[LLVMdev] Platform-independent Exception Handling

[Bill Wendling]

On Feb 11, 2013, at 7:39 AM, Anthony Bryant <antjbryant at gmail.com> wrote:

> 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,

That was the intention, anyway. :)

> 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?

They should use the same system. The code to emit DWARF exceptions and debug information is in one place and doesn't have code in it that's target-dependent.

> 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?
> 
I'm not familiar with COFF, but no we would generate the same EH tables regardless of the file format.

>> 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?
> 
Some of the intrinsics are generated by the front-end and the rest are generated during code-generation. The one that the front-end generates is the llvm.eh.typeid.for intrinsic.
However, that intrinsic is used by both DWARF and SjLj exceptions.

> 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.
> 
That's a problem, because bitcode isn't platform independent. :-) But as far as the exception handling in LLVM, it should be platform independent until you get to the code-generation phase.

-bw

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