CMM-to-ASM: Register allocation wierdness

[Ben Gamari]

Ccing David Spitzenberg, who has thought about proc-point splitting, which
is relevant for reasons that we will see below.


Harendra Kumar <harendra.kumar at gmail.com> writes:

> On 16 June 2016 at 13:59, Ben Gamari <ben at smart-cactus.org> wrote:
>>
>> It actually came to my attention while researching this that the
>> -fregs-graph flag is currently silently ignored [2]. Unfortunately this
>> means you'll need to build a new compiler if you want to try using it.
>
> Yes I did try -fregs-graph and -fregs-iterative both. To debug why nothing
> changed I had to compare the executables produced with and without the
> flags and found them identical.  A note in the manual could have saved me
> some time since that's the first place to go for help. I was wondering if I
> am making a mistake in the build and if it is not being rebuilt
> properly. Your note confirms my observation, it indeed does not change
> anything.
>
Indeed; I've opened D2335 [1] to reenable -fregs-graph and add an
appropriate note to the users guide.

>> All-in-all, the graph coloring allocator is in great need of some love;
>> Harendra, perhaps you'd like to have a try at dusting it off and perhaps
>> look into why it regresses in compiler performance? It would be great if
>> we could use it by default.
>
> Yes, I can try that. In fact I was going in that direction and then stopped
> to look at what llvm does. llvm gave me impressive results in some cases
> though not so great in others. I compared the code generated by llvm and it
> perhaps did a better job in theory (used fewer instructions) but due to
> more spilling the end result was pretty similar.
>
For the record, I have also struggled with register spilling issues in
the past. See, for instance, #10012, which describes a behavior which
arises from the C-- sinking pass's unwillingness to duplicate code
across branches. While in general it's good to avoid the code bloat that
this duplication implies, in the case shown in that ticket duplicating
the computation would be significantly less code than the bloat from
spilling the needed results.

> But I found a few interesting optimizations that llvm did. For example,
> there was a heap adjustment and check in the looping path which was
> redundant and was readjusted in the loop itself without use. LLVM either
> removed the redundant  _adjustments_ in the loop or moved them out of the
> loop. But it did not remove the corresponding heap _checks_. That makes me
> wonder if the redundant heap checks can also be moved or removed. If we can
> do some sort of loop analysis at the CMM level itself and avoid or remove
> the redundant heap adjustments as well as checks or at least float them out
> of the cycle wherever possible. That sort of optimization can make a
> significant difference to my case at least. Since we are explicitly aware
> of the heap at the CMM level there may be an opportunity to do better than
> llvm if we optimize the generated CMM or the generation of CMM itself.
>
Very interesting, thanks for writing this down! Indeed if these checks
really are redundant then we should try to avoid them. Do you have any
code you could share that demosntrates this?

It would be great to open Trac tickets to track some of the optimization
opportunities that you noted we may be missing. Trac tickets are far
easier to track over longer durations than mailing list conversations,
which tend to get lost in the noise after a few weeks pass.

> A thought that came to my mind was whether we should focus on getting
> better code out of the llvm backend or the native code generator. LLVM
> seems pretty good at the specialized task of code generation and low level
> optimization, it is well funded, widely used and has a big community
> support. That allows us to leverage that huge effort and take advantage of
> the new developments. Does it make sense to outsource the code generation
> and low level optimization tasks to llvm and ghc focussing on higher level
> optimizations which are harder to do at the llvm level? What are the
> downsides of using llvm exclusively in future?
>

There is indeed a question of where we wish to focus our optimization
efforts. However, I think using LLVM exclusively would be a mistake.
LLVM is a rather large dependency that has in the past been rather
difficult to track (this is why we now only target one LLVM release in a
given GHC release). Moreover, it's significantly slower than our
existing native code generator. There are a number of reasons for this,
some of which are fixable. For instance, we currently make no effort to tell
LLVM which passes are worth running and which we've handled; this is
something which should be fixed but will require a rather significant
investment by someone to determine how GHC's and LLVM's passes overlap,
how they interact, and generally which are helpful (see GHC #11295).

Furthermore, there are a few annoying impedance mismatches between Cmm
and LLVM's representation. This can be seen in our treatment of proc
points: when we need to take the address of a block within a function
LLVM requires that we break the block into a separate procedure, hiding
many potential optimizations from the optimizer. This was discussed
further on this list earlier this year [2]. It would be great to
eliminate proc-point splitting but doing so will almost certainly
require cooperation from LLVM.

Cheers,

- Ben


[1] https://phabricator.haskell.org/D2335
[2] https://mail.haskell.org/pipermail/ghc-devs/2015-November/010535.html
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