[llvm-dev] LLVM Alias Analysis (original) (raw)

Sean Perry via llvm-dev llvm-dev at lists.llvm.org
Wed Dec 12 10:36:31 PST 2018

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]

LLVM Developers mailing list llvm-dev at lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181212/d614e61b/attachment.html> -------------- next part -------------- A non-text attachment was scrubbed... Name: graycol.gif Type: image/gif Size: 105 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181212/d614e61b/attachment.gif>

More information about the llvm-dev mailing list