switch clang to -fno-common by default

Common variables will be flagged as errors by default.  This can
be overridden with -fcommon.

Cherry-pick this change that will be part of LLVM11.  We bring it
forward so we can finish fixing the fallout in the ports tree.

ok deraadt@ sthen@ mglocker@

3 files changed, 6 insertions, 21 deletions

diff --git a/gnu/llvm/clang/include/clang/Driver/Options.td b/gnu/llvm/clang/include/clang/Driver/Options.td
index ff76bcb7ef7..a370a522e84 100644
--- a/gnu/llvm/clang/include/clang/Driver/Options.td
+++ b/gnu/llvm/clang/include/clang/Driver/Options.td
@@ -840,7 +840,8 @@ def fno_record_command_line : Flag<["-"], "fno-record-command-line">,
   Group<f_clang_Group>;
 def : Flag<["-"], "frecord-gcc-switches">, Alias<frecord_command_line>;
 def : Flag<["-"], "fno-record-gcc-switches">, Alias<fno_record_command_line>;
-def fcommon : Flag<["-"], "fcommon">, Group<f_Group>;
+def fcommon : Flag<["-"], "fcommon">, Group<f_Group>,
+  Flags<[CoreOption, CC1Option]>, HelpText<"Place uninitialized global variables in a common block">;
 def fcompile_resource_EQ : Joined<["-"], "fcompile-resource=">, Group<f_Group>;
 def fcomplete_member_pointers : Flag<["-"], "fcomplete-member-pointers">, Group<f_clang_Group>,
    Flags<[CoreOption, CC1Option]>,
diff --git a/gnu/llvm/clang/lib/Driver/ToolChains/Clang.cpp b/gnu/llvm/clang/lib/Driver/ToolChains/Clang.cpp
index 03f637113d1..69900484f61 100644
--- a/gnu/llvm/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/gnu/llvm/clang/lib/Driver/ToolChains/Clang.cpp
@@ -1404,20 +1404,6 @@ static bool isSignedCharDefault(const llvm::Triple &Triple) {
   }
 }
 
-static bool isNoCommonDefault(const llvm::Triple &Triple) {
-  switch (Triple.getArch()) {
-  default:
-    if (Triple.isOSFuchsia())
-      return true;
-    return false;
-
-  case llvm::Triple::xcore:
-  case llvm::Triple::wasm32:
-  case llvm::Triple::wasm64:
-    return true;
-  }
-}
-
 static bool hasMultipleInvocations(const llvm::Triple &Triple,
                                    const ArgList &Args) {
   // Supported only on Darwin where we invoke the compiler multiple times
@@ -5638,11 +5624,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (!Args.hasFlag(options::OPT_Qy, options::OPT_Qn, true))
     CmdArgs.push_back("-Qn");
 
-  // -fcommon is the default unless compiling kernel code or the target says so
-  bool NoCommonDefault = KernelOrKext || isNoCommonDefault(RawTriple);
-  if (!Args.hasFlag(options::OPT_fcommon, options::OPT_fno_common,
-                    !NoCommonDefault))
-    CmdArgs.push_back("-fno-common");
+  // -fno-common is the default, set -fcommon only when that flag is set.
+  if (Args.hasFlag(options::OPT_fcommon, options::OPT_fno_common, false))
+    CmdArgs.push_back("-fcommon");
 
   // -fsigned-bitfields is default, and clang doesn't yet support
   // -funsigned-bitfields.
diff --git a/gnu/llvm/clang/lib/Frontend/CompilerInvocation.cpp b/gnu/llvm/clang/lib/Frontend/CompilerInvocation.cpp
index e33e80f5b37..f47906798a0 100644
--- a/gnu/llvm/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/gnu/llvm/clang/lib/Frontend/CompilerInvocation.cpp
@@ -808,7 +808,7 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.DwarfDebugFlags = Args.getLastArgValue(OPT_dwarf_debug_flags);
   Opts.RecordCommandLine = Args.getLastArgValue(OPT_record_command_line);
   Opts.MergeAllConstants = Args.hasArg(OPT_fmerge_all_constants);
-  Opts.NoCommon = Args.hasArg(OPT_fno_common);
+  Opts.NoCommon = !Args.hasArg(OPT_fcommon);
   Opts.NoInlineLineTables = Args.hasArg(OPT_gno_inline_line_tables);
   Opts.NoImplicitFloat = Args.hasArg(OPT_no_implicit_float);
   Opts.OptimizeSize = getOptimizationLevelSize(Args);