Define __GCC_HAVE_DWARF2_CFI_ASM if applicable

In -fno-exceptions -fno-asynchronous-unwind-tables -g0 mode,
GCC does not emit `.cfi_*` directives.

```
% diff <(gcc -fno-asynchronous-unwind-tables -dM -E a.c) <(gcc -dM -E a.c)
130a131
> #define __GCC_HAVE_DWARF2_CFI_ASM 1
```

This macro is useful because code can decide whether inline asm should include `.cfi_*` directives.
`.cfi_*` directives without `.cfi_startproc` can cause assembler errors
(integrated assembler: `this directive must appear between .cfi_startproc and .cfi_endproc directives`).

Differential Revision: https://reviews.llvm.org/D97743

Author: MaskRay

clang/lib/Driver/ToolChains/Clang.cpp

@@ -404,7 +404,7 @@ shouldUseExceptionTablesForObjCExceptions(const ObjCRuntime &runtime,
 /// master flag, -fexceptions and also language specific flags to enable/disable
 /// C++ and Objective-C exceptions. This makes it possible to for example
 /// disable C++ exceptions but enable Objective-C exceptions.
-static void addExceptionArgs(const ArgList &Args, types::ID InputType,
+static bool addExceptionArgs(const ArgList &Args, types::ID InputType,
                              const ToolChain &TC, bool KernelOrKext,
                              const ObjCRuntime &objcRuntime,
                              ArgStringList &CmdArgs) {
@@ -419,7 +419,7 @@ static void addExceptionArgs(const ArgList &Args, types::ID InputType,
     Args.ClaimAllArgs(options::OPT_fno_objc_exceptions);
     Args.ClaimAllArgs(options::OPT_fcxx_exceptions);
     Args.ClaimAllArgs(options::OPT_fno_cxx_exceptions);
-    return;
+    return false;
   }
 
   // See if the user explicitly enabled exceptions.
@@ -462,6 +462,7 @@ static void addExceptionArgs(const ArgList &Args, types::ID InputType,
 
   if (EH)
     CmdArgs.push_back("-fexceptions");
+  return EH;
 }
 
 static bool ShouldEnableAutolink(const ArgList &Args, const ToolChain &TC,
@@ -4971,14 +4972,15 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   // This is a coarse approximation of what llvm-gcc actually does, both
   // -fasynchronous-unwind-tables and -fnon-call-exceptions interact in more
   // complicated ways.
-  bool AsynchronousUnwindTables =
+  bool UnwindTables =
       Args.hasFlag(options::OPT_fasynchronous_unwind_tables,
                    options::OPT_fno_asynchronous_unwind_tables,
                    (TC.IsUnwindTablesDefault(Args) ||
                     TC.getSanitizerArgs().needsUnwindTables()) &&
                        !Freestanding);
-  if (Args.hasFlag(options::OPT_funwind_tables, options::OPT_fno_unwind_tables,
-                   AsynchronousUnwindTables))
+  UnwindTables = Args.hasFlag(options::OPT_funwind_tables,
+                              options::OPT_fno_unwind_tables, UnwindTables);
+  if (UnwindTables)
     CmdArgs.push_back("-munwind-tables");
 
   // Prepare `-aux-target-cpu` and `-aux-target-feature` unless
@@ -6039,8 +6041,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back("-fapplication-extension");
 
   // Handle GCC-style exception args.
+  bool EH = false;
   if (!C.getDriver().IsCLMode())
-    addExceptionArgs(Args, InputType, TC, KernelOrKext, Runtime, CmdArgs);
+    EH = addExceptionArgs(Args, InputType, TC, KernelOrKext, Runtime, CmdArgs);
 
   // Handle exception personalities
   Arg *A = Args.getLastArg(
@@ -6600,6 +6603,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                        !TC.getTriple().isAndroid() && TC.useIntegratedAs()))
     CmdArgs.push_back("-faddrsig");
 
+  if ((Triple.isOSBinFormatELF() || Triple.isOSBinFormatMachO()) &&
+      (EH || UnwindTables || DebugInfoKind != codegenoptions::NoDebugInfo))
+    CmdArgs.push_back("-D__GCC_HAVE_DWARF2_CFI_ASM=1");
+
   if (Arg *A = Args.getLastArg(options::OPT_fsymbol_partition_EQ)) {
     std::string Str = A->getAsString(Args);
     if (!TC.getTriple().isOSBinFormatELF())

clang/test/Preprocessor/unwind-tables.c

@@ -0,0 +1,10 @@
+// RUN: %clang %s -dM -E -target x86_64-windows | FileCheck %s --check-prefix=NO
+// RUN: %clang %s -dM -E -target x86_64 -fno-asynchronous-unwind-tables | FileCheck %s --check-prefix=NO
+
+// RUN: %clang %s -dM -E -target x86_64 | FileCheck %s
+// RUN: %clang %s -dM -E -target aarch64-apple-darwin | FileCheck %s
+// RUN: %clang %s -dM -E -target x86_64 -fno-asynchronous-unwind-tables -g | FileCheck %s
+// RUN: %clang %s -dM -E -target x86_64 -fno-asynchronous-unwind-tables -fexceptions | FileCheck %s
+
+// NO-NOT: #define __GCC_HAVE_DWARF2_CFI_ASM
+// CHECK: #define __GCC_HAVE_DWARF2_CFI_ASM 1