[LLVMdev] parallel loop metadata question

[Humphreys, Jonathan]

Thanks for the link.  I understand your concern of caution with metadata.  I cannot, though, imagine how the dependence relation (independence) of two memory references can be affected by a third memory reference.  If two references are independent across loop iterations, then they are independent, and any other load or store cannot change that.  Right?

Jon

-----Original Message-----
From: Pekka Jääskeläinen [mailto:pekka.jaaskelainen at tut.fi] 
Sent: Friday, May 02, 2014 3:37 AM
To: Humphreys, Jonathan; llvmdev at cs.uiuc.edu
Cc: Daniel Berlin
Subject: Re: [LLVMdev] parallel loop metadata question

On 05/02/2014 01:49 AM, Humphreys, Jonathan wrote:
> Here's where I have a question:
>
> - if not all memory instructions in a loop have this metadata, can the 
> compiler still infer that the instructions having this metadata do not 
> have any loop carried dependencies with any other instruction also 
> having this metadata (for the common loops referenced by the metadata)?
>
> If so, it would be nice if we documented the actual meaning of 
> llvm.mem.parallel_loop_access as such.  And the conclusion that a loop 
> is 'parallel' if all its memory instructions have this metadata would 
> be a natural consequence of that meaning.
>
> This will allow us to potentially vectorize and/or software pipeline 
> large portions of loops even when some transformation introduces a 
> memory operation whose dependence effects are unknown.

This would be certainly useful.

We discussed a similar issue when the original parallel loop MD was sketched last year:

http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-February/059256.html

In this case the question is whether we can be sure the (new) accesses inserted between the parallel annotated accesses cannot not ruin the parallel (independence) semantics between the annotated ones.

While I cannot give you a breaking example/optimization immediately, I'm not confident there isn't one. The problem is that we need to be extra careful to not break the metadata rules; if an optimization does not understand a metadata, it should not be able to accidentally break any additional semantics implied by it.

--
Pekka