llvm dev - Sep 2015 - Alias set with only one element having may-alias property

Looks like missing some basics here.

Consider following test:

     1  int escape(int **ptr);
     2  int foo(int * ptr)
     3  {
     4     return escape(&ptr);
     5  }

At line number 4 address of pointer passed to another routine.

When I print alias set:

Alias Set Tracker: 1 alias sets for 1 pointer values.
  AliasSet[0x43be490, 2] may alias, Mod/Ref   Pointers: (i32** %ptr.addr, 8)
    1 Unknown instructions: i32 %call

Here ptr.addr is only in the set, and set property is may-alias.

I'm not sure it may-alias with what, if its aliasing with self then why not
must-alias ?

Is may-alias indicating ptr.addr is unsafe to optimize, and it can possibly
alias with something.

Regards,
Ashutosh

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----- Original Message -----
> From: "Ashutosh via llvm-dev Nema" <llvm-dev at
lists.llvm.org>
> To: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Tuesday, September 22, 2015 5:37:48 AM
> Subject: [llvm-dev] Alias set with only one element having may-alias
property
> 
> Looks like missing some basics here.
> 
> Consider following test:
> 
> 1 int escape(int **ptr);
> 2 int foo(int * ptr)
> 3 {
> 4 return escape(&ptr);
> 5 }
> 
> At line number 4 address of pointer passed to another routine.
> 
> When I print alias set:
> 
> Alias Set Tracker: 1 alias sets for 1 pointer values.
> AliasSet[0x43be490, 2] may alias, Mod/Ref Pointers: (i32** %ptr.addr,
> 8)
> 1 Unknown instructions: i32 %call
> 
> Here ptr.addr is only in the set, and set property is may-alias.
> 
> I’m not sure it may-alias with what, if its aliasing with self then
> why not must-alias ?
> 
> Is may-alias indicating ptr.addr is unsafe to optimize, and it can
> possibly alias with something.
Regarding optimization, this depends on how the AST is used. But, if other
pointer accesses are found that might alias with %ptr.addr, then Mod/Ref seems
like a default state for the alias set given the unknown behavior of 'int
escape(int **ptr)'.

 -Hal
> 
> Regards,
> Ashutosh
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory