[llvm-dev] Clarifying noalias with metadata merging

[Jeroen Dobbelaere via llvm-dev]

Hi Modi,

for !noalias/!alias.scope, merging metadata for from two instructions means:
- !noalias: taking the intersection (= the merged instruction can only 'noalias'
  with the scopes for which all of the original instructions are 'noalias')
- !alias.scope: taking the union (= the merged instruction can alias with any
  of the 'aliases of the original instructions')

This ensures that, at worst, a previous 'NoAlias' alias analysis result will
now become 'MayAlias', which is still correct, but maybe less efficient.

Note: if you are interested in alias analysis and restrict support, you
should take a look at the 'Full Restrict' patches[0]. 

Greetings,

Jeroen Dobbelaere

[0] Full Restrict Patches: https://reviews.llvm.org/D68484


> -----Original Message-----
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Modi Mo via
> llvm-dev
> Sent: Tuesday, November 17, 2020 03:36
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] Clarifying noalias with metadata merging
> 
> Hi,
> 
> Looking at how noalias is defined:
> https://llvm.org/docs/LangRef.html#noalias-and-alias-scope-metadata
> "When evaluating an aliasing query, **if for some domain**, the set of scopes
> with that domain in one instruction’s alias.scope list is a subset of (or
> equal to) the set of scopes for that domain in another instruction’s noalias
> list, then the two memory accesses are assumed not to alias."
> 
> 
> This corresponds to Hal's change from llvm-svn: 213948
> "...
> The subset/superset condition now
> applies within each domain independently, and we only need it to hold in one
> domain. Each time we inline, we add the new scopes in a new scope domain, and
> everything now composes nicely. In addition, this simplifies the
> implementation.
> ..."
> 
> I think I understand the intent here for inlining: domains are compared in
> nested order and the first one in noalias which is a superset of alias.scope
> means regardless of further inlining there can't be aliasing.
> 
> However, when other optimizations like memcpyopt merge instructions they merge
> metadata which in the case of alias.scope means taking the union between the 2
> instructions. However, this appears to break the nesting paradigm since what's
> getting merged in doesn't need to have an inline-inliner relationship. This
> can then lead to incorrect aliasing information being computed.
> 
> Is the semantics for noalias/alias.scope in use still that which is documented
> in the LangRef? And if so, is metadata merging as it is now doing something
> wrong here?
> 
> Thanks,
> Modi
>