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

[Evan Cheng]

On Sep 24, 2010, at 12:07 PM, Evan Cheng wrote:

> 
> 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.

Tried it. No miscompilations on Darwin.

Evan

> 
> Evan
> 
>> 
>> /jakob
>> 
> 
> 
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