llvm dev - Aug 2017 - 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

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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

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