[clang] Fix catching pointers by reference on mingw targets (llvm#162546)

For this specific case, when catching a pointer data type, by reference,
Clang generates a special code pattern, which directly accesses the
exception data by skipping past the `_Unwind_Exception` manually (rather
than using the return value of `__cxa_begin_catch`).

On most platforms, `_Unwind_Exception` is 32 bytes, but in some
configurations it's different. (ARM EHABI is one preexisting case.) In
the case of SEH, it's also different - it is 48 bytes in 32 bit mode and
64 bytes in 64 bit mode. (See the SEH ifdef in `_Unwind_Exception` in
`clang/lib/Headers/unwind.h`.)

Handle this case in `TargetCodeGenInfo::getSizeOfUnwindException`,
fixing the code generation for catching pointers by reference.

This fixes mstorsjo/llvm-mingw#522.

(cherry picked from commit 10be254)

Author: mstorsjo

clang/lib/CodeGen/TargetInfo.cpp

@@ -75,6 +75,8 @@ TargetCodeGenInfo::~TargetCodeGenInfo() = default;
 // If someone can figure out a general rule for this, that would be great.
 // It's probably just doomed to be platform-dependent, though.
 unsigned TargetCodeGenInfo::getSizeOfUnwindException() const {
+  if (getABIInfo().getContext().getLangOpts().hasSEHExceptions())
+    return getABIInfo().getDataLayout().getPointerSizeInBits() > 32 ? 64 : 48;
   // Verified for:
   //   x86-64     FreeBSD, Linux, Darwin
   //   x86-32     FreeBSD, Linux, Darwin

clang/test/CodeGenCXX/sizeof-unwind-exception.cpp

@@ -3,6 +3,8 @@
 // RUN: %clang_cc1 -triple x86_64-apple-darwin10 -emit-llvm -fcxx-exceptions -fexceptions %s -O2 -o - | FileCheck %s --check-prefix=ARM-DARWIN
 // RUN: %clang_cc1 -triple arm-unknown-gnueabi -emit-llvm -fcxx-exceptions -fexceptions %s -O2 -o - | FileCheck %s --check-prefix=ARM-EABI
 // RUN: %clang_cc1 -triple mipsel-unknown-unknown -emit-llvm -fcxx-exceptions -fexceptions %s -O2 -o - | FileCheck %s --check-prefix=MIPS
+// RUN: %clang_cc1 -triple x86_64-windows-gnu -emit-llvm -fcxx-exceptions -fexceptions -exception-model=seh %s -O2 -o - | FileCheck %s --check-prefix=MINGW-X86-64
+// RUN: %clang_cc1 -triple thumbv7-windows-gnu -emit-llvm -fcxx-exceptions -fexceptions -exception-model=seh %s -O2 -o - | FileCheck %s --check-prefix=MINGW-ARMV7
 
 void foo();
 void test() {
@@ -25,9 +27,15 @@ void test() {
 // ARM-EABI-NEXT:   [[T1:%.*]] = getelementptr i8, ptr [[EXN]], i32 88
 // MIPS:            [[T0:%.*]] = tail call ptr @__cxa_begin_catch(ptr [[EXN:%.*]]) [[NUW:#[0-9]+]]
 // MIPS-NEXT:       [[T1:%.*]] = getelementptr i8, ptr [[EXN]], i32 24
+// MINGW-X86-64:     [[T0:%.*]] = tail call ptr @__cxa_begin_catch(ptr [[EXN:%.*]]) [[NUW:#[0-9]+]]
+// MINGW-X86-64-NEXT:[[T1:%.*]] = getelementptr i8, ptr [[EXN]], i64 64
+// MINGW-ARMV7:      [[T0:%.*]] = tail call arm_aapcs_vfpcc ptr @__cxa_begin_catch(ptr [[EXN:%.*]]) [[NUW:#[0-9]+]]
+// MINGW-ARMV7-NEXT: [[T1:%.*]] = getelementptr i8, ptr [[EXN]], i32 48
 
 // X86-64: attributes [[NUW]] = { nounwind }
 // X86-32: attributes [[NUW]] = { nounwind }
 // ARM-DARWIN: attributes [[NUW]] = { nounwind }
 // ARM-EABI: attributes [[NUW]] = { nounwind }
 // MIPS: attributes [[NUW]] = { nounwind }
+// MINGW-X86-64: attributes [[NUW]] = { nounwind }
+// MINGW-ARMV7: attributes [[NUW]] = { nounwind }