[LLVMdev] noalias and alias.scope metadata notations

[Hal Finkel]

Hi Whitney,

The aliasing information conveyed by alias.scope and noalias is static, but each do provide lists of alias scopes (in different distinct domains). I think that this is easiest to understand in the context of translating noalias function parameters into the representation of the metadata.

Let's assume we have a function like this:
void foo(float* noalias p, float *noalias q, float* r, bool b) {
  float *t = b ? p : q;
  *r = *t;
}

To translate the 'noalias' function parameter attributes into metadata (which we do when inlining, see AddAliasScopeMetadata in lib/Transforms/Utils/InlineFunction.cpp). We add one alias scope for each noalias function parameter, so here we have two scopes: one for p and one for q. These scopes are used to indicate pointer accesses that derive from those pointers.

In this example, consider the access *t. This pointer might be derived from p or from q, and so it gets both scopes in its !alias.scope metadata. Now consider the access to *r. We know that it does not derive from either p or q, so it gets both scopes in its !noalias metadata list. Conceptually speaking, the !alias.scope list is an "or", and the !noalias list is an "and".

 -Hal

----- Original Message -----
> From: "Whitney Tsang" <whitneyt at ca.ibm.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Monday, September 22, 2014 10:04:20 AM
> Subject: [LLVMdev] noalias and alias.scope metadata notations
> 
> 
> 
> 
> Hi all,
> 
> In LLVM language reference I read that one can use noalias and
> alias.scope metadata to provide more detailed information about
> pointer aliasing. However, I am not clear about how the notations
> should be. Am I right that all load instruction should have
> !alias.scope at the end and all store instruction should have
> !noalias at the end? If yes, why? If I understand it correctly,
> aliasing information should be static (i.e. one !alias.scope and
> !noalias should be enough for each variable).
> 
> Thanks,
> Whitney
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory