Re: Linux and aliasing?

[Andi Kleen <ak@muc.de>]

toon@moene.indiv.nluug.nl (Toon Moene) writes:

> Jeffrey A Law wrote:
> 
> > Mark Mitchell wrote:
> 
> >   > Either
> >   > 1. Leave it as it is (the Linux kernel will need -fno-strict-aliasing).
> 
> > This is my strong preference.
> > 
> > I see no need to make conforming, portable code run slower.
> 
> Exactly.  Remember that a standard is a contract between producer and
> (end-)user, in our case:  between compiler writer and C programmer.
> 
> 	"We won't optimize your constructs away as long as you program 
> 	 according to said standard"


Erm, there seem to be some misunderstandings about the C standard in
this discussion.

My C9x draft says: 

6.2.6.1
 [#5] Certain object representations  need  not  represent  a
       value  of the object type.  If the stored value of an object
       has such a representation  and  is  accessed  by  an  lvalue
       expression  that  does not have character type, the behavior
       is undefined.  If such a representation  is  produced  by  a
       side  effect  that modifies all or any part of the object by
       an lvalue expression that does not have character type,  the
       behavior is undefined.37)  Such a representation is called a
       trap representation.


Now it says undefined behaviour is:

       3.18
       [#1] undefined behavior
       behavior, upon use of a  nonportable  or  erroneous  program
       construct,  of  erroneous data, or of indeterminately valued
       objects, for which this International  Standard  imposes  no
       requirements

So it imposes no requirements on what to do when it happen. This means gcc
is free to do what it wants. This includes unreasonable things, or reasonable
things. I think turning alias analysis off in this case is reasonable, and
of course fully standards compliant. Also the argument "that will slow
down legal programs" is non sense, because there are no strictly conforming
programs which can do this. 


-Andi

P.S.: Toon, this is not Fortran ;)

-- 
This is like TV. I don't like TV.