[llvm-dev] Generating a loop from llvm bitcode

[David Greene via llvm-dev]

"Ejjeh, Adel via llvm-dev" <llvm-dev at lists.llvm.org> writes:

> Hello 
>
> I am developing a backend that generates OpenCL from llvm bitcode. I
> start with a revived fork of the original LLVM C-Backend and have been
> modifying it. 

How far do you intend to take this?  Reconstructing high-level control
flow from a CFG is not possible in the general case without code
duplication.  One can do quite a lot but the nature of low-level
optimization means that at some point you'll likely need gotos, unless
code expansion is not a concern.  And even then, the result won't look
much like the source program (though you probably don't care).

Beyond that, reconstructing high-level characteristics of values
(finding induction variables, getting proper array dimensions, etc.) is
even more challenging.  One can't reconstruct original C struct types,
for example, because many of them get merged into a single LLVM type.
One might be able to do a bit better with TBAA but frontends would have
to save a lot more information in the IR to allow general translation.

There are many things in LLVM that cannot be expressed in C.  C has no
"shufflevector" operation, for example.  At best one can try to emit
(non-portable) intrinsics which may or may not be acceptable.  Or emit
some kind of awful loop, I guess.

The C backend was riddled with problems even beyond the above issues.
If I were going to attempt something like this I don't think I'd start
with that.

It's not a hopeless project as long as you manage your expectations.  :)

                          -David