llvm dev - Apr 2010 - [LLVMdev] AnalysisUsage: addRequired vs. addRequiredTransitive

On Mar 31, 2010, at 3:13 PM, Owen Anderson wrote:
> Some analyses, like Andersen's AA, do all their computation in their  
> runOnFunction().  Therefore, anything they depended on can be  
> destroyed after the runOnFunction() returns.
What about AA itself? Would addRequired<AliasAnalysis> keep  
AliasAnalysis alive (but allow AliasAnalysis's dependencies to die)?
>  Others, like MemoryDependenceAnalysis, are "lazy."  MDA  
> specifically does NOT compute results in its runOnFunction(),  
> instead computing results on-demand when a user queries it.
> Because MDA depends on AA, we must ensure that, as long as MDA is  
> alive and responding to queries, AA is alive as well. That is what  
> addRequiredTransitive does.
I'm not sure if I follow this. So let's say I'm writing a pass Foo,
and Foo depends on MDA. You're saying that Foo's getAnalysisUsage must  
call addRequiredTransitive< MemoryDependenceAnalysis>, not  
addRequired<MemoryDependenceAnalysis>, because otherwise the  
dependencies of Foo's dependencies might be destroyed. Do I have that  
right? Thanks,

Trevor

On Fri Apr 02 00:37:03 +0200 2010, Trevor Harmon wrote:
> On Mar 31, 2010, at 3:13 PM, Owen Anderson wrote:
> > Others, like MemoryDependenceAnalysis, are "lazy."  MDA
> > specifically does NOT compute results in its runOnFunction(),
> > instead computing results on-demand when a user queries it. Because
> > MDA depends on AA, we must ensure that, as long as MDA is  alive and
> > responding to queries, AA is alive as well. That is what
> > addRequiredTransitive does.
> 
> I'm not sure if I follow this. So let's say I'm writing a pass
Foo,
> and Foo depends on MDA. You're saying that Foo's getAnalysisUsage
must
> call addRequiredTransitive< MemoryDependenceAnalysis>, not
> addRequired<MemoryDependenceAnalysis>, because otherwise the
> dependencies of Foo's dependencies might be destroyed. Do I have that
> right?
Nope. If Foo depends on MDA, then Foo does addRequired<MDA>(). But when
Foo uses MDA, MDA itself needs AA. So MDA does
addRequiredTransitive<AA>() as AA does not only need to be around within
MDA's runOnFunction but also within each runOnFunction of a pass
depending on MDA.

So if MDA does addRequired<AA>, AA is only guaranteed to be alive for
MDA's runOnFunction. If MDA does addRequiredTransitive<AA> instead, AA
is not only alive for MDA's runOnFunction, but also for all
runOnFunction-s of passes requiring MDA.

Clear as mud?

-- 
Regards,
Andreas

On Apr 1, 2010, at 4:59 PM, Andreas Bolka wrote:
> Nope. If Foo depends on MDA, then Foo does addRequired<MDA>(). But  
> when
> Foo uses MDA, MDA itself needs AA. So MDA does
> addRequiredTransitive<AA>() as AA does not only need to be around  
> within
> MDA's runOnFunction but also within each runOnFunction of a pass
> depending on MDA.
>
> So if MDA does addRequired<AA>, AA is only guaranteed to be alive for
> MDA's runOnFunction. If MDA does addRequiredTransitive<AA>
instead, AA
> is not only alive for MDA's runOnFunction, but also for all
> runOnFunction-s of passes requiring MDA.
I get it now; thanks.

By the way, how can I help put this MDA/AA example into the LLVM  
documentation? There are a lot of explanations like this that should  
be preserved in the docs someplace, but I don't know what the usual  
procedure is for getting them there.

Trevor