[clang] [llvm] Enable fexec-charset option (PR #138895)

[Sergei Barannikov via cfe-commits]

s-barannikov wrote:

> The -fexec-charset option used for the c++ library may not be the same as in the user source. The design point is to have one typeinfo object per type. In a simple example we can have two source files defining two types compiled with two different -fexec-charset options. This will result in typeinfo objects being defined in three different places:
> 
> 1. the basic types in standard c++ library
> 2. type T1 defined in the first source file
> 3. type T2 defined in the second source file
> 
> If these are all in different code pages, users won't be able to know what code page is being returned by name().

Linking objects compiled with different -fexec-charset option would be a programmer mistake because the objects are not ABI compatible (in some sense). You can't safely pass strings across object boundaries if the objects are compiled with different -fexec-charset options. For example, if your program compiled with -fexec-charset=ascii tries to examine an ebcdic-encoded string returned by type_info::name() (e.g., tries to demangle it), it will fail. If you compile libc++-/abi with -fexec-charset=ascii, you will get rubbish in "std::terminate called after throwing an exception of type std::bad_alloc", because the "std::bad_alloc" part will be demangled using the wrong (ASCII) decoder.

These are the same problems you get when passing any strings hardcoded in any library to another library compiled with different -fexec-charset option. I still don't see how type_info::name() is different.


https://github.com/llvm/llvm-project/pull/138895