[LLVMdev] [Caml-list] Ocaml(opt) & llvm

[Gordon Henriksen]

On Nov 28, 2007, at 15:18, Chris Lattner wrote:

> On Tue, 27 Nov 2007, Gordon Henriksen wrote:
>>
>
>> The codegen for raise is simple. It just reads a saved return  
>> address from the caml_exception_pointer global and returns through  
>> several stack frames in one go.
>
> Nice.

Yup.

>> The try-with expression is where the trickery lies. The expression  
>> try body with pattern1 -> catch1 | patternN -> catchN is compiled to:
>
> This seems pretty straight-forward to normal landing pads.

Certainly the primitives are more or less identical to LLVM's. There  
is a strict stack of handlers, though, and I'm not sure how easy that  
is to reconstruct.

> The only significant difference is how it dynamically registers the  
> callback and how it continues unwinding.

I thought the implications with respect to frame object displacements  
seemed rather invasive. Within the try block, frame offsets are  
progressively increased with each level of nesting.

> If you were interested in adding this to llvm as some new EH  
> intrinsics, I wouldn't be opposed to that.

Okay. I imagine the SJLJ EH would be a good place to start poking  
around if I were to give it a shot?

> It's basically a form of sjlj exceptions.

Right.

> It only works if values are not held in registers across throws  
> though, which is kinda lame ...


Though I'm primarily interested in this model only from an  
interoperability perspective, reloading the register file for a throw  
seems a comparatively small price to pay compared to, say,  
symbolically unwinding the stack. :) More importantly, the common case  
through code does not require a register file save/restore.

— Gordon