llvm dev - Nov 2009 - [LLVMdev] Removing instanceof tests

Hi all,

    I wrote you some days ago about one project that I want to do on vmkit:
I want to remove redundant instanceof tests. I am right now looking at the
LLVM code that vmkit produces for java files, but I am finding it very
difficult to identify the code that is produced by each instanceof. Would it
be possible for you guys to give me some pointers on how to attack this
problem? Should I focus on the vmkit front-end, or should I perform this
type of optimization directly in LLVM, as a function pass?

Just to give you context, the optimization that I want to perform is to
remove redundant instanceof's that are inserted by vmkit to ensure that the
code does not do unsafe casts at runtime. For instance, if I code the
program:

if (o instanceof String)
  String s = (String)o;

then vmkit will most likely insert a second test in the code, like this:

if (o instanceof String) {
  if (!o instanceof String)
    throw ClassCastException
  String s = (String)o;
}

Thanks in advance,

Alysson
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One thing that might fix it is to tell LLVM that the class field of
all objects is constant.  Maybe vmkit has already done this, though.
I think this involves writing a custom alias analysis pass.  LLVM
needs to have enough information to know that between the instanceof
and the cast o's pointer hasn't been modified and that o->class is
the
same.

It may be easier to fix in your frontend if you do a dataflow analysis
where you build up type information about objects based on instanceof
checks and ifs and eliminate the check, though.

Reid

On Thu, Nov 19, 2009 at 11:59 AM, Alysson <aishofpf at gmail.com>
wrote:
> Hi all,
>
>     I wrote you some days ago about one project that I want to do on vmkit:
> I want to remove redundant instanceof tests. I am right now looking at the
> LLVM code that vmkit produces for java files, but I am finding it very
> difficult to identify the code that is produced by each instanceof. Would
it
> be possible for you guys to give me some pointers on how to attack this
> problem? Should I focus on the vmkit front-end, or should I perform this
> type of optimization directly in LLVM, as a function pass?
>
> Just to give you context, the optimization that I want to perform is to
> remove redundant instanceof's that are inserted by vmkit to ensure that
the
> code does not do unsafe casts at runtime. For instance, if I code the
> program:
>
> if (o instanceof String)
>   String s = (String)o;
>
> then vmkit will most likely insert a second test in the code, like this:
>
> if (o instanceof String) {
>   if (!o instanceof String)
>     throw ClassCastException
>   String s = (String)o;
> }
>
> Thanks in advance,
>
> Alysson
>
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