Re: [Algorithms] How to get 3dvector largest coordinate index?

[Charles N. <cha...@gm...>]

On Wed, Mar 4, 2009 at 10:23 PM, 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:
>

This C99 TC3 note you keep quoting doesn't guarantee that it will always
work, though, right?  The last sentence in the note reads:

"This might be a trap representation."

To me that means that you can't just trust it to always work, it might blow
up and is compiler-and-platform specific.  I don't think you have the
blessing of the C99 standards committee to cast-through-union whenever you
want and they promise it will be ok.

Plus I don't believe it's part of the C++03 standard, so even if gcc
implements it there's no guarