[LLVMdev] Platform-independent Exception Handling

[Anthony Bryant]

Hi Bill,

Thanks for the response, I just have a couple more questions.

On 11 February 2013 06:45, Bill Wendling <wendling at apple.com> wrote:
> As far as which OS/architectures support zero-cost EH and which
> support SJLJ, LLVM is designed to assume that Intel supports zero-cost
> exceptions and ARM supports SjLj exceptions. However, it's all in the
> personality function and unwind library. So it's not required that ARM
> use only SJLJ, it's just how it works because of the personality
> function and unwind library. :)

So it sounds like it's completely OS independent, but I just want to
confirm - do the COFF and Mach-O outputs use the same system? Do I need
to do anything special in order to get them to work?
For example, do they generate different exception handling tables (using
something other than the DWARF format), meaning that I'd have to write
separate personality functions?

> The invoke/landingpad/resume instructions are meant to be platform
> independent. However, there is a "lowering" phase that occurs before
> code generation that converts those instructions into the form needed
> for the EH support on the architecture we're compiling for. And during
> code generation, we generate different EH tables based on that
> decision. So basically it's converted into platform-specific code
> right before code generation happens.
>
> That's a long-winded way of saying that you should use our current EH
> model and assume zero-cost exceptions unless you're on ARM (there is
> an ARM EABI which isn't SJLJ-based (from my understanding), but I
> don't know the status of that in LLVM) or have been proven otherwise.

Do I need to do anything different to generate SJLJ exception-handling
bitcode? There are some intrinsics documented at
http://llvm.org/docs/ExceptionHandling.html#llvm-eh-sjlj-setjmp - but
there doesn't seem to be any documentation about how to use them. Are
calls to them generated by an LLVM pass, or do I need to call them
myself?

My problem is that I want my bitcode to be platform independent, so if
SJLJ requires me to generate special calls to intrinsics then I can't
use LLVM's exception handling at all until LLVM supports one way of
doing it on all platforms.

Thanks,
Anthony.