Re: [Caml-list] Status of Flambda in OCaml 4.03

[Markus Mottl <markus.mottl@gmail.com>]

I've just tested Flambda, and it seems to already be doing a pretty
decent job on some non-trivial examples (e.g. inlining combinations of
functors and first class functions).  I hope there will be a stable
4.03 OPAM switch that enables it.  I'm looking forward to being able
to write more elegant, abstract code that's still efficient.

Regards,
Markus

On Wed, Mar 9, 2016 at 2:14 AM, Mark Shinwell <mshinwell@janestreet.com> wrote:
> It will not be enabled by default in 4.03.  For the majority of
> programs, in the current state, it should improve performance (mainly
> by lowering allocation).  It should never generate wrong code.
> However we know of examples that don't improve as much as we would
> like, which we will try to address for 4.04.
>
> There will be a draft version of the new Flambda manual chapter
> available shortly (hopefully this week).  Amongst other things this
> documents what you found about the configure options and the flags'
> operation.
>
> Mark
>
> On 9 March 2016 at 03:55, Markus Mottl <markus.mottl@gmail.com> wrote:
>> Hi Alain,
>>
>> I see, thanks.  It was a little confusing, because the command line
>> options for tuning flambda were still available even without Flambda
>> being enabled.
>>
>> Will Flambda be enabled by default in OCaml 4.03 or is it still
>> considered to be too experimental?  It could turn out to become one of
>> the most impactful new features in terms of how I write code.
>>
>> Regards,
>> Markus
>>
>> On Tue, Mar 8, 2016 at 5:53 PM, Alain Frisch <alain.frisch@lexifi.com> wrote:
>>> Hi Markus,
>>>
>>> flambda needs to be enabled explicitly at configure time with the "-flambda"
>>> flag.  The new optimizer will then be used unconditionally, and you can
>>> tweak it using command-line parameters passed to ocamlopt (see "ocamlopt
>>> -h").
>>>
>>>
>>> Alain
>>>
>>>
>>> On 08/03/2016 23:10, Markus Mottl wrote:
>>>>
>>>> Hi,
>>>>
>>>> I'm trying out OCaml 4.03.0+beta1 right now and wanted to test Flambda
>>>> optimizations.  But looking at the generated assembly, it doesn't seem
>>>> to be doing much if anything on the simple test examples that I
>>>> thought would benefit.
>>>>
>>>> To give an example of what I expected to see, lets consider this code:
>>>>
>>>> -----
>>>> let map_pair f (x, y) = f x, f y
>>>>
>>>> let succ x = x + 1
>>>> let map_pair_succ1 pair = map_pair succ pair
>>>> let map_pair_succ2 (x, y) = succ x, succ y
>>>> -----
>>>>
>>>> I would have thought that the "succ" function would be inlined in
>>>> "map_pair_succ1" as the compiler would do for "map_pair_succ2".
>>>> But the generated code looks like this:
>>>>
>>>> -----
>>>> L101:
>>>>    movq  %rax, %rdi
>>>>    movq  %rdi, 8(%rsp)
>>>>    movq  %rbx, (%rsp)
>>>>    movq  8(%rbx), %rax
>>>>    movq  (%rdi), %rsi
>>>>    movq  %rdi, %rbx
>>>>    call  *%rsi
>>>> L102:
>>>>    movq  %rax, 16(%rsp)
>>>>    movq  (%rsp), %rax
>>>>    movq  (%rax), %rax
>>>>    movq  8(%rsp), %rbx
>>>>    movq  (%rbx), %rdi
>>>>    call  *%rdi
>>>> -----
>>>>
>>>> Is Flambda supposed to work out of the box with the current beta?
>>>> What flags or annotations should I use for testing?  Any showcase
>>>> examples I should try out that are expected to be improved?
>>>>
>>>> Regards,
>>>> Markus
>>>>
>>>
>>
>>
>>
>> --
>> Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com
>>
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-- 
Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com