Re: [ublas] Aliasing for += and -=

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2005-06-14 20:55:35

christopher diggins wrote:

>> I don't think the tradeoff's work that way.  Whether aliases appear in an
>> expression depends purely on how the user writes it,
> 
> It also depends on whether the library actually uses aliases in the first
> place.

What does it mean for the library to *use* aliases?  How does the library
know whether arguments are aliases (except in some special cases) ?

> 
>>  the question is
>> whether the library gets the correct answer or not.  The choices are:
>> always make a copy of the right-hand-side before assigning to the
>> left-hand-side.  This will always give the correct answer, but at a
>> performance cost.  This is the way uBLAS works.  OR never make a copy of
>> the right-hand-side and require the user to handle potential aliasing
>> issues.
> 
> Or never alias anything. Always correct and as I demonstrated, is more
> efficient than ublas for all of the basic operations.

What do you mean by "never alias anything"?  How can the *library* choose
that?

> 
>> In both cases, the default can be overridden - in the former by
>> providing an alternate syntax to perform the operation with a noalias
>> assumption, and in the latter by choosing a different algorithm (eg, to
>> copy backwards instead of forwards) or making an explicit copy.
>>
>> In one case, the default operation is slow and safe.  In the other, the
>> default operation is fast and dangerous.  None of the choices is both
>> unsafe and with a performance penalty.
> 
> Having a library which uses aliasing doesn't buy you anything except
> saving against the occasional superflous copy (which isn't a problem when
> using blas routines). My point is that the superflous copy is less common
> than the basic operation (like indexing, or scalar multiplication, or
> matrix addition, or matrix multiplication) which I showed alread was more
> efficient doing naively.

Now I am lost: I thought the (often) superfluous copy that uBLAS performs is
what you were arguing against?

Cheers,
Ian