !RawTriple.isNVPTX() && !RawTriple.isOSAIX())
     CmdArgs.push_back("-mconstructor-aliases");
 
   // Darwin's kernel doesn't support guard variables; just die if we
   // try to use them.
   if (KernelOrKext && RawTriple.isOSDarwin())
     CmdArgs.push_back("-fforbid-guard-variables");
 
   if (Args.hasFlag(options::OPT_mms_bitfields, options::OPT_mno_ms_bitfields,
-                   false)) {
+                   Triple.isWindowsGNUEnvironment())) {
     CmdArgs.push_back("-mms-bitfields");
   }
 
   if (Args.hasFlag(options::OPT_mpie_copy_relocations,
                    options::OPT_mno_pie_copy_relocations,
                    false)) {
     CmdArgs.push_back("-mpie-copy-relocations");
   }

clang/test/Driver/ms-bitfields.c

-// RUN: %clang -### %s 2>&1 | FileCheck %s -check-prefix=NO-MSBITFIELDS
+// RUN: %clang -### -target x86_64-linux-gnu %s 2>&1 | FileCheck %s -check-prefix=NO-MSBITFIELDS
+// RUN: %clang -### -target x86_64-windows-gnu %s 2>&1 | FileCheck %s -check-prefix=MSBITFIELDS
 // RUN: %clang -### -mno-ms-bitfields -mms-bitfields %s 2>&1 | FileCheck %s -check-prefix=MSBITFIELDS
 // RUN: %clang -### -mms-bitfields -mno-ms-bitfields %s 2>&1 | FileCheck %s -check-prefix=NO-MSBITFIELDS
 
 // MSBITFIELDS: -mms-bitfields
 // NO-MSBITFIELDS-NOT: -mms-bitfields
 
 // REQUIRES: clang-driver