[LLVMdev] The most efficient way to compile to LLVM IR?

[David Blaikie]

On Mon, Apr 15, 2013 at 3:32 PM, Hongbo Zhang <bobzhang1988 at gmail.com> wrote:
> Hi all,
>    I am trying to compile my toy language to LLVM back end. (I am new to
> LLVM, so my questions may sound naive)
>    I am looking at some tutorials about LLVM, most are about how to use LLVM
> IRBuilder, however, I find the API provided by IRBuilder is quite imperative
> and verbose, and the API changes so fast that most of the tutorials are out
> of dated.
>    So I am wondering what's the benefit of emitting LLVM IR using IRBuilder
> compared with designing my own abstract syntax in a high-level programming
> language(e.g. Haskell or OCaml) and unparsing it to LLVM IR.

To be clear you're suggesting having your frontend (say, for
argument's sake, written in C++) parse your toy language and then emit
a (say) Haskell representation of IR? Using some Haskell APIs you'll
write that will emit LLVM bitcode? And then running the resulting
Haskell program to produce your bitcode that you'll load back in to
LLVM to optimize/compile?

> Is there some
> benefit of using IRBuilder I ignored here?

It'll be more efficient to keep the IR in memory rather than to go out
to a source file, run that file to produce bitcode, then load that
bitcode in to LLVM.

Also I'm not sure I see quite how that scheme would be less verbose.

> And I have some follow-up
> questions: 1. How stable is the IR format?

There's in-built autoupgrade so that you can load old IR in newer
versions of LLVM (any 3.* series should be compatible, I believe).

> 2. Is the binary representation
> of IR format (*.bc) stable and the same across different platforms?

It's the same format, but the actual bitcode isn't retargetable, as
such. (ie: don't expect to be able to produce bitcode that you can
compile for different architectures)

> 3. Is
> there any previous work of building a declarative`interface instead of using
> IRBuilder?

Not that I know of. The frontends tend to use IRBuilder.

>    Thank you in advance!
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