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

Friedman, Eli via llvm-dev llvm-dev at lists.llvm.org
Tue Nov 14 11:56:18 PST 2017

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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