[LLVMdev] Upcoming Changes/Additions to Scoped-NoAlias metadata

[Hal Finkel]

----- Original Message -----
> From: "Philip Reames" <listmail at philipreames.com>
> To: "Chandler Carruth" <chandlerc at google.com>, "Hal Finkel" <hfinkel at anl.gov>
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Friday, November 14, 2014 11:57:54 AM
> Subject: Re: [LLVMdev] Upcoming Changes/Additions to Scoped-NoAlias metadata
> 
> 
> 
> On 11/13/2014 05:02 PM, Chandler Carruth wrote:
> 
> This intrinsic needs to carry control dependencies (it cannot be
> hoisted out of a loop, for example) -- in this sense it is very much
> like @llvm.assume. And like @llvm.assume, we'll need to add logic to
> various passes to ignore it as appropriate so that it does not block
> optimizations unnecessarily.
> We should do something to make this simpler. I think we should have
> an intrinsic inst base class that assume, lifetime, and other
> intrinsics which do not represent actual code in the final program
> derive from so that we don't have to update these lists all over the
> place. If we need 2 tiers to model assume & noalias as distinct from
> the lifetime or other intrinsics, fine. We should have high-level
> categories that can be tested and updated. Agreed.
> 
> Specific to this point, I've seen a number of cases in discussion
> recently where a notion of a intrinsic which is control dependent on
> surrounding control flow, but does not read or write memory would be
> useful. It really feels like that's what the current implementation
> of llvm.assume has become and likely what this proposal would
> require. Maybe it's time to introduce a notion of an arbitrary
> non-memory control dependence?
> 
> Cases that come to mind:
> - invariant.*
> - lifetime.*

But these two do have a memory control dependence, right? You can't reorder a store to x and a lifetime.end(x).

 -Hal

> - float point state changes (this last one might need a stronger
> restriction than the others)
> 
> Philip
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory