[LLVMdev] How to describe a pointer that points to All memory(include global memory, heap, stack)?
Daniel Berlin
dberlin at dberlin.org
Thu Mar 21 21:42:27 PDT 2013
On Thu, Mar 21, 2013 at 12:29 AM, Steven Su <steven_known at yahoo.com.cn> wrote:
> Hi, Daniel, thank you for your advice.
> Yes, ALL_MEMORY points to ALL_MEMORY.
>
> We use MD(memory descriptor) to abstract a memory location.
> MD contains 4 main fields: id, base, offset, size.
> For these special MD (ALL_MEMORY, GLOBAL_MEMORY, STACK_MEMORY, HEAP_MEMORY),
> we give them id 1, 2, 3, 4, that means MD1 is ALL_MEMORY, MD2 is GLOBAL_MEMORY, the same goes for the rest.
> Then we maintain a BITSET class to encapsulate the 'union', 'intersect', 'diff' etc to simply the operations bewteen special
> MD and other general MDs.
Okay.
> e.g: union operation bewteen special MD and general MD.
>
> Given BITSET a, b;
> a={MD1}
> b={MD10}
> c={MD20}
>
> Here MD10, MD20 are general MD. They described the actually variable.
>
> b.union(c) equal to {MD10, MD20}
> a.union(b) equal to {MD1}
> b.union(a) equal to {MD1}
>
> Our flow-sensitive method is divided into 2 step:
> 1. Iterative solve flow-equation to compute POINT-TO for each stmt.
> 2. According to POINT-TO info, compute MayDef BITSET and MayUse BITSET for each stmt.
>
> So, I have an question, after step 2 each stmt should own two BITSETs to record MayDef and MayUse.
> Is that right or necessary?
It's fine, it's not strictly necessary, but it's not bad or harmful.
> How to describe both MayDef, MayUse info if just have a single BITSET in each stmt?
Cut the universe in half. Assuming 64 bit ids:
bits 1-9223372036854775808 represent mayuses
bits 9223372036854775809-18446744073709551616 represent maydefs
(If your bitsets are sparse, and allow negative indexes, like gcc's,
you could just use positive and negative numbers).
This would need to be abstracted properly so that the
union/intersect/diff/iterators operated only on the proper portions of
the bitset.
I'm not sure why you'd want to do this as opposed to keeping two
bitsets, using bdds, or using sparse bitmaps.
>
>
> --- 13年3月21日，周四, Daniel Berlin <dberlin at dberlin.org> 写道：
>
>> 发件人: Daniel Berlin <dberlin at dberlin.org>
>> 主题: Re: [LLVMdev] How to describe a pointer that points to All memory(include global memory, heap, stack)?
>> 收件人: "Steven Su" <steven_known at yahoo.com.cn>
>> 抄送: "John Criswell" <criswell at illinois.edu>, "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
>> 日期: 2013年3月21日,周四,下午2:15
>> On Wed, Mar 20, 2013 at 7:33 PM,
>> Steven Su <steven_known at yahoo.com.cn>
>> wrote:
>> > Hi, John
>> > I am building a
>> flow sensitive intra-procedural alias analysis(without
>> interprocedural info).
>> > So, the first
>> thing I have to consider is where a parameter-pointer or a
>> global-pointer might point to.
>> > Then I defined
>> several special Virtual Memory Locations: ALL_MEMORY,
>> GLOBAL_MEMORY, STACK_MEMORY, HEAP_MEMORY. ALL_MEMORY
>> contains GLOBAL_MEMORY, STACK_MEMORY, HEAP_MEMORY.
>>
>>
>> Contains or points to?
>>
>> ALL_MEMORY should point to ALL_MEMORY, otherwise *ALL_MEMORY
>> !=
>> ALL_MEMORY, which will break things like linked lists.
>>
>>
>> In any case, the final representation in GCC of points to
>> anything is
>> a single bit flag.
>> We don't keep sparse bit vectors around just to have a
>> single bit set in them.
