[llvm-dev] AliasAnalysis: may-alias subcategory

[Nema, Ashutosh via llvm-dev]

There are function which does have optimization opportunities but because of may-alias memory dependencies sometimes optimization is not effective. May be runtime checks kills the gains of optimization. For such cases aiming to do interprocedural function specialization optimization where in the clone function version no-alias assumption can be assumed and the original function version will hold the default alias assumption.

i.e. 
void foo(int *A, int *B, int *C) {
  for (int i=0; i<N; i++)
       A[i] = B[i] + C[i];
}
void callFoo() {
   foo (A1, B1, C1); // A1 no-alias with B1 & C1
   foo (A2, B2, C2); // A2 no-alias with B1 & C1
   foo (A3, B3, C3); // A3 must-alias with B3/C3
}

Will be transformed to:
void foo(int *A, int *B, int *C) {
       // default loop version
}
void foo.clone(int * restrict A, int *B, int *C) {
       // Optimal loop version
}
void callFoo() {
   foo.clone(A1, B1, C1); // Call to optimal version
   foo.clone(A2, B2, C2); // Call to optimal version
   foo (A3, B3, C3); // Call to default version
}

For such cases I like to differentiate between “may-alias” and “may-alias-because-it's-input”.

Regards,
Ashutosh

-----Original Message-----
From: Nuno Lopes [mailto:nunoplopes at sapo.pt] 
Sent: Monday, August 7, 2017 6:12 PM
To: Nema, Ashutosh <Ashutosh.Nema at amd.com>; llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] AliasAnalysis: may-alias subcategory

You're right that stating may-alias(A, B) and may-alias(A, C) are the only possible answers right now, since the compiler cannot possibly know if these alias or not.
LLVM supports the noalias attribute for function parameters, which may be what you're looking for?  http://llvm.org/docs/LangRef.html#noalias

If not, how differentiating between a may-alias and a may-alias-because-it's-input would help?  Do you have a particular optimization that would be enabled?

Nuno

-----Original Message-----
From: Nema, Ashutosh via llvm-dev
Sent: Monday, August 7, 2017 12:29 PM
Subject: [llvm-dev] AliasAnalysis: may-alias subcategory

Hi,

The current AliasAnalysis marks may-alias for cases when memory passed as function argument.

i.e.
void foo(int *A, int *B, int *C) {
  for (int i=0; i<N; i++)
       A[i] = B[i] + C[i];
}

In the above example, it’s may-alias for memory ‘A’, because ’A’ is not known at ‘foo’ call sites.
Alias analysis is able to figure out memory ‘A’ is no-alias, if I modify the above test by making ‘A’ as an stack variable.

I’m interested to find the cases where memory is always may-alias because of input function parameter.
Probably a sub category(may-alias-because-of-function-parameter) of may-alias may help.

Not sure llvm already has something to sub categorize may-alias cases.

Thanks,
Ashutosh