[PATCH] Clarify wording in the LangRef around !invariant.load

[Pete Cooper]

> On Nov 20, 2014, at 5:20 PM, Philip Reames <listmail at philipreames.com> wrote:
> 
> 
> On 11/20/2014 04:36 PM, Pete Cooper wrote:
>> Hi Philip
>> 
>> I added invariant.load in this commit: http://llvm.org/viewvc/llvm-project?view=revision&revision=150257 <http://llvm.org/viewvc/llvm-project?view=revision&revision=150257>
>> 
>> I’m not arguing against the changes you are proposing, but I just wanted to give some clarification on the original intention of this metadata, just incase that changes things.
>> 
>> So in Objective-C, globals such as “*SELECTOR_REFERENCES*” are initialized once, by the runtime.  They are then essentially constant.  Unfortunately they aren’t compile time constants or we could just put them in the appropriate section.
>> 
>> This metadata was meant to solve exactly this situation, but never considered for any others.  In particular, its undefined behavior in Objective-C if anyone did chase pointers through the runtime and find a way to modify one of these.  It’s therefore never expected that you’ll see a store, call, or anything else, in any way modify one of these.
> Makes sense.  Just to be clear, did you intend for invariant.load to imply that the address of the selector variable was always dereferenceable?  The current *implementation* doesn't seem to     assume that, but what's the history?
In every example I ever saw, the address was actually a @global so was always dereferencable..  I guess i would be possible to have a PHI over selector references and then you aren’t pointing directly at a global, but the load was always assumed to read from a global one way or another.
>> 
>> TBH this metadata is probably the wrong thing for selector references, or at least its name is.  What we really needed was some way to describe 'externally initialized once before first use’ which implies that its always invariant anyway.  That just never occurred to me at the time of writing.
> I'm trying to figure out how to support "initialized once (in visible IR) before first use".  The goal of this thread is really just to clarify what the current semantics are.  I think the current semantics actually implement both; I'm just trying to confirm this.  
Agreed.  I think they do implement both right now.

Thanks,
Pete
> 
> 
>> 
>> Thanks,
>> Pete
>>> On Nov 20, 2014, at 4:25 PM, Philip Reames <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>>> 
>>> 
>>> On 11/20/2014 04:14 PM, Sanjoy Das wrote:
>>>>> To clarify that the invariant nature of a given pointer does not preclude the modification of that location through a pointer which is unrelated to the load operand. In particular, initializing a location and then passing a pointer through an opaque intrinsic which produces a new unrelated pointer, should behave as expected provided that the intrinsic is memory dependent on the initializing store.
>>>> Does this mean you cannot common loads for escaped memory over calls?  The call could have modified the pointed-to location using another pointer.
>>> No, it does not.  The optimizer is free to assume that the location as seen through *this* pointer is invariant.  Whether another copy of the pointer escaped is irrelevant to that assumption.  The call may contain a "potentially aliasing store" in my proposed wording.
>>> 
>>> The scheme I was trying to describe in my clarification (not LangRef wording!) was this:
>>> %p = ...
>>> store i32 5, i32* %p
>>> %p2 = call i32* @opaque_call(i32* %p)
>>> %v = load i32* %p2, !invariant !{}
>>> 
>>> The fact that %p and %p2 might happen to be the same location is irrelevant to the fact that the location as seen by %p2 is invariant and the location as seen by %p is not.  In this case, I would expect the load from %p2 to see the value stored to %p, but that's only because the optimizer can't provide that %p == %p2.  If it could, then the value of the load is unclear.  (5, or whatever value that field had before...)
>>> 
>>>> 
>>>> http://reviews.llvm.org/D6346 <http://reviews.llvm.org/D6346>
>>>> 
>>>> 
>>> 
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>> 
> 

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