[LLVMdev] Landing my new development on the trunk ...

[Daniel Berlin]

On Mon, Nov 15, 2010 at 10:38 AM, Brian West <bnwest at rice.edu> wrote:

>  On 11/13/10 11:05 PM, Daniel Berlin wrote:
>
>  A big downside of the current LSR algorithm is it's slow.  I had
>> initially
>> hoped that some of the heuristics would protect it better, but the problem
>> is
>> more complex than I had expected.  I haven't done any measurements,
>> but it's likely that OSR is faster, which may interest some people
>> regardless
>> of how the output compares.
>>
>
> There is one thing both the original paper, the original MSCP
> implementation did (too bad the links to this point to ftp.cs.rice.edu,
> which no longer works, the web files were a great implementation resource) ,
> and my GCC implementation did, which is LFTR (Linear Function Test
> Replacement). LFTR after OSR can help reduce register pressure since it
> enables eliminating the IV's that no longer serve any useful purpose.  I
> don't see any implementation in this code.
>
>  --Dan
>
> Dan,
>
> LFTR (Linear Function Test Replacement) was mentioned in the original
> paper.  I considered including LFTR with OSR, but decided to get OSR to
> trunk first and then add LFTR (-lftr) as a separate pass later.
>

I'm not sure why you'd add it as a separate pass, it is about 80-150 lines
of code, and adding it as as a separate pass requires you to do things like
induction variable detection + etc all over again.

See http://gcc.gnu.org/ml/gcc-patches/2007-01/msg01035.html, record_edge,
apply_lftr_edge, follow_lftr_edge and perform_lftr

The LLVM development documentation suggests that new work be committed
> piecemeal over time.
>
> Sure, but that doesn't mean you should commit something that is going


> LLVM does have an optimization pass, -instcombine, which will delete unused
> induction variables.
>

LFTR does not delete unused IV's directly, it does reductions to transform
the IV into something else.
instcombine could only do this if it knew the sequence of reductions we
applied to strength reduce the IV in the first place, or if it computed
equivalence of ivs itself.
Both of these are not cheap operations.

I recommend that -instcombine be run after OSR.
>

In general, there is a careful balance between leaving the IR in a state
that requires expensive cleanup, and doing some of that cleanup yourself
where it's cheap and easy.
If you were to simply fall on the extreme of running cleanup passes after
every optimization, your compiler would be much slower.


> It is my understanding that LFTR would attempt to remove induction
> variables whose only use is to participate in an end-loop-of-loop test
> condition.
>

Well, no, any IV whose only use is a comparison and a linear function of an
existing IV.  This is mostly end of loop test conditions, but you'd be
surprised where else this pops up.

Logging or progress tracking, for example, where you do things like

if (i % 100 == 0) {
   printf(".");
}

etc

Anyway, it looks like the consensus so far is that you need to produce some
compile time and benchmark numbers showing OSR is worth it as it's own pass
as opposed to replacing/augmenting the LSR implementation that exists now
with it.

--Dan
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