Re: strict aliasing

[Joe Buck <Joe dot Buck at synopsys dot COM>]

On Tue, Nov 06, 2007 at 05:44:44AM +1100, skaller wrote:
> 
> On Mon, 2007-11-05 at 10:20 -0800, Joe Buck wrote:
> > On Mon, Nov 05, 2007 at 10:15:55AM -0800, Ian Lance Taylor wrote:
> > > skaller <skaller@users.sourceforge.net> writes:
> > > > Ah, I see. So turning [strict aliasing] off isn't really all that bad
> > > > for optimisation.
> > > 
> > > It depends on the processor.  For an in-order processor with a deep
> > > pipeline (e.g., Itanium, but there are others), the ability to reorder
> > > loads and stores is very important to permit scheduling flexibility.
> > > Strict aliasing reportedly makes a difference for x86, but I don't
> > > think it makes a huge difference.
> > 
> > It also depends on the input: for scientific codes that process both
> > integer and real data, strict aliasing could make a large difference
> > on any processor.
> 
> Hmm .. the problem is that C is fairly brain dead, and people
> regularly use casts: strict aliasing almost seems to break
> a fundamental feature of the language.

Strict aliasing has been the rule since 1989 and is a fundamental feature.
Many programmers believe that they can cast with abandon, but this is not
the case.

> [Hmm .. how strict is it? int* and unsigned* have to be aliased,
> there's even a rule that guarantees that for the common subset
> of values the representation is the same (that rule would be
> useless if you couldn't alias an int and an unsigned!)

This isn't the place to teach people the exact rule; it's in the standard.
As you say, the unsigned and signed versions of integral types can
alias.