llvm dev - Mar 2018 - Removing the LoopInstSimplify pass

Hi,

I think we should remove the LoopInstSimplify pass, as it has no test coverage
and no users (afaik).

If you are using the pass, or think that it should stay in tree for some other
reason, please let me know.

Here's the patch: https://reviews.llvm.org/D44053
<https://reviews.llvm.org/D44053>

vedant
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We're not actively using this, but from a design perspective I'm 
wondering if we should be using this or something like it.  At the 
moment, our various loop optimization assume mostly canonical input.  
Some of the passes have been taught to deal with limited amounts of 
non-canonical-ism, but there's a strong code simplicity argument in 
favor of only handling canonical input and then having something else 
canonicalize if necessary.  Given our loop passes are iterated to a 
fixed point, having that canonicalization done in another loop pass 
seems appropriate.  This pass seems like a start down that path.

Since we don't really document this anywhere, let me describe what I see 
as a canonical loop:

  * Use LCSSA, and LoopSimplify form.  Meaning no-uses of instructions
    defined in loop outside of loop and phis of loop exit blocks. 
    Preheaders available.  Unique (unshared) exit blocks.
  * All trivial CSE done.  I include both arithmetic and load
    elimination for constant memory.
  * All instsimplify style simplifications available within the loop and
    preheader done.  There's an argument for doing instcombine style
    optimizations within the loop as well, but that's less clear to me.
  * All trivial LICM done.  By this I mean LICM which does not require
    aliasing or speculation safety logic.  This is Loop::makeInvariant.
  * All trivial branches discharged.  By this I mean both CFGSimplify
    style elimination of constant branch conditions, but also CVP,
    KnownBits, and SCEV.  (Today, this is often true on entry to a loop
    pass manager, but is not upheld as passes run.)

To be clear, the above list is aspirational.  We definitely don't do all 
of the above today.  :)

Philip

On 03/02/2018 04:36 PM, Vedant Kumar via llvm-dev wrote:
> Hi,
>
> I think we should remove the LoopInstSimplify pass, as it has no test 
> coverage and no users (afaik).
>
> If you are using the pass, or think that it should stay in tree for 
> some other reason, please let me know.
>
> Here's the patch: https://reviews.llvm.org/D44053
>
> vedant
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Thanks for sharing this background information :). If you've got the time, I
think it'd be great to check this bulleted list into docs/. I see that we
don't have a Canonicalizations.rst or a LoopOptimizations.rst -- your notes
look like a good starting point.

Given that the pass seems to be doing the right thing from a design perspective,
should it stay in tree? Since it's been off-by-default since it was
introduced in 2011, I'm still in favor of removing it unless someone is
actively working on it.

[+ Chad, one of the pass contributors, for comment.]

thanks,
vedant

> On Mar 5, 2018, at 8:47 AM, Philip Reames <listmail at
philipreames.com> wrote:
> 
> We're not actively using this, but from a design perspective I'm
wondering if we should be using this or something like it.  At the moment, our
various loop optimization assume mostly canonical input.  Some of the passes
have been taught to deal with limited amounts of non-canonical-ism, but
there's a strong code simplicity argument in favor of only handling
canonical input and then having something else canonicalize if necessary.  Given
our loop passes are iterated to a fixed point, having that canonicalization done
in another loop pass seems appropriate.  This pass seems like a start down that
path.
> 
> Since we don't really document this anywhere, let me describe what I
see as a canonical loop:
> 
> Use LCSSA, and LoopSimplify form.  Meaning no-uses of instructions defined
in loop outside of loop and phis of loop exit blocks.  Preheaders available. 
Unique (unshared) exit blocks.
> All trivial CSE done.  I include both arithmetic and load elimination for
constant memory.
> All instsimplify style simplifications available within the loop and
preheader done.  There's an argument for doing instcombine style
optimizations within the loop as well, but that's less clear to me.
> All trivial LICM done.  By this I mean LICM which does not require aliasing
or speculation safety logic.  This is Loop::makeInvariant.
> All trivial branches discharged.  By this I mean both CFGSimplify style
elimination of constant branch conditions, but also CVP, KnownBits, and SCEV. 
(Today, this is often true on entry to a loop pass manager, but is not upheld as
passes run.)
> To be clear, the above list is aspirational.  We definitely don't do
all of the above today.  :)
> 
> Philip
> 
> On 03/02/2018 04:36 PM, Vedant Kumar via llvm-dev wrote:
>> Hi,
>> 
>> I think we should remove the LoopInstSimplify pass, as it has no test
coverage and no users (afaik).
>> 
>> If you are using the pass, or think that it should stay in tree for
some other reason, please let me know.
>> 
>> Here's the patch: https://reviews.llvm.org/D44053
<https://reviews.llvm.org/D44053>
>> 
>> vedant
>> 
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
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