[LLVMdev] memory lifetime and invariance

[Chris Lattner]

On May 19, 2009, at 7:58 PM, Nick Lewycky wrote:

> Chris Lattner wrote:
>> On May 19, 2009, at 12:32 AM, Nick Lewycky wrote:
>>
>>> Chris Lattner wrote:
>>>> FYI, I wrote up some thoughts on this here:
>>>> http://nondot.org/sabre/LLVMNotes/MemoryUseMarkers.txt
>>>>
>>>> The intention is to allow front-ends to express things like "I know
>>>> this memory is a constant in this region" and to allow the  
>>>> optimizer/
>>>> codegen to perform more aggressive stack slot merging.
>>> Very nice!
>>>
>>> Why does @llvm.invariant.end restate the size when it already takes
>>> the
>>> call to @llvm.invariant.start as its first argument? Did you really
>>> mean
>>> to allow for the case where part of an object is still invariant and
>>> part isn't? If so, isn't it missing an offset as well?
>>
>> It is basically because the invariant.start may not be the immediate
>> operand of the invariant.end.  You may end up having phi nodes
>> connecting the two when various phi translation cases occur.
>
> The verifier rejects phi nodes of {} type. Do you actually have a case
> where you want this to happen?

That sounds like a serious bug then: {} should be a first class value  
just like {i32}.

-Chris