Re: Strange performance enhancement when setting -inline 320

On Mon, Sep 16, 2013 at 6:34 AM, Matthew Fluet <[email protected]> wrote:
> On Thu, Sep 12, 2013 at 4:31 AM, René Neumann <[email protected]> wrote:
>> according to the man page for "-inline" the default value is 320. But
>> when setting this explicitly, I get a better performance (albeit not much):
>>
>> -> w/o "-inline 320": 2600 ms
>> -> w/  "-inline 320": 2500 ms
>>
>> Why is this? (Those numbers are reproducable, so probably not a
>> temporary effect).
>>
>> Used MLton: 2013715
>
> Looks to be primarily a documentation error.
>
> MLton employs a few different inlining strategies; see http://mlton.org/Inline.
>
> The "inlineLeaf1" and "inlineLeaf2" passes run at the beginning of the
> SSA optimization pipeline, and inlines leaf functions (functions that
> make no calls to other functions).  Both of these passes limit
> inlining up to a maximum function size; inlineLeaf1 uses a max size of
> 20 and inlineLeaf2 uses a max size of 40.  Both of these passes also
> work their way up the whole-program call graph so that any function
> that has all its calls to other functions inlined is itself considered
> for leaf inlining.  Neither of these passes are affected by
> (non-expert) compile-time options.
>
> The "inlineNonRecursive" pass runs about half-way through the SSA
> optimization pipeline, and inlines arbitrary non-recursive functions.
> This pass limits inlining through two parameters: 'product' and
> 'small'.  It inlines any function satisfying '(numCalls - 1) * (size -
> small) <= product', where 'numCalls' is the static number of calls to
> the function and 'size' is the size of the function.  The default
> value of 'product' is 320 and the default value of 'small' is 60; so,
> it will inline any function with size less than or equal to 60 and
> will inline any function that has exactly one static call site and
> will otherwise moderate the inlining by the 'product'.
>
> The documentation bug is that, while '320' is the default value for
> 'product', the "-inline <n>" compile-time option actually sets the
> value for the 'small' parameter.  So, compiling with "-inline 320" is
> changing behavior from the default --- allowing larger functions to be
> inlined.

I meant to add: there is no compile-time option for setting the value
for the 'product' parameter.

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