[llvm-commits] [llvm] r114681 - /llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

[Evan Cheng]

On Sep 24, 2010, at 11:49 AM, Jakob Stoklund Olesen wrote:

> 
> On Sep 24, 2010, at 10:48 AM, Evan Cheng wrote:
> 
>> There is definitely a codegen bug downstream. A unconditional branch to a fallthrough block is correct the last time I checked. If someone has some time to look into this, I would appreciate a test case.
> 
> It looks like MachineBasicBlock::canFallThrough affects codegen which could trigger this bug, and MachineBasicBlock::updateTerminator looks like it is going to undo it anyway by removing the unconditional branch.
> 
> Is any code assuming the presence of an unconditional branch?

I sure hope not. The reason for this change some DAG transformations require the presence of the unconditional branch following a conditional one so it can flip them. With this case, a lot of fcmp_oeq can be optimized to a pair of conditional branches, rather than two setcc and an and.

I've verified machine instruction passes will eliminate the branch to fallthrough. I wonder if we could disable branch folding and see if more miscompilations show up.

Evan

> 
> /jakob
>