[llvm-dev] LLVM Alias Analysis

[Sean Perry via llvm-dev]

The variables ptra and ptrb don't actually alias.  A simple way to think of
aliasing is to to say two l-values alias if assigning into one changes the
other.  In your program changing the value of ptra does not change the
value of ptrb.  What you do have is two pointers that point to the same
l-value.  The aliasing analysis will show that variable a (which is address
taken) is aliased to a shadow of type int.  Any indirections through ptra
or ptrb will use that same shadow.  This is how clang and llvm get the
aliasing correct.

This is a common mistake when learning about aliasing.  People often think
the pointer should alias the object.  It's really the shadow (i.e. a
pointer indirection) that aliases the object.  In fact you can never have
two variables ever alasing each other.

Regards
--
Sean Perry
Compiler Development
IBM Canada Lab
(905)-413-6031 (tie 313-6031), fax (905)-413-4839




From:	"Артём Вопилов via llvm-dev" <llvm-dev at lists.llvm.org>
To:	llvm-admin at lists.llvm.org, llvm-dev at lists.llvm.org,
            mailman at lists.llvm.org, bugs-admin at lists.llvm.org
Date:	12/12/18 11:54 AM
Subject:	[llvm-dev] LLVM Alias Analysis
Sent by:	"llvm-dev" <llvm-dev-bounces at lists.llvm.org>



Dear LLVM Developers,

My name is Artem Vopilov, I am a student at TU Darmstadt. I am writing to
you to ask about Alias Analysis.

I am using llvm to analyze alias between variables in programs. I am using
Alias Analysis implemented in llvm with command "opt -analyze -aa-eval
-print-all-alias-modref-info" and for printing sets of alias "opt -analyze
-aa-eval -print-alias-sets".

I execute these commands on the small test program I attached to this
email.
If you take a look at it, it will be obvious, that variables "ptra" and
"ptrb" alias. However, the results of Alias Analysis do not indicate, that
these variable alias. I would like to ask you, if it is possible to change
the source code of Alias Analysis, so that it will show that "ptra" and
"ptrb" variables from the test program alias.

I am looking forward to hearing from you and hope, that you can help me
solve my problem.

Respectfully yours,
Artem Vopilov
[attachment "Test.c" deleted by Sean Perry/Toronto/IBM]
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