[LLVMdev] How to describe a pointer that points to All memory(include global memory, heap, stack)?
Steven Su
steven_known at yahoo.com.cn
Sat Mar 23 00:44:52 PDT 2013
Daniel,
> I'm not sure why you'd want to do this as opposed to keeping two bitsets, using bdds, or using sparse bitmaps.
It looks like your method is more concisely, using one bitset.
And as you said, if our two bitsets method can also make alias analysis and DU analysis work well, and do not harm compiler performance,
we choose to keep the current design method, then put more attention
into other work. : )
--- 13年3月22日，周五, 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: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> 日期: 2013年3月22日,周五,下午12:42
> 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?
> >> >> >
> >> >> >
> >> >> >
> >> >> >
> >> _______________________________________________
> >> >> > LLVM Developers mailing list
> >> >> > LLVMdev at cs.uiuc.edu
> >> >> http://llvm.cs.uiuc.edu
> >> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >> >>
> >> >>
> >> >
> >> >
> _______________________________________________
> >> > LLVM Developers mailing list
> >> > LLVMdev at cs.uiuc.edu
> >> http://llvm.cs.uiuc.edu
> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >>
>
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