Recommit #2 "[Driver] Default to -fno-common for all targets"

After a first attempt to fix the test-suite failures, my first recommit
caused the same failures again. I had updated CMakeList.txt files of
tests that needed -fcommon, but it turns out that there are also
Makefiles which are used by some bots, so I've updated these Makefiles
now too.

See the original commit message for more details on this change:
0a9fc92

Author: Sjoerd Meijer

clang/docs/ClangCommandLineReference.rst

@@ -1307,6 +1307,10 @@ Use colors in diagnostics
 
 .. option:: -fcommon, -fno-common
 
+Place definitions of variables with no storage class and no initializer
+(tentative definitions) in a common block, instead of generating individual
+zero-initialized definitions (default -fno-common).
+
 .. option:: -fcompile-resource=<arg>, --resource <arg>, --resource=<arg>
 
 .. option:: -fconstant-cfstrings, -fno-constant-cfstrings

clang/docs/ReleaseNotes.rst

@@ -84,6 +84,13 @@ future versions of Clang.
 Modified Compiler Flags
 -----------------------
 
+- -fno-common has been enabled as the default for all targets.  Therefore, C
+  code that uses tentative definitions as definitions of a variable in multiple
+  translation units will trigger multiple-definition linker errors.  Generally,
+  this occurs when the use of the ``extern`` keyword is neglected in the declaration
+  of a variable in a header file. In some cases, no specific translation unit
+  provides a definition of the variable. The previous behavior can be restored by
+  specifying ``-fcommon``.
 
 New Pragmas in Clang
 --------------------

clang/include/clang/Driver/Options.td

@@ -848,7 +848,8 @@ 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]>,

clang/lib/Driver/ToolChains/Clang.cpp

@@ -1408,20 +1408,6 @@ 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
@@ -5692,11 +5678,9 @@ 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.

clang/lib/Frontend/CompilerInvocation.cpp

@@ -811,7 +811,7 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.RecordCommandLine =
       std::string(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);

clang/test/CodeGen/2008-07-21-mixed-var-fn-decl.c

@@ -3,6 +3,6 @@
 int g0, f0();
 int f1(), g1;
 
-// CHECK: @g0 = common {{(dso_local )?}}global i32 0, align 4
-// CHECK: @g1 = common {{(dso_local )?}}global i32 0, align 4
+// CHECK: @g0 = {{(dso_local )?}}global i32 0, align 4
+// CHECK: @g1 = {{(dso_local )?}}global i32 0, align 4
 

clang/test/CodeGen/2009-10-20-GlobalDebug.c

@@ -2,10 +2,10 @@
 // RUN: %clang -target i386-apple-darwin10 -flto -S -g %s -o - | FileCheck %s
 
 // CHECK: @main.localstatic = internal global i32 0, align 4, !dbg [[L:![0-9]+]]
-// CHECK: @global = common global i32 0, align 4, !dbg [[G:![0-9]+]]
+// CHECK: @global = global i32 0, align 4, !dbg [[G:![0-9]+]]
 
 int global;
-int main() { 
+int main() {
   static int localstatic;
   return 0;
 }

clang/test/CodeGen/aarch64-sve.c

@@ -16,7 +16,7 @@
 // CHECK-DEBUG: cannot yet generate debug info for SVE type '__SVFloat64_t'
 // CHECK-DEBUG: cannot yet generate debug info for SVE type '__SVBool_t'
 
-// CHECK: @ptr = common global <vscale x 16 x i8>* null, align 8
+// CHECK: @ptr = global <vscale x 16 x i8>* null, align 8
 // CHECK: %s8 = alloca <vscale x 16 x i8>, align 16
 // CHECK: %s16 = alloca <vscale x 8 x i16>, align 16
 // CHECK: %s32 = alloca <vscale x 4 x i32>, align 16

clang/test/CodeGen/address-space.c

@@ -1,13 +1,13 @@
 // RUN: %clang_cc1 -triple x86_64-apple-darwin -emit-llvm < %s | FileCheck -enable-var-scope -check-prefixes=CHECK,X86 %s
 // RUN: %clang_cc1 -triple amdgcn -emit-llvm < %s | FileCheck -enable-var-scope -check-prefixes=CHECK,AMDGCN %s
 
-// CHECK: @foo = common addrspace(1) global
+// CHECK: @foo = addrspace(1) global
 int foo __attribute__((address_space(1)));
 
-// CHECK: @ban = common addrspace(1) global
+// CHECK: @ban = addrspace(1) global
 int ban[10] __attribute__((address_space(1)));
 
-// CHECK: @a = common global
+// CHECK: @a = global
 int a __attribute__((address_space(0)));
 
 // CHECK-LABEL: define i32 @test1()

clang/test/CodeGen/alias.c

@@ -5,8 +5,13 @@
 // RUN: %clang_cc1 -triple aarch64-linux-gnu -emit-llvm -o - %s | FileCheck -check-prefix=CHECKGLOBALS %s
 
 int g0;
-// CHECKBASIC-DAG: @g0 = common global i32 0
-// CHECKASM-DAG: .comm g0,4,4
+// CHECKBASIC-DAG: @g0 = global i32 0
+// CHECKASM-DAG:  .bss
+// CHECKASM-DAG:  .globl  g0
+// CHECKASM-DAG:  .p2align  2
+// CHECKASM-DAG:  g0:
+// CHECKASM-DAG:  .long 0
+// CHECKASM-DAG:  .size g0, 4
 __thread int TL_WITH_ALIAS;
 // CHECKBASIC-DAG: @TL_WITH_ALIAS = thread_local global i32 0, align 4
 // CHECKASM-DAG: .globl TL_WITH_ALIAS