Re: RFC: vectorizer cost model

[Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>]

Hello,

> "Linthicum, Tony" <tony.linthicum@amd.com> writes:
> 
> >   * Would using a tree-level API like estimate_num_insns be superior
> >     to using a simple cost of 1 per statment?
> 
> For this sort of calculation, I would guess not.  I would imagine that
> on most processors the cost of a single vector instruction is
> comparable to the cost of the same operation on scalar operands.  So
> simply counting instructions should give you a good indication of
> whether vectorization is profitable, even though each instruction will
> of course have a different cost.
> 
> If this turns out not to be the case, then I think the next step would
> be to have a target defined multiplier for vector instructions,
> indicating that for that target vector instructions were more or less
> expensive than scalar instructions according to some ratio.
> 
> I doubt that using the full fledged cost infrastructure would give you
> better results than that in practice.

on the other hand, using it does not cost you anything, and would avoid
risk of creating yet another code duplication.

> >   * What is the best way to access target level cost information?
> 
> I'm sure you know that the loop code does this by generating RTL and
> asking for the cost of it (computation_cost in tree-ssa-loop-ivopts.c).

Which should be removed (I have some patches for that, but I somehow
forgot on them because of other issues).

Zdenek