[MLton-user] more optimization questions

[brian denheyer]

On Nov 19, 2005, at 9:21 PM, Stephen Weeks wrote:

>
> Speaking of common-subexpression elimination and loop optimizations, I
> misspoke when I said that there would be no record destructuring when
> doing 3-d array subscript.  Looking at the code again:
>
>>      fun index (A3{n1, n2, n3, ...}, i, j, k) =
>>          (* 2 mults/2 adds for each index count *)
>>          k + n3 * (j + n2 * i)
>>
>>      fun sub (arr as A3{elems, ...}, i, j, k) =
>>          Array.sub(elems, index(arr, i, j, k))
>
> There will be no tuple construction/select for getting arr, i, j, k.
> However, there may be record selects to get elems, n1, n2, n3.  MLton
> doesn't do loop-invariant code motion, so those selects will not get
> lifted outside the loop automatically.  Since MLton does
> common-subexpression elimination, it should be possible to convince it
> to move the selects of elems, n1, n2, n3 outside the loop by doing
> those same selects in some code that dominates the loop (e.g. by
> creating a loop header that is the only entry) and doesn't get
> optimized away.  Easier said than done, and I don't have a ready idea
> at hand.  But it might be worth looking into, after first verifying
> from the generated code that the selects indeed aren't lifted out.
>
I suppose I can figure this out from the intermediate files.

I'm making an unguided attempt to do so, but finding something  
specific like the selects will require a little guidance :-)


> Another alternative is to write a simple loop-invariant code motion
> pass for MLton :-).  It shouldn't be too hard to get this sort of
> thing; it's just that none of us has ever found the time.

I've got the source and I'm looking through it.

Any docs ? :-)

Where in the compiler would the SLICMP live ?

My guess is that it would take 85% of my time to understand the  
compiler program structure and  15% of my time to actually code  
something.

such an effort probably means having a good understanding of the  
intermediate representation.  So I think I'll just have to spend some  
time looking through the listing.

Brian