llvm dev - May 2010 - [LLVMdev] Separate loop condition and loop body

Hi,

Is it possible to get the list of BasicBlocks building the condition of a
loop and the list of BasicBlocks that form the body? My first approach was
to iterate over the list of all Basicblocks of a loop and separate the
header as the condition and the remaining as the body of the loop. However,
this is not correct for instance in the case of a while loop in the form:
while( A & B) do
{  body  }

My aim is to manipulate for loops, while and do-while loops unitary, but I
did not find an easy way to do this. I tried also to apply passes like
"Canonicalize natural loops", "Canonicalize Induction
Variables" or "Natural
Loop Construction" such that the loops were represented in the same form.
Still I do not find a one-to-one link between the loop condition, body and
end,  and the BasicBlocks returned by the getHeader, getLoopLatch etc.

My next strategy would be to modify the front-end, clang, and attach
metadata to indicate whether the BasicBlock is part of the condition or of
the body of the loop.

I would very much appreciate if you could suggest an easier way to
differentiate between the parts of the loop.

Thank you,
Alexandra
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On May 10, 2010, at 8:43 AM, Xinfinity wrote:
> Is it possible to get the list of BasicBlocks building the condition  
> of a
> loop and the list of BasicBlocks that form the body?
Based on my (limited) experience with Loop and LoopInfo, this isn't  
possible. (But don't take my word for it.)
> My aim is to manipulate for loops, while and do-while loops unitary,  
> but I
> did not find an easy way to do this.
I think the problem here is that LLVM's CFG is low-level enough such  
that the distinction between the loop header expression and the loop  
body is lost.

Why exactly do you need to identify the blocks that form the loop  
header expression? I'm wondering if you could find some workaround  
that doesn't require this...

Trevor

On Mon, May 10, 2010 at 8:05 PM, Trevor Harmon <Trevor.W.Harmon at
nasa.gov> wrote:
> On May 10, 2010, at 8:43 AM, Xinfinity wrote:
>
>> Is it possible to get the list of BasicBlocks building the condition
>> of a loop and the list of BasicBlocks that form the body?
>
> Based on my (limited) experience with Loop and LoopInfo, this isn't
> possible. (But don't take my word for it.)
>
>> My aim is to manipulate for loops, while and do-while loops unitary,
>> but I did not find an easy way to do this.
>
> I think the problem here is that LLVM's CFG is low-level enough such
> that the distinction between the loop header expression and the loop
> body is lost.
>
> Why exactly do you need to identify the blocks that form the loop
> header expression? I'm wondering if you could find some workaround
> that doesn't require this...
>
To me it looks like any basic block from the loop body with a
successor not in the loop body is a BB "building the condition" (an
"exit" block).
If you already have the loop body, it should be pretty easy to find
out about those nodes.

cheers,

Benoit