[LLVMdev] Directly emit LLVM IR?

[Gordon Henriksen]

On Feb 22, 2008, at 11:11, Eric Jonas wrote:

> Hello! I'm interested in using LLVM as a target for a compiler I've  
> written in common lisp (SBCL). While I looked at perhaps wrapping  
> the LLVM C++ interface, wrapping C++ in, well, anything not C++ is a  
> pain. Someone on IRC mentioned that they didn't think I'd miss out  
> on any functionality by directly emitting IR, but suggested I query  
> the list.
>
> Do I miss out on any optimizations or other neat trickery by using  
> the IR directly? Does anyone know of any other platforms which  
> directly target LLVM via emitting IR?

Hi Eric,

There are 3 major ways to tackle generating LLVM IR from a front-end:

• Embed the LLVM C++ code.
for: best tracks changes to the LLVM IR, .ll syntax, and .bc format
for: enables running LLVM optimization passes without a emit/parse cycle
for: adapts well to a JIT context
against: lots of ugly glue code to write

• Emit LLVM assembly from your compiler's native language.
for: very straightforward to get started
against: the .ll parser is slower than the bitcode reader when  
interfacing to the middle end
against: you'll have to re-engineer the LLVM IR object model and asm  
writer in your language
against: it may be harder to track changes to the IR

• Emit LLVM bitcode from your compiler's native language.
for: can use the more-efficient bitcode reader when interfacing to the  
middle end
against: you'll have to re-engineer the LLVM IR object model and  
bitcode writer in your language
against: it may be harder to track changes to the IR

If you go with the first option, the C bindings in include/llvm-c  
should help a lot, since most languages have C FFIs. The C interface  
was designed to require very little manual memory management, and so  
is fairly straightforward to talk to with most FFIs.

— Gordon

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