[Driver] Default to -fno-common for all targets

This makes -fno-common the default for all targets because this has performance
and code-size benefits and is more language conforming for C code.
Additionally, GCC10 also defaults to -fno-common and so we get consistent
behaviour with GCC.

With this change, 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.

As GCC has switched already, we benefit from applications already being ported
and existing documentation how to do this. For example:
- https://gcc.gnu.org/gcc-10/porting_to.html
- https://wiki.gentoo.org/wiki/Gcc_10_porting_notes/fno_common

Differential revision: https://reviews.llvm.org/D75056

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
@@ -5675,11 +5661,9 @@ 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

@@ -809,7 +809,7 @@ 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