[LLVMdev] How to insert basic block in a loop

[Philip Reames]

On 06/16/2015 07:22 AM, marwayusuf at feng.bu.edu.eg wrote:
>
> Thanks a million. Now I managed to insert the block into the loop 
> using the mentioned function. However I have another issue now. I've 
> inserted a block (named foo) in the start of the loop. I want to make 
> the predecessors of the the previous start block (for.cond) to jump to 
> foo instead, and foo jumps to for.cond. Predecessors of for.cond are 
> entry and for.inc. When I run my pass, only for.inc is changed, entry 
> is not. Here is code:
>
>
>           pred_iterator PI = pred_begin(loopCond), PE = 
> pred_end(loopCond);
>           while (PI != PE) {
>             if((*PI)!=foo){
>               BasicBlock::iterator it = (*PI)->end();
>               it--;
>               BranchInst *bi = dyn_cast<BranchInst>(it);
>               bi->setSuccessor(0, foo);
>             }
>             PI++;
>           }
>
> I guessed that the problem is that entry block is not part of the 
> loop, and this is a loop pass. So, if this is this the problem, how 
> can I overcome? And if not, what could be the problem?
You've identified the problem correctly.  The notion your looking for is 
a loop preheader (For LoopSimplify form), or a non-loop contained 
predecessor of the loop header (for generalized loops). You need to 
explicitly locate and visit those blocks.  See LoopInfo.h.

I would strongly suggest you use some of the existing loop transforms 
(LICM, LoopUnswitch, etc...) as examples.  They all have to do similar 
things.
>
>
>
> Regards,
> Marwa Yusuf
> Teaching Assistant - Computer Engineering Department
> Faculty of Engineering - Benha University
> E-JUST PhD Student
> Computer Science & Engineering Dept.
> ------------------------------------------------------------------------
> *From:* Philip Reames <listmail at philipreames.com>
> *Sent:* Friday, June 12, 2015 11:55 PM
> *To:* marwayusuf at feng.bu.edu.eg
> *Cc:* llvmdev at cs.uiuc.edu
> *Subject:* Re: [LLVMdev] How to insert basic block in a loop
> Given lack of context in your question, it's hard to figure out what 
> you're asking, but as a guess, I'd suggest you look at 
> addBasicBlockToLoop on LoopBase in LoopInfo.h
>
> Philip
>
> On 06/12/2015 01:53 PM, marwayusuf at feng.bu.edu.eg wrote:
>>
>> Dear All
>>
>> I'm making a transformation pass that inserts a new basic block at 
>> the start of a loop. However when I try to change 
>> predecessor/successor relations, it does not consider the new block 
>> in the loop at all. So I got that just inserting a loop in a function 
>> before another loop is not enough. So how exactly to do this job?
>>
>>
>> Regards,
>> Marwa Yusuf
>> Teaching Assistant - Computer Engineering Department
>> Faculty of Engineering - Benha University
>> E-JUST PhD Student
>> Computer Science & Engineering Dept.
>>
>>
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>
>
>
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