Erratic failure to specialize function

[Aleksey Khudyakov]

On Mon, Dec 19, 2011 at 12:26 AM, Antoine Latter <aslatter at gmail.com> wrote:
> On Sun, Dec 18, 2011 at 2:57 PM, Alexey Khudyakov
> <alexey.skladnoy at gmail.com> wrote:
>> Hello!
>>
>>
>> I've found a puzzling performance problem with code which uses vector
>> library and relies heavily on GHC to perform inlining and
>> specialization. In some cases compiler refuses to specialize function
>> and just copies there generic version which is slow.
>>
>> Here is smallest test case I've manages to make:
>>
>
> This is a guess, but based on what I've read the GHC inliner only
> fires when the function is fully saturated as declared - so if you
> declare a function with one argument to the left of the '=' symbol,
> the inliner only then inlines when it is applied to one value.
>
> This means that the un-inlined function is passed to criterion in the
> first case, but not the second.
>
Yes. That's the case. I've checked the core and indeed in the first
case function wasn't inlined.

> Does adding a SPECIALIZE pragma help?
>
Yes. Since GHC have a optimized version it chooses it.

So it looks like that functions which must be inlined (e.g. to
eliminate dictionaries) should be written with no parameters

> variance = \vec -> G.sum vec

This variant gets inlined. Otherwise SPECIALIZE pragma
could help. Although it's annoying to write so many pragmas