[LLVMdev] [LLVMDev] Has anyone written this?

Bob Wilson
Thu, 07 Oct 2010 00:04:30 -0000

I thought that the BranchFolder pass already handled that.  Did you check?

On Oct 6, 2010, at 5:03 PM, Jeff Kunkel wrote:

> It would go something like like the code below. The goal would be to
> turn the basic blocks which the graph looks like
"...->x->y->..."
> where the instructions of x and y could live in the same basic block
> without a jump or fall through in between.
>
>    bool runOnMachineFunction(MachineFunction &mf) {
>      BitVector seen( mf.size() );
>      for( unsigned i = 0, e = mf.size(); i != e; ++i ) {
>        if( seen[i] )
>          continue;
>        seen[i] = true;
>        MachineBasicBlock * start, *block;
>        start = block = mf.getBlockNumbered(i);
>        std::vector< MachineBasicBlock* > blocks;
>        while( block->succ_size() == 1 &&
> (*block->succ_begin())->pred_size() == 1 ) {
>          block = *block->succ_begin();
>          seen[block->getNumber()] = true;
>          blocks.push_back( block );
>        }
>        // TODO:
>        // For each basic block bb in blocks in order of insersion:
>        // 1. Remove basic blocks in the block vector from the machine
function.
>        // 2. Remove the jump from the start block if it exists.
>        // 3. Add the instruction from bb into the start block.
>      }
>
> Thanks,
> Jeff Kunkel
>
> On Wed, Oct 6, 2010 at 7:49 PM, Bob Wilson <bob.wilson at apple.com>
wrote:
>>
>> On Oct 6, 2010, at 4:31 PM, Jeff Kunkel wrote:
>>
>>> Has anyone written a pass at the MachineFunction level which
combines
>>> machine basic blocks which is guaranteed to be the single
predecessor
>>> to another block? Or is there a reason not to combine them?
>>
>> I'm not sure exactly what transformation you're referring to,
but BranchFolder::OptimizeBranches does a lot of things like that.