[LLVMdev] CFG modifcations and code gen

[Dale Johannesen]

On Oct 13, 2008, at 5:14 PMPDT, Villmow, Micah wrote:

> As stated in an earlier email, I am working on getting break/ 
> continue to work correctly for my backend, but I ran into another  
> issue with codegen and the CFG. It seems that code gen is not done  
> based on the CFG, but rather on the block numbers, and the function  
> call MachineFunction::RenumberBlocks doesn’t renumber the blocks  
> based on the CFG. So how can I modify the CFG so that when codegen  
> occurs, it follows the graph and not just does a linear scan over  
> the MachineBasicBlock vector?

It's not intended to work that way.  The BranchFolding pass, which  
runs later, is responsible for rearranging things into a more  
reasonable order.

> In the attached dot file, block 2 gets printed before block 3 and 5  
> and block 4 gets printed between blocks 3 and 5. This obviously is  
> not what the CFG is saying it should be done. The code I’m using to  
> do this transformation, which takes 2 & 4 and places them after  
> block 5 is:
>     MachineBasicBlock* newExitPath = findLowestNumSuccBB(root);
>                 pHigh->addSuccessor(newExitPath);
>                 root->removeSuccessor(newExitPath);
>                 root->CorrectExtraCFGEdges(*root->succ_begin(),  
> NULL, false);
>                 pHigh->CorrectExtraCFGEdges(*pHigh->succ_begin(),  
> newExitPath, false);
>                 newExitPath->moveAfter(pHigh);
>                 root->getParent()->RenumberBlocks();
>
> So what am I doing wrong?
>
> Thanks,
>
>
> Micah Villmow
> Systems Engineer
> Advanced Technology & Performance
> Advanced Micro Devices Inc.
> 4555 Great America Pkwy,
> Santa Clara, CA. 95054
> P: 408-572-6219
> F: 408-572-6596
>
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