[LLVMdev] newbie question - selecting the write kind of pass

[Chris Lattner]

On Sun, 3 Apr 2005, Sameer D. Sahasrabuddhe wrote:

> On Sat, Apr 02, 2005 at 11:35:30AM -0600, Chris Lattner wrote:
>
>> On Sat, 2 Apr 2005, Sameer D. Sahasrabuddhe wrote:
>>> I want to create a simple map from a Value to the instruction that
>>> defines it. Such a map is present inside SchedGraph, but I need it in
>>> a much simpler context.
>>
>> Is this in the context of the code generator?
>
> No ... I am just trying to feel the waters ... simply trying to create
> a "data flow graph" from the SSA form. I meant "context" in a very
> loose, plain-English way.

Ah, ok.  Well in this case, the function describing the mapping from a 
Value* to the instruction that defines it is the identity function.  Given 
a Value*, you can do something like this:

Value *V = ...

if (Instruction *I = dyn_cast<Instruction>(V)) {
   ... V is the result of the instruction I ...
} else {
   ... V is some other sort of value, such as a constant or a formal
   argument ...
}

>>> The documentation says that "a FunctionPass is not allowed to retain
>>> state across functions. What does that imply for an Analysis pass
>>> derived from FunctionPass?
>>
>> Yes.
>
> Ummm ... let me rephrase my question :)
> An analysis pass derived from a FunctionPass is not expected to retain
> state across functions. What exactly does that mean when the passes
> are being run on the program? I assume that analysis will be
> instantiated only once, and used everywhere. If that is so, as the
> pass writer, do I have to "flush" all data-structures from the
> previous run, in runOnFunction?

Ah whoops, right, I uh did an incredibly poor job answering your question 
:)

You have it exactly right.  Currently if you have a function pass, it is 
created once per function, and any other functionpass that requires it 
will see its results.  The pass manager tells your pass that all clients 
are done with it by calling the releaseMemory method:
http://llvm.cs.uiuc.edu/docs/WritingAnLLVMPass.html#releaseMemory

That is the place you should "flush" the contents of the analysis.

-Chris

-- 
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