[LLVMdev] AliasAnalysis calling failed in Pass interaction

[Daniel Berlin]

On Wed, Apr 29, 2015 at 2:05 PM, Shen Liu <shl413 at lehigh.edu> wrote:
> Hi Daniel, thanks for your reply! Yes I didn't implement AA by myself but
> called builtin -basicaa interface for temporary use. Just add the -basicaa
> option in my opt command.
>
> I hope -basicaa can be called to process to check some simple aliases. And
> when I do
>
> DataDependencyGraph &ddgGraph = getAnalysis<DataDependencyGraph>(*F);
>
> inside a Function Pass, it works.What make me confused is when my Pass is a
> Module Pass, --basicaa cannot be called anymore. It seems the -basicaa
> option cannot be seen by getAnalysis() if I call it in a Module Pass.

Using it as a command line option makes it run a pass. It does not
make it available to your pass.
Essentially, you are getting lucky.
The point at which basicaa gets runs when added as a command line
option happens to make it hang around at the point you are using a
function pass.

If you want this to work properly, always, the right thing to do is to
tell it you want alias analysis in your AnalysisUsage.

IE
  AU.addRequiredTransitive<AliasAnalysis>();


>
> Do you have any suggestions?
>
> Best Regards,
>
> Shen
>
>
>
> On Wed, Apr 29, 2015 at 4:45 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>>
>> On Wed, Apr 29, 2015 at 1:27 PM, Shen Liu <shl413 at lehigh.edu> wrote:
>> > Dear LLVM developers,
>> >
>> > I am Shen, a PhD student at Lehigh Univ. PA.  Now I am implementing a
>> > Program Dependence Graph(PDG) on LLVM. I have 4 passes here:
>> >
>> > 1. ProgramDependenceGraph (a ModulePass on the highest level)
>> > 2. DataDependenceGraph (a Intermediate FunctionPass).
>> > 3. FlowDependenceAnalysis Pass (a intermediate FunctionPass) which uses
>> > llvm
>> > built-in AliasAnalysis (-basicaa)
>>
>> Does this implement AliasAnalysis?
>
>
>
>
>>
>>
>> > 4. AliasAnalysis Pass  (LLVM built-in Pass)
>> >
>> > When my call chain is
>> >
>> > DatadependenceGraph <-- FlowDependenceAnalysis <-- AliasAnalysis
>> >
>> > everything is fine and basicaa is executed successfully.
>>
>> BasicAA is not AliasAnalysis. BasicAA is *an* AliasAnalysis.
>>
>> If you have not added BasicAliasAnalysis to the pass list, it will not
>> be called when you call AliasAnalysis API.
>>
>> >
>> > My problem is, when I use a Module Pass to call a Function Pass, the low
>> > level AliasAnalysis does not work anymore, all return values for
>> > location
>> > comparisons are "May Alias". In other words, the call chain
>> >
>> > ProgramDependenceGraph(Module Pass) <-- DatadependenceGraph(Function
>> > Pass)
>> > <-- FlowDependenceAnalysis <-- AliasAnalysis if failed.
>> >
>> > However, if I change ProgramDependenceGraph into a Function Pass, this
>> > call
>> > chain works and AliasAnalysis can be executed successfully.
>> >
>> > So, i guess there may be an error in my interaction between Module Pass
>> > and
>> > Function Pass. Could you give me some hints to help me solve it? Thank
>> > you
>> > very much!
>> >
>> >
>> > Here are some key code for Pass interaction in my implementation:
>> >
>> > bool ProgramDependencyGraph::runOnModule(Module &M)
>> > {
>> >  ...
>> >   for(Module::iterator F = M.begin(), E = M.end(); F != E; ++F)
>> >     {
>> >        DataDependencyGraph &ddgGraph =
>> > getAnalysis<DataDependencyGraph>(*F);
>> >        ....
>> >     }
>> > }
>> >
>> > ...
>> > void ProgramDependencyGraph::getAnalysisUsage(AnalysisUsage &AU) const
>> > {
>> >    ...
>> >   AU.addRequired<DataDependencyGraph>();
>> >   AU.setPreservesAll();
>> > }
>> >
>>
>> This does not require alias analysis or it's analysis group, which is
>> wrong if you want those to work?
>>
>> >
>> > Best Regards,
>> >
>> > Shen
>> >
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >
>
>