[LLVMdev] Mapping document questions

[Sean Silva]

On Tue, Dec 17, 2013 at 2:35 PM, Mikael Lyngvig <mikael at lyngvig.org> wrote:

> These questions are all related to the "Mapping High-Level Constructs to
> LLVM IR" document:
>
> 1. What are the \01 characters that are prefixed to dllimport Windows
> symbols for?  I have never seem them before so I am unsure if they are
> Clang/LLVM-specific?
>

I expect it's some form of mangling that is recognized by the
linker/loader, and I doubt it's LLVM-specific. You'll probably want to look
on MSDN. btw, ObjC also uses something like this in its mangling.


> 2. Anybody interested in explaining the zero cost exception handling
> mechanism using an existing example from the Mapping document?  This
> requires coding up a working example in LLVM IR or simplifying a
> compiler-generated example so that it becomes easy to read.
>

Maybe try looking at the C++ abi spec? <
http://mentorembedded.github.io/cxx-abi/abi-eh.html> Alternatively, try
looking at the source code of some existing frontends which use zero cost
exception handling. As far as I understand it, there is a nontrivial
runtime component needed for doing this.


>
> 3. Anybody interested in explaining the various forms of constant
> expressions that exist in LLVM IR?
>

I don't know very much about them, but they basically mirror the regular
instructions. I don't think it's the best-thought-out part of the IR.


>
> 4. Do any of you have a simple yet realistic example of an address-space
> cast?
>

addrspacecast is relatively new and serves a pretty specialized purpose.
Don't worry about it.

>
> 5. Anybody interested in documenting the insertvalue and extractvalue
> instructions?  I haven't figured out how to get a structure into a register
> yet.
>

I would expect a struct-typed `load` would do the trick.


>
> 6. Anybody interested in documenting closures?
>

The first thing you want to do is to survey the different approaches to
closures and their respective pros/cons. That's a lot more important than
giving a specific example of a particular approach. Again, looking at
existing language frontends is probably the right approach.

-- Sean Silva


>
> P.S. I sent off the document to the LLVM-commits list earlier today, but
> David Blaikie pointed out that the document should be in a state where it
> can be published before committing it, so that's what I am working on now.
>
> For the time being, the document can be found at:
>
>
> https://github.com/archfrog/llvm-doc/blob/master/MappingHighLevelConstructsToLLVMIR.rst
>
> As soon as it is ready to be published, it will be moved to the llvm/docs
> folder.
>
>
> -- Mikael
>
>
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