Re: [Caml-list] Re: Ant: Efficiency of let/and

[Brian Hurt <bhurt@spnz.org>]

On Mon, 26 Sep 2005, Stefan Monnier wrote:

>> Syntactically and semantically there is no difference.  I was wondering if
>> the ocamlopt compiler took advatange of the implicit paralellism at all.
>
> If someone tries to use such things as `and' or
> unspecified-argument-evaluation-order in the hopes that the compiler will
> extract some imagined parallelism is simply deluding himself.
> In some cases, the freedom to execute in any order does lead to better
> code, but that code rarely if ever uses any kind of parallelism.

I was thinking of instruction-level parallelism- the ability of the 
compiler to reorder instructions to better use the available functional 
units, not thread-level parallelism.  Sorry for not being clear there.

I'm not even sure how much extra efficiency is there to be had.  Obviously 
it'd be hard "thread" calls to complex functions, so code like:

let foo lst1 lst2 =
     let len1 = List.length lst1
     and len2 = List.length lst2
     in
     ...

wouldn't be helped- it'd be computationally infeasible for the compiler to 
interleave the two different calls to List.length.  So you'd pretty 
obviously be limited to "simple" expressions, at which point the CPU's own 
prefetching and reordering is likely to do the work for you wether the 
compiler does it or not.  In fact, the CPU's reordering can start 
executing the code to List.length lst2 speculatively before the call to 
List.length lst1 is complete, and in that sense the CPU's reordering is 
more capable then what the compiler can do (this, BTW, is the fundamental 
problem with the Itanium).

Brian