[MLton-devel] improvement for Int.{div,mod}

[Matthew Fluet]

> So, "x div 16" simplifies to
>
>   if x >= zero
>     then I.quot (x, 16)
>     else I.quot (x + one, 16) -? one
>
> The assembly I see for the then branch is
>
> 	movl %esi,%edi
> 	sarl $3,%edi
> 	shrl $28,%edi
> 	addl %edi,%esi
> 	sarl $4,%esi
>
> The assembly I see for the else branch is
>
> 	incl %esi
> 	movl %esi,%edi
> 	sarl $3,%edi
> 	shrl $28,%edi
> 	addl %edi,%esi
> 	sarl $4,%esi
> 	decl %esi
>
> Both of these branches appear to be doing stuff to handle both
> negative and positive values of x (aka %esi).  For example, the then
> branch could be simplified to a single shrl, since we know x >= 0.

O.k., now I understand.  Unfortunately, this simplification is performed
as a peephole optimization, so the "context" is exactly  idiv x,$16.
I.e., we don't have any information about the sign of x.

If we wanted to perform such an optimization, I think we would want to
introduce integer shift primitives and do the transformation at the SSA
IL.




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