[Driver] Automatically enable -munwind-tables if -fseh-exceptions is enabled

[mstorsjo]

Summary

For targets where SEH exceptions are used by default (on MinGW, only x86_64 so far), -munwind-tables are added automatically. If
-fseh-exeptions is enabled on a target where SEH exeptions are availble but not enabled by default yet (aarch64), we need to pass -munwind-tables if -fseh-exceptions was specified.

Once things have settled and this works fine in practice, the default can be switched from dwarf to SEH. Or is this unnecessary churn and we should just keep off this until the default can be changed?

[rnk]

lib/Driver/ToolChains/Clang.cpp

3929 ↗

(On Diff #178410)

Can you put this logic in the Windows or mingw toolchain instead by overriding IsUnwindTablesDefault? -fseh-exceptions feels fairly target specific.

If not, this boolean default feels fairly complicated. Here's how I would restructure it:

bool UnwindTables = false;
if (!Freestanding)
  UnwindTables = TC.IsUnwindTablesDefault(Args) || TC.getSanitizerArgs().needsUnwindTables();
UnwindTables = Args.hasFlag(OPT_fasync..., OPT_fno_async..., UnwindTables);
UnwindTables = Args.hasFlag(OPT_munwind_tables, OPT_mno_unwind_tables, UnwindTables);

Basically, we have some defaults dictated by the toolchain, and then two boolean flags, where one takes precedence over the other. -fno-async by itself will disable unwind tables, but -munwind-tables -fno-asynchronous-unwind-tables will leave them enabled, despite not following the usual "last flag wins" logic.

[mstorsjo]

lib/Driver/ToolChains/Clang.cpp

3929 ↗

(On Diff #178410)

Can you put this logic in the Windows or mingw toolchain instead by overriding IsUnwindTablesDefault? -fseh-exceptions feels fairly target specific.

Yes, that seems to be doable. Thanks!

[mstorsjo]

Moved the added code to toolchains::MinGW::IsUnwindTablesDefault.

[rnk]

lgtm

lib/Driver/ToolChains/MinGW.cpp

Tool *toolchains::MinGW::buildLinker() const {
   return new tools::MinGW::Linker(*this);
 }
 
 bool toolchains::MinGW::HasNativeLLVMSupport() const {
   return NativeLLVMSupport;
 }
 
 bool toolchains::MinGW::IsUnwindTablesDefault(const ArgList &Args) const {
+  Arg *ExceptionArg = Args.getLastArg(options::OPT_fsjlj_exceptions,
+                                      options::OPT_fseh_exceptions,
+                                      options::OPT_fdwarf_exceptions);
+  if (ExceptionArg &&
+      ExceptionArg->getOption().matches(options::OPT_fseh_exceptions))
+    return true;
   return getArch() == llvm::Triple::x86_64;
 }
 
 bool toolchains::MinGW::isPICDefault() const {
   return getArch() == llvm::Triple::x86_64;
 }
 
 bool toolchains::MinGW::isPIEDefault() const { return false; }

test/Driver/windows-exceptions.cpp

 // RUN: %clang -target i686-windows-msvc -c %s -### 2>&1 | FileCheck -check-prefix=MSVC %s
 // RUN: %clang -target x86_64-windows-msvc -c %s -### 2>&1 | FileCheck -check-prefix=MSVC %s
 // RUN: %clang -target i686-windows-gnu -c %s -### 2>&1 | FileCheck -check-prefix=MINGW-DWARF %s
 // RUN: %clang -target x86_64-windows-gnu -c %s -### 2>&1 | FileCheck -check-prefix=MINGW-SEH %s
+// RUN: %clang -target aarch64-windows-gnu -c %s -### 2>&1 | FileCheck -check-prefix=MINGW-DWARF %s
+// RUN: %clang -target aarch64-windows-gnu -fseh-exceptions -c %s -### 2>&1 | FileCheck -check-prefix=MINGW-SEH %s
 
 MSVC-NOT: -fdwarf-exceptions
 MSVC-NOT: -fseh-exceptions
 MINGW-DWARF: -fdwarf-exceptions
+MINGW-SEH: -munwind-tables
 MINGW-SEH: -fseh-exceptions