[Algorithms] Strict Aliasing ( was Re: How to get 3dvector largest coordinate index? )

[Justin S. <jus...@ho...>]

FYI: I believe Mike Acton covered the issue pretty thoroughly here:
http://www.cellperformance.com/mike_acton/2006/06/understanding_strict_aliasing.html

No explanation of the specialness of char* however.

Cheers,
Justin.

On 05/03/2009, at 5:37 PM, Jeff Russell wrote:

> yea i was gonna say, use of unions to reinterpret types is fairly  
> widespread from what i can tell. if its wrong, a lot of software  
> could have a lot of trouble with a "correct" compiler. or is this  
> one of those cases where no compiler does it "right" anyway (at  
> least by default) because so much existing code would break?
>
> the performance benefit of strict aliasing seems straightforward and  
> hard to argue with. but it also seems like there should be  
> *something* though. reinterpret_cast doesn't do the job either?  
> seems odd that somehow the char* would be magical...
>
> jeff
>
> On Thu, Mar 5, 2009 at 12:23 AM, Matt J <mjo...@gm...>  
> wrote:
>
> Well, I guess it depends on what standard you use (so many to choose  
> from). But like I mentioned before in the latest C99 TC3 standard it  
> adds a clause that says it is okay to do "type punning" through a  
> union. Here is an FFMPEG post that discusses it in detail:
>
> http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/2008-December/056962.html
>
> Finally, even CERT thinks its okay:
>
> https://www.securecoding.cert.org/confluence/download/attachments/26017980/SD+West+Dangerous+Optimizations.pdf?version=1
>
> See page 75. Which was written in 2009. I understand that is an  
> argument based on an appeal to authority, but the assumption here is  
> these four guys that wrote this understand good coding practices  
> better than most. GCC holds C++ code to C99 standards, from what I  
> read. So.... code that tried to optimize unions would be non- 
> conforming not to mention break tons of legacy code in C and C++
>
>
> > if cast-through-union violates the standard, then why are unions in
> > the language? isn't storing bytes and interpreting them as different
> > types what unions are for?
>
> No, that's not what unions are for. Unions are there to allow
> you to store different type data in the same memory location
> at DIFFERENT (nonoverlapping) TIMES.
>
> Standard-violating aliasing is when you have different type
> data in the same memory location at the SAME TIME.
>
> The standard disallows same-time different-type aliasing,
> because disallowing it allows for a lot of optimization
> opportunities. This is a GOOD THING -- except programmers
> need to learn to write standard-conforming code (which is
> also a GOOD THING).
>
>
>
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