[LLVMdev] Analysis Passes

[Devang Patel]

On Jul 25, 2008, at 1:57 PM, John Criswell wrote:

> Devang Patel wrote:
>> On Jul 25, 2008, at 12:58 PM, Marc de Kruijf wrote:
>>
>> I'd like to write a pass that does both: implements a code  
>> transformation and saves information that can be accessed by  
>> subsequent passes.
>>
>> Ideally, we want to use two separate pass. However, it is quiet  
>> possible that your requirement is unique. Would it be possible to  
>> provide more info. on what your pass does ?
>>
> We actually have a similar situation with Automatic Pool Allocation
> (APA) and SAFECode.  Some of the SAFECode passes need to query the
> Automatic Pool Allocation transform pass to determine what it has  
> done.
> Examples include:
>
> 1) Given a pointer, what pool did APA use to allocate objects to which
> the pointer can point?
> 2) Is a function a clone of an original function, and if so, which
> function is the original function?
> 3) What is the LLVM type of a pool descriptor?  Different versions of
> APA use a different type.
>
> I think we could refactor APA into an analysis pass and transform  
> pass.
> The analysis pass does nothing on its own but has update methods that
> other passes can use to update it's information.  When APA is run, it
> updates the analysis pass, and when the SAFECode passes run, they get
> the APA information from the analysis pass.
>
> Just FYI.

Let's say you do not refactor APA into two passes. And continue to  
request APA through getAnalysisUsage(). How does requesting APA ( a  
transformation pass) through getAnalysisUsage() help here ? Note,  
getAnalysisUsage() is a pass manager contract  that ensures that APA  
pass is available. It does not handle how information is stored and  
extracted.

Let's say your APA pass implements

APA {
  void runOnFunction();  // do pool allocation. A
  const PoolInfo *getPoolInfo(); // provides pool info
};

If pass A1 and A2 wants to use info collected during APA and request  
it through  getAnalysisUsage() then following might happen


A1 // requires APA
SomeOtherPass // does not preserve APA
A2 // requires APA

will instruction pass manager to schedule

APA
A1
SomeOtherPass
APA
A2

Is it appropriate to run APA transformation pass twice ?
-
Devang