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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-codegen

Author: Martin Storsjö (mstorsjo)

<details>
<summary>Changes</summary>

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 https://github.com/mstorsjo/llvm-mingw/issues/522.

---
Full diff: https://github.com/llvm/llvm-project/pull/162546.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/TargetInfo.cpp (+2) 
- (modified) clang/test/CodeGenCXX/sizeof-unwind-exception.cpp (+8) 


``````````diff
diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index 1e58c3f217812..342a3af0ac1ee 100644
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -82,6 +82,8 @@ 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().getCodeGenOpts().hasSEHExceptions())
+    return getABIInfo().getDataLayout().getPointerSizeInBits() > 32 ? 64 : 48;
   // Verified for:
   //   x86-64     FreeBSD, Linux, Darwin
   //   x86-32     FreeBSD, Linux, Darwin
diff --git a/clang/test/CodeGenCXX/sizeof-unwind-exception.cpp b/clang/test/CodeGenCXX/sizeof-unwind-exception.cpp
index 4fb977a5367e7..e40b2d7ae43ea 100644
--- a/clang/test/CodeGenCXX/sizeof-unwind-exception.cpp
+++ b/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 }

``````````

</details>


https://github.com/llvm/llvm-project/pull/162546