[LLVMdev] Alias Analysis accuracy

[Welson Sun]

Here is the result of running mem2reg then basicaa, it is even worse: (%a
should be alias to %0, and partial alias to %3)

opt -mem2reg -basicaa -aa-eval -print-all-alias-modref-info < foo.s >
/dev/null
Function: foo: 6 pointers, 0 call sites
  NoAlias:      i32* %a, i32* %b
  NoAlias:      i32* %a, i32* %c
  NoAlias:      i32* %b, i32* %c
  PartialAlias: i32* %1, i32* %a
  NoAlias:      i32* %1, i32* %b
  NoAlias:      i32* %1, i32* %c
  NoAlias:      i32* %4, i32* %a
  PartialAlias: i32* %4, i32* %b
  NoAlias:      i32* %4, i32* %c
  NoAlias:      i32* %1, i32* %4
  NoAlias:      i32* %8, i32* %a
  NoAlias:      i32* %8, i32* %b
  PartialAlias: i32* %8, i32* %c
  NoAlias:      i32* %1, i32* %8
  NoAlias:      i32* %4, i32* %8


On Fri, Sep 21, 2012 at 3:18 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> On Fri, Sep 21, 2012 at 3:08 PM, Welson Sun <welson.sun at gmail.com> wrote:
> > OK, with the restrict type qualifier, it is a little bit better:
> >
> > The IR's function signature becomes:
> > define void @foo(i32* noalias %a, i32* noalias %b, i32* noalias %c)
> nounwind
> > {
> >
> > Now the AA result:
> > Function: foo: 13 pointers, 0 call sites
> >   NoAlias:      i32* %a, i32* %b
> >   NoAlias:      i32* %a, i32* %c
> >   NoAlias:      i32* %b, i32* %c
> >   NoAlias:      i32* %a, i32** %a_addr
> >   NoAlias:      i32* %b, i32** %a_addr
> >   NoAlias:      i32* %c, i32** %a_addr
> >   NoAlias:      i32* %a, i32** %b_addr
> >   NoAlias:      i32* %b, i32** %b_addr
> >   NoAlias:      i32* %c, i32** %b_addr
> >   NoAlias:      i32** %a_addr, i32** %b_addr
> >   NoAlias:      i32* %a, i32** %c_addr
> >   NoAlias:      i32* %b, i32** %c_addr
> >   NoAlias:      i32* %c, i32** %c_addr
> >   NoAlias:      i32** %a_addr, i32** %c_addr
> >   NoAlias:      i32** %b_addr, i32** %c_addr
> >   NoAlias:      i32* %a, i32* %i
> >   NoAlias:      i32* %b, i32* %i
> >   NoAlias:      i32* %c, i32* %i
> >   NoAlias:      i32* %i, i32** %a_addr
> >   NoAlias:      i32* %i, i32** %b_addr
> >   NoAlias:      i32* %i, i32** %c_addr
> >   MayAlias:     i32* %0, i32* %a
> >   MayAlias:     i32* %0, i32* %b
> >   MayAlias:     i32* %0, i32* %c
> >   NoAlias:      i32* %0, i32** %a_addr
> >   NoAlias:      i32* %0, i32** %b_addr
> >   NoAlias:      i32* %0, i32** %c_addr
> >   NoAlias:      i32* %0, i32* %i
> >   MayAlias:     i32* %3, i32* %a
> >   MayAlias:     i32* %3, i32* %b
> >   MayAlias:     i32* %3, i32* %c
> >   NoAlias:      i32* %3, i32** %a_addr
> >   NoAlias:      i32* %3, i32** %b_addr
> >   NoAlias:      i32* %3, i32** %c_addr
> >   NoAlias:      i32* %3, i32* %i
> >   PartialAlias: i32* %0, i32* %3
> >   MayAlias:     i32* %5, i32* %a
> >   MayAlias:     i32* %5, i32* %b
> >   MayAlias:     i32* %5, i32* %c
> >   NoAlias:      i32* %5, i32** %a_addr
> >   NoAlias:      i32* %5, i32** %b_addr
> >   NoAlias:      i32* %5, i32** %c_addr
> >   NoAlias:      i32* %5, i32* %i
> >   MayAlias:     i32* %0, i32* %5
> >   MayAlias:     i32* %3, i32* %5
> >   MayAlias:     i32* %8, i32* %a
> >   MayAlias:     i32* %8, i32* %b
> >   MayAlias:     i32* %8, i32* %c
> >   NoAlias:      i32* %8, i32** %a_addr
> >   NoAlias:      i32* %8, i32** %b_addr
> >   NoAlias:      i32* %8, i32** %c_addr
> >   NoAlias:      i32* %8, i32* %i
> >   MayAlias:     i32* %0, i32* %8
> >   MayAlias:     i32* %3, i32* %8
> >   PartialAlias: i32* %5, i32* %8
> >   MayAlias:     i32* %11, i32* %a
> >   MayAlias:     i32* %11, i32* %b
> >   MayAlias:     i32* %11, i32* %c
> >   NoAlias:      i32* %11, i32** %a_addr
> >   NoAlias:      i32* %11, i32** %b_addr
> >   NoAlias:      i32* %11, i32** %c_addr
> >   NoAlias:      i32* %11, i32* %i
> >   MayAlias:     i32* %0, i32* %11
> >   MayAlias:     i32* %11, i32* %3
> >   MayAlias:     i32* %11, i32* %5
> >   MayAlias:     i32* %11, i32* %8
> >   MayAlias:     i32* %14, i32* %a
> >   MayAlias:     i32* %14, i32* %b
> >   MayAlias:     i32* %14, i32* %c
> >   NoAlias:      i32* %14, i32** %a_addr
> >   NoAlias:      i32* %14, i32** %b_addr
> >   NoAlias:      i32* %14, i32** %c_addr
> >   NoAlias:      i32* %14, i32* %i
> >   MayAlias:     i32* %0, i32* %14
> >   MayAlias:     i32* %14, i32* %3
> >   MayAlias:     i32* %14, i32* %5
> >   MayAlias:     i32* %14, i32* %8
> >   PartialAlias: i32* %11, i32* %14
> >
> > a, b and c are no longer alias, as expected. However, they all may alias
> to
> > %0. I guess basicaa doesn't handle data flow analysis?
>
> No.  Try running mem2reg.
>
> -Eli
>



-- 
Welson

Phone: (408) 418-8385
Email:  welson.sun at gmail.com
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