[clang] [MinGW] Use SEH for unwind info on ARM by default

[mstorsjo]

Summary

This goes hand in hand with D126870.

[efriedma]

LGTM

[efriedma]

Oh, just remembered, we should probably release-note this. (You can post that as a followup.)

[mstorsjo]

In D126871#3560924, @efriedma wrote:

Oh, just remembered, we should probably release-note this. (You can post that as a followup.)

Sure, can do.

Btw, thanks a lot for your reviews, you've spent a lot of time on them! They did improve the end result a lot, and your pointers to e.g. specific passes that do things was very valuable - that's something that is hard to figure ouy by just grepping around in the code.

Also regarding announcing completion - at this point, the unwind info generation is complete, and itanium style unwinding works. But for MSVC style (C++ exceptions and __try/__except) there's still a couple backend things that need to be implemented. Do you happen to know roughly how much effort that is? I've essentially achieved what I wanted here, but it would be nice just for completeness to have those parts implemented too, but it's a lower priority for me.

[efriedma]

Btw, thanks a lot for your reviews, you've spent a lot of time on them!

You're welcome.

But for MSVC style (C++ exceptions and try/except) there's still a couple backend things that need to be implemented. Do you happen to know roughly how much effort that is?

Getting funclets working is a significant amount of code; see D51524 .

Beyond that, it's not that much work, I think? There's probably some random little bits I'm not remembering, but most of the code is shared across x64/aarch64/arm, I think.

[mstorsjo]

In D126871#3561243, @efriedma wrote:

But for MSVC style (C++ exceptions and try/except) there's still a couple backend things that need to be implemented. Do you happen to know roughly how much effort that is?

Getting funclets working is a significant amount of code; see D51524 .

Yikes - ok, thanks, that's good to know. Are funclets also needed for __try/__except?

[efriedma]

Funclets are necessary for both C++ and SEH exception handling, yes.

clang/lib/Driver/ToolChains/MinGW.cpp

 
 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 ||
-         getArch() == llvm::Triple::aarch64;
+  return getArch() == llvm::Triple::x86_64 || getArch() == llvm::Triple::arm ||
+         getArch() == llvm::Triple::thumb || getArch() == llvm::Triple::aarch64;
 }
 
 bool toolchains::MinGW::isPICDefault() const {
   return getArch() == llvm::Triple::x86_64 ||
          getArch() == llvm::Triple::aarch64;
 }
 
 bool toolchains::MinGW::isPIEDefault(const llvm::opt::ArgList &Args) const {
   return false;
 }
 
 bool toolchains::MinGW::isPICDefaultForced() const { return true; }
 
 llvm::ExceptionHandling
 toolchains::MinGW::GetExceptionModel(const ArgList &Args) const {
-  if (getArch() == llvm::Triple::x86_64 || getArch() == llvm::Triple::aarch64)
+  if (getArch() == llvm::Triple::x86_64 || getArch() == llvm::Triple::aarch64 ||
+      getArch() == llvm::Triple::arm || getArch() == llvm::Triple::thumb)
     return llvm::ExceptionHandling::WinEH;
   return llvm::ExceptionHandling::DwarfCFI;
 }
 
 SanitizerMask toolchains::MinGW::getSupportedSanitizers() const {
   SanitizerMask Res = ToolChain::getSupportedSanitizers();
   Res |= SanitizerKind::Address;
   Res |= SanitizerKind::PointerCompare;

clang/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 armv7-windows-gnu -fdwarf-exceptions -c %s -### 2>&1 | FileCheck -check-prefix=MINGW-DWARF %s
+// RUN: %clang -target armv7-windows-gnu -c %s -### 2>&1 | FileCheck -check-prefix=MINGW-SEH %s
 // RUN: %clang -target aarch64-windows-gnu -fdwarf-exceptions -c %s -### 2>&1 | FileCheck -check-prefix=MINGW-DWARF %s
 // RUN: %clang -target aarch64-windows-gnu -c %s -### 2>&1 | FileCheck -check-prefix=MINGW-SEH %s
 
 MSVC-NOT: -exception-model=dwarf
 MSVC-NOT: -exception-model=seh
 MINGW-DWARF: -exception-model=dwarf
 MINGW-SEH: -funwind-tables=2
 MINGW-SEH: -exception-model=seh