[LLVMdev] Pool Allocation and DSA

[John Criswell]

Patrick Alexander Simmons wrote:
> Thank you, and I apologize for conflating the two passes.
>
> My main remaining concern is about what happens when there is a node in 
> a function's DSGraph that points to a pool (or pools, if there are 
> multiple callers) that is not in the DSGraph of that function.  For 
> example, on page 3 of the 2005 PLDI paper, the DSGraph given for 
> createnode(), Data is pointing to an unlabeled node in the graph.  I 
> would like to know specifically what this node is and what would happen 
> if getPool() were called on it.  My best guess is that getPool() would 
> return NULL.
>   
Ah.  I think I see the issue now.  There may be some instances in which
pool allocation may not need to pass a pointer's pool into a function
because there are no allocations or deallocations from that pool.  In
that case, what does getPool() return?

I have no idea.  We added the getPool() method to support SAFECode , and
SAFECode requires that the pool for a pointer always be available, so
the above situation cannot (or at least, should not) occur.  I didn't
think of the above case when adding getPool() because it's not supposed
to happen for SAFECode, so what it does is anybody's guess.

If your use of pool allocation requires that you can always get the pool
handle for a pointer, you need to make sure that pool allocation is
configured so that it doesn't generate the above scenario.  I think
there is an option to make all pools be global pools.
 
> Moreover, it seems like it should be possible to call a Function with a 
> pointer without passing in the pointer's pool, as long as no allocations 
> are done using the pointer.  What will the DSNode for that pointer in 
> the function's graph look like?  What edges will be coming out of it?
>   
In the DSGraph for the function, you should have a DSNode for the
pointer that is passed into the function as a parameter.  Whether there
is an explicit pool for it is another matter, but there will be a DSNode
for the pointer.

> Also, after thinking about it for a while, I don't think it's ever 
> possible for a node in the global DSGraph to have pointers to a pool 
> that is not also global, thus preventing the problem of a global node 
> having pointers into some function's DSGraph.  Am I right?
>   
I don't know.  Global variables can point to stack and heap objects, so
DSA must have a way to represent this in the DSGraphs.  However, if you
configure pool allocation to allow for context sensitivity (i.e., it can
allocate pool descriptors on the stack), then I don't know if it will
place stack/heap objects pointed to by global variables in a global pool
or in a stack-allocated pool.

-- John T.

> Thank you again,
> --Patrick
>
> John Criswell wrote:
>   
>> I don't believe this is correct, or, at the very least, you are using 
>> confusing terminology.
>>
>> First, DSA and Pool Allocation are two completely different things.  DSA 
>> is a points-to analysis.  It creates a graph (one per function + 1 
>> global graph for global variables) where each node represents a set of 
>> memory objects and the edges represent pointers within one memory object 
>> that point to another memory object (i.e. if node A points to node B, 
>> that means a field in one of the objects in node A points to an object 
>> in node B).  The DSGraph also contains a ScalarMap which maps LLVM 
>> Values to nodes in the graph (these nodes are called DSNodes).  DSA is 
>> somewhat special because it attempts to disambiguate different linked 
>> data structures within programs (hence the name Data Structure Analysis, 
>> or DSA).
>>
>> Automatic Pool Allocation (APA) is a transform that is built on top of 
>> DSA.  It uses the points-to graphs computed by DSA to group allocations 
>> into various pools.  The original idea was to place nodes of the same 
>> pointer-based data structure into the same pool to get better cache 
>> locality.  The pools that Automatic Pool Allocation creates can either 
>> be global variables or local alloca'ed memory, depending upon how long 
>> the pool must be alive, what heuristics Automatic Pool Allocation is 
>> using, and whether the DSA results are context sensitive or 
>> insensitive.  When a pool is created based on context sensitive results, 
>> it is stack allocated and passed around as an argument to functions.
>>
>> However, Automatic Pool Allocation has another use beyond improving 
>> performance.  It turns out that if you allocate memory based on the 
>> points-to analysis results, you can speed up run-time checks for memory 
>> safety.  This leads to SAFECode: a transform built on top of Automatic 
>> Pool Allocation that adds additional information to each pool to 
>> implement fast run-time checks.
>>
>> Transforms like SAFECode need to know the mapping between the pointers 
>> in a program and the pool in which those pointers were assigned (more 
>> accurately, the pool in which the memory objects pointed to by those 
>> pointers were allocated).  The easiest way to do this was to have 
>> SAFECode query Automatic Pool Allocation.  This is, therefore, the 
>> purpose of getPool(): it allows other LLVM passes to ask Automatic Pool 
>> Allocation what pool a particular LLVM Value lives in.  The pool is 
>> returned as an LLVM Value *.  Depending on whether the pool is global, 
>> stack allocated, or passed into the function determines whether it is an 
>> LLVM GlobalVariable *, LLVM AllocaInst *, or LLVM Argument *.
>>
>> -- John T.
>>   
>>     
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