[llvm-dev] Update control flow graph when splitting a machine basic block?

[Friedman, Eli via llvm-dev]

On 11/13/2017 6:58 PM, 章明 wrote:
>> It looks like what's happening is that IfConversion makes the return
>> conditional, then that gets merged with the following block.  I mean, I
>> would say it's a bug in that there's a "terminator" in a position not at
>> the end of the block, but a return doesn't have any CFG successors, so
>> I'm not sure it has any practical effect.
>>
>> I think we won't merge with the following block for an indirect branch
>> which is not a return.
> I believe MachineBasicBlock::getFirstTerminator, MachineBasicBlock::getFirstInstrTerminator, and
> Methods/functions that use these methods will break on a machine basic block like this,
> although I have not tested these with such a machine basic block. I don't know whether
> MachineBasicBlock::getFirstTerminator and MachineBasicBlock::getFirstInstrTerminator are
> intended to be used in a late stage like just before code emission or not.

If you really want to prevent the merge, it's probably not hard. See the 
handling for isReturn() in ARMBaseInstrInfo::analyzeBranch, which 
intentionally ignores predicated return instructions.  But it's probably 
simpler from your perspective to do the same thing that analyzeBranch 
does, and ignore predicated return instructions.

-Eli

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