Fixing LLVM_BUILTIN_TRAP on Windows

[Aaron Ballman]

On Mon, Feb 9, 2015 at 1:49 PM, Reid Kleckner <rnk at google.com> wrote:
> How is raising a 0xdeadd0d0 exception code better than an access violation?

It's considerably more explicit, isn't a hack, and has the extra-added
bonus of not being UB (since we use LLVM_BUILTIN_TRAP from signal
handlers, for instance). I'm not tied to the exception code, but
picking one that's unlikely to be handled by some other exception
filter is a wise choice.

> The only way I think we could functionally improve over the access violation
> is to have something that's known noreturn (like __builtin_trap) and encodes
> smaller, like ud2a.

I disagree, and would point out this isn't a functional improvement
but a bug fix. Assuming that accessing 0x11 as a volatile piece of
memory may suffice for platforms without any way to fail quickly to
our abnormal termination handlers, but that doesn't apply on Windows
where RaiseException will definitely result in calling our vectored
exception handler (unlike __fastfail() on Windows 8 and higher), and
won't be UB even from crazy places like signal handlers.

~Aaron

>
> On Mon, Feb 9, 2015 at 9:02 AM, Aaron Ballman <aaron at aaronballman.com>
> wrote:
>>
>> The implementation for LLVM_BUILTIN_TRAP on Windows leaves quite a bit
>> to be desired. Instead of relying on writing to an invalid volatile
>> pointer, which is hoped to result in raising an exception we can catch
>> with the vectored exception handler, simply raise an exception
>> manually.
>>
>> To prevent having to #include a Win32 header from Compiler.h, this
>> patch forward declares the RaiseException call.
>>
>> The only functional difference between the current implementation and
>> the new one is that RaiseException will appear on the stack traces now
>> when invoking this macro. Eg)
>>
>> e:\llvm\2013>clang -fsyntax-only "E:\Aaron Ballman\Desktop\test.cpp"
>> 0x755EC42D (0xDEADD0D0 0x00000001 0x00000000 0x00000000), RaiseException()
>> + 0x5
>> 8 bytes(s)
>> 0x03D9F222 (0x00692018 0x00000000 0x0685E3C4 0x0685E3A0), `anonymous
>> namespace':
>> :PragmaDebugHandler::HandlePragma() + 0xC2 bytes(s),
>> e:\llvm\llvm\tools\clang\li
>> b\lex\pragma.cpp, line 870 + 0x13 byte(s)
>> 0x03D997DC (0x00692018 0x00000000 0x0685E3C4 0x0685E494),
>> clang::PragmaNamespace
>> ::HandlePragma() + 0xBC bytes(s),
>> e:\llvm\llvm\tools\clang\lib\lex\pragma.cpp, l
>> ine 95 + 0x1B byte(s)
>> ...
>>
>> instead of:
>>
>> e:\llvm\2013>clang -fsyntax-only "E:\Aaron Ballman\Desktop\test.cpp"
>> 0x03CDF1CF (0x00302018 0x00000000 0x0679E5F8 0x0679E5D4), `anonymous
>> namespace':
>> :PragmaDebugHandler::HandlePragma() + 0xAF bytes(s),
>> e:\llvm\llvm\tools\clang\li
>> b\lex\pragma.cpp, line 870
>> 0x03CD979C (0x00302018 0x00000000 0x0679E5F8 0x0679E6C8),
>> clang::PragmaNamespace
>> ::HandlePragma() + 0xBC bytes(s),
>> e:\llvm\llvm\tools\clang\lib\lex\pragma.cpp, l
>> ine 95 + 0x1B byte(s)
>> ...
>>
>> ~Aaron
>
>