>>
>>
>> >
>> > e.g1:
>> > extern int * q;
>> > void f(int * p)
>> > {
>> > int
>> a;
>> > *p
>> = 0; //STMT1
>> > *q
>> = a; //STMT2
>> > }
>> > For above case,
>> both p and q pointed to ALL_MEMORY: p->ALL_MEMORY,
>> q->ALL_MEMORY.
>> > And each STMT has
>> two BitVectors to describe MayDef, MayUse.
>> > (I think the
>> BitVector must be sparse, otherwise the alias-analysis
>> module need too much memory to allocate for all
>> BitVectors.)
>>
>> Well, actually, this is what led to bdd representations.
>>
>> GCC uses sparse bitmaps, which work fine for intraprocedural
>> use.
>> We do a large amount of presolving to discover equivalences
>> before
>> ever filling in points-to sets.
>>
>> I'm not sure your mechanism of solving, but if you are
>> constraint
>> solving, you should special case ALL_MEMORY (and a few
>> others).
>> There is never a reason to add anything to a points-to set
>> that
>> contains ALL_MEMORY.
>>
>>
>> > For STMT1, the
>> MayDef={ALL_MEMORY}, MayUse={}
>> > For STMT2, the
>> MayDef={ALL_MEMORY}, MayUse={'a'}
>> >
>> ---------------------
>> > e.g2:
>> > extern int
>> A[100];
>> > int * q;
>> > void f2(int * p)
>> > {
>> > *p
>> = 0; //STMT1
>> > p =
>> &A; //STMT2
>> >
>> p[0] = *q; //STMT3
>> >
>> bar(); //STMT4
>> > }
>> > In e.g2,
>> > for STMT1, the
>> MayDef={ALL_MEMORY}, MayUse={}
>> > for STMT2,
>> p->A, so the MayDef={'p'}, MayUse={}
>> > for STMT3, the
>> MayDef={'A'}, MayUse={ALL_MEMORY}
>> > for STMT4, the
>> MayDef={ALL_MEMORY}, MayUse={ALL_MEMORY}
>> >
>> > Is that right? Or there are another better methods. :)
>> >
>> >
>> >
>> > --- 13年3月14日，周四, John Criswell <criswell at illinois.edu>
>> 写道：
>> >
>> >> 发件人: John Criswell <criswell at illinois.edu>
>> >> 主题: Re: [LLVMdev] How to describe a pointer
>> that points to All memory(include global memory, heap,
>> stack)?
>> >> 收件人: "Steven Su" <steven_known at yahoo.com.cn>
>> >> 抄送: llvmdev at cs.uiuc.edu
>> >> 日期: 2013年3月14日,周四,上午12:14
>> >> On 3/13/13 4:06 AM, Steven Su wrote:
>> >> > Hello, could any one point me following
>> question.
>> >>
>> >> Without any context, your question is difficult to
>> >> answer. Are you
>> >> building a points-to analysis and wanting to know
>> how an
>> >> alias analysis
>> >> might encode the fact that a pointer could alias
>> any other
>> >> pointer?
>> >>
>> >> -- John T.
>> >>
>> >> >
>> >> > e.g:
>> >> > void foo(int * p)
>> >> > {
>> >> > *p =
>> >> 0;
>> >> > }
>> >> > Here 'p' may
>> point to
>> >> all memory location.
>> >> > Could you
>> tell me how
>> >> to represent the POINT TO set of 'p'?
>> >> >
>> >>
>> >> > Here is my
>> solution:
>> >> > Introduce a
>> >> memory-class named: Global_Mem, then p pointed to
>> >> global_mem.
>> >> > And the MOD
>> set of
>> >> '*p=0' is Global_Mem.
>> >> >
>> >> > But how to present the overlapping alias set:
>> >> > e.g2:
>> >> > extern
>> A[100];
>> >> > void foo(int * p, int
>> i)
>> >> > {
>> >> >
>> *p
>> >> = 0;
>> >> >
>> >> A[i] = 10;
>> >> > }
>> >> >
>> >> > 'p' may point to anywhere. So p may point to
>> A. How to
>> >> describe the relation?
>> >> >
>> >> >
>> >> >
>> >> >
>> _______________________________________________
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>> >> http://llvm.cs.uiuc.edu
>> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>
>> >>
>> >
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