[LLVMdev] Problem Adding New Pass to Alias Analysis Group

John Criswell criswell at illinois.edu
Wed Jul 10 14:06:20 PDT 2013

On 7/10/13 3:43 PM, Cristianno Martins wrote:
> Hello John,
> What opt command line arguments are you using?

I'm not using opt.  I'm manually scheduling a pipline within a tool.  
The code looks like this:

PassManager pm;
MyAlias * aa = new MyAlias();
pm.add(new MyAliasUsingPass());

Both MyAlias and MyAliasUsingPass are now ModulePass'es.  MyAlias is an 
alias analysis pass while MyAliasUsingPass is a pass that requires an 
alias analysis and performs a test query.

The output of -debug-pass=Structure is the following:

No Alias Analysis (always returns 'may' alias)
   ModulePass Manager

I've changed MyAlias to call abort() when it is queried, but the program 
never crashes when running MyAliasUsingPass, which indicates that my 
MyAlias is never being used for queries.

I've also tried making MyAlias an ImmutablePass, but that didn't appear 
to work either.

> If you follow this link 
> <http://llvm.org/docs/Passes.html#no-aa-no-alias-analysis-always-returns-may-alias>, 
> you can see that -no-aa is the default alias analysis implementation 
> if you do not manually specify which AA passes you want to use. Note 
> that you can pass as many different implementations of AA as you want, 
> and each of them will be chained together for each function, like a 
> pipeline, if the previous one was not able to determine if there is a 
> dependence or not.

Yes, I am aware of how analysis groups are *supposed* to work.  :) I'm 
just not getting the advertised functionality and am at a loss as to 
what I could be doing wrong.

-- John T.

> Hope this help,
> --
> Cristianno Martins
> PhD Student of Computer Science
> University of Campinas
> cmartins at ic.unicamp.br <mailto:cmartins at ic.unicamp.br>
> On Wed, Jul 10, 2013 at 1:24 PM, John Criswell <criswell at illinois.edu 
> <mailto:criswell at illinois.edu>> wrote:
>     Dear All,
>     I'm trying to add a new alias analysis to the alias analysis group
>     in LLVM 3.2.  This new pass is linked statically into a tool that
>     lives outside the LLVM source tree, so I'm trying to avoid making
>     patches to the LLVM sources.
>     I've added the INITIALIZE_AG_PASS_BEGIN() and
>     INITIALIZE_AG_PASS_END() code to the pass, manually scheduled it
>     before the MemoryDependenceAnalysis pass, and have tried making it
>     a FunctionPass and an ImmutablePass, but no matter what I do, it
>     seems like MemoryDependenceAnalysis and other passes keep using
>     the -no-aa default pass instead.
>     1) Does anyone have ideas on how to verify that my pass is part of
>     the alias analysis group?
>     2) Does anyone have any ideas on what I might be doing wrong?
>     Any ideas would be appreciated.
>     Thanks in advance,
>     -- John T.
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