Incompatible behavior when catching exceptions

[hebasto]

Here is a minimal reproducible example:

#include <iostream>

int main()
{
    try {
        throw "EX";
    } catch (const char* const& e) {
        std::cout << "e=" << e << '\n';
    }
}

The binary produced by the llvm-mingw 20250924 with LLVM 21.1.2 toolchain prints garbage output, unlike binaries built with other toolchains.

This issue breaks the BOOST_<level>_EXCEPTION macros.

[mstorsjo]

Thanks for the nicely reduced bug report; I've managed to reproduce it, and I understand what the issue is.

In short, the following patch is needed to fix it:

diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index 1e58c3f21781..342a3af0ac1e 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

This is required because Clang seems to generate slightly more clever (or at least attempting to be clever) code for catches with references to pointer variables. (I don't entirely understand why the same couldn't be done for references to any other type, but I didn't try to dig into that bit.) https://github.com/llvm/llvm-project/blob/llvmorg-21.1.3/clang/lib/CodeGen/ItaniumCXXABI.cpp#L4869-L4891 GCC doesn't seem to do the same trick; it generates the exact same code here, regardless of whether we're doing catch (const char* &e) or catch (const char* e). And only in this case, Clang seems to need the value of getSizeOfUnwindException().

I'll send a patch with this for upstream (and I'll try to get it backported into later LLVM 21.x releases) when I get it complete for review (i.e. including testcases).

[mstorsjo]

Fix posted in llvm/llvm-project#162546.

[mstorsjo]

Reopening this issue here until the fix is available in an actual release. The fix will be available in tomorrow's nightly (not today's which probably appears in a couple hours) at https://github.com/mstorsjo/llvm-mingw/releases/nightly (or whenever there's the next fully successful nightly), and hopefully also backported to a 21.x point release soon thereafter.