[llvm-commits] [llvm] r75567 - /llvm/trunk/lib/VMCore/Instruction.cpp

[Dan Gohman]

On Jul 15, 2009, at 2:20 AM, Owen Anderson wrote:


>
> On Jul 15, 2009, at 2:07 AM, Eli Friedman wrote:
>
>
>> On Wed, Jul 15, 2009 at 1:39 AM, Duncan Sands<baldrick at free.fr>  
>> wrote:
>>
>>>> FreeInst is trapping.
>>>>
>>>
>>>
>>> Really?
>>>
>>
>>
>> Anything that touches memory is trapping because it'll crash on an
>>
>> invalid pointer.
>>
>
> Does free() actually dereference the pointer?  I don't think so,  
> ergo no trapping.
>
> It's probably undefined what happens when you free an invalid  
> pointer, so we can choose to do whatever we want with it, but I see  
> no particular reason to require preserving trapping behavior for it.

isTrapping() also includes things like div and rem which have undefined
behavior when the remainder is 0. The point of isTrapping is not to
test for actual trapping, but rather to test for anything that would
make naively moving an instruction around the CFG dangerous.

mayHaveSideEffects() doesn't do the same thing, since it only tests for
mayWriteToMemory() || mayThrow().

Perhaps mayHaveSideEffects() should be renamed to
mayHaveDefinedSideEffects(), and isTrapping() should be renamed to
mayHaveUndefinedBehavior()?

Dan