[LLVMdev] Google Summer of Code Idea

Daniel Berlin
Wed, 05 Mar 2008 19:27:37 -0000

On Wed, Mar 5, 2008 at 3:44 AM, Xi Wang <xi.wang at gmail.com>
wrote:
> On Mon, Mar 3, 2008 at 6:55 PM, Richard Warburton
>
> <richard.warburton at gmail.com> wrote:
>
> > >  No
>  > >  I've got a BDD based version of andersen's around
already (uncommitted).
>  > >  It's slower in time (by a factor of 8 or so), but very
slightly (1-2
>  > >  megabytes of memory) more memory efficient on some cases.
Nowadays,
>  > >  the bitmap version is actually more memory efficient than the
BDD
>  > >  version, even if you turn off BDD caching, etc.
>  > >
>  > >  The thing you have to understand is that these context
sensitive BDD
>  > >  algorithms work okay on java, but fall down pretty badly on
C/C++,
>  > >  because it is a much bigger problem.   Whaley's is just
brute force,
>  > >  and Zhu's is pretty darn complex.
>  >
>  > I was interested to see whether Whaley's algorithm really works
>  > particularly, given its simplicity and the claims made in the papers.
>
>  I have implemented a tool based on the Phoenix compiler framework at
>  MSR, which can dump relations as BDD from C/C++ code and feed them
>  into John Whaley's bddbddb for further analysis.  As Dan said,
>  currently the performance is not good enough for C/C++ code (due to
>  more complex relations, BDD variable orders, etc.).  I am still trying
>  several optimizations.  John also told that it took quite a lot of
>  effort to get the Java version to scale.

My guess is that if you try doing variable substitution style
optimizations, you will get very very serious speedups.

Usually, out of 80-100k constraint variables, you can unify 60k of them.
In my testing, it reduces the number of constraint variables by 80+%
in non-context sensitive analysis.
bddbddb is already going to be rather good at solving the query
relations, so to get faster, you have to start reducing the size of
the problem itself.