[LLVMdev] not to break 'for' statement into basic blocks

[David A. Greene]

On Saturday 14 July 2007 17:23, Anton Korobeynikov wrote:

> > 1) First, I tried to re-unite basic blocks which llvm-gcc spits out to
> > make 'for' again. But this is quite tricky. Generalizing it for the
> > optimzed llvm bytecode is not easy.
>
> I'd say 'is not possible at all'.

No, it certainly is possible.  One does this, for example, when constructing
a control dependence graph.  It can't be represented in llvm, so one needs
a higher-level abstraction.  A control dependence graph is one such 
abstraction.

Even llvm has a notion of "loops" that it extracts from the control flow 
graph.  Now, these are very low-level abstractions and probably won't work
for the purposes of this ISA.

And when you get difficult control behavior...

> However, I really don't see, how you can transform C code (in general)
> to your target, because C *has* explicit goto statement. Maybe you can
> split the whole CFG into "cycles" and "paths" and try to "lower" them.
> But this can be pretty complicated.

Again, gotos can be eliminated with some program analysis and transformation.
See, for example, http://citeseer.ist.psu.edu/22521.html.

                                           -Dave