CFG modifcations and code gen

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

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);



                root->CorrectExtraCFGEdges(*root->succ_begin(), NULL,

newExitPath, false);



So what am I doing wrong?


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 (2.4 KB)

It’s not intended to work that way. The BranchFolding pass, which runs later, is responsible for rearranging things into a more reasonable order. (2.4 KB)

You don’t have to run the branch folding pass, do you?


But, the branch folding pass, or whatever passes are supposed to reorder
the blocks based on the CFG, are not doing so in this case. Otherwise
there is no way that blocks 2 and 4 should be printing out before blocks
3 & 5. Renumber blocks just seems to reorder the values based on their
pre-set block number, but when the CFG is modified these number should
modified also to follow the new ordering, which is not occurring.

Did you implement TargetInstrInfo::AnalyzeBranch for your target? Check out the large comment above it in TargetInstrInfo.h



I took a look at AnalyzeBranch and I don’t see how it can solve my problem. The issue itself isn’t with branching, as I can handle branches fairly well in my custom pass(see the before and after dot files attached). I can take a bunch of branches and construct high level control flow for my backend since I have no ability to do goto/jump, only whileloop and ifs. So analyzing the branch’s isn’t the problem. The problem comes with emit’ing the code. Even though I’ve renumbered the blocks and re-ordered the CFG into a more sane control flow. The code emitter still processes the blocks in the old order. So instead of going from 0-5, it prints out the instructions in the order, 0, 1, 2, 5, 3, 4. This is the order that the old CFG is in, not the new one after this pass is done.

I’ve added this analysis pass as a PreEmitPass, is this the correct location to implement this? Or should I be implementing it earlier? Later?

Thanks for any advice, (2.83 KB) (2.4 KB)

After a bunch more investigate, I’ve seem to have figured out what is going on here. The MachineFunction holds a vector of MachineBasicBlocks and it is this vector that is traversed by the MachineFunction iterator when printing out instructions. The problem is occurring when a modification to the CFG moves around so that the ordering of them is different. Even if the pred/succ blocks are modified, their position in the MBBNumbering vector does not change. This causes the CFG graph and the MachineFunction vector to become out of sync causing all sorts of issues with code generation. (i.e. having my return block being generated in the middle of a loop).

I figured that sorting the vector based on the block id’s would fix this issue, but the constructor required to sort correctly is marked as private.

So, my basic need is this. How do I re-order the MachineBasicBlocks vector based on their Block ID numbers via the current API(2.3)?

The branch folding pass moves around blocks, please take a look at its implementation to see how it works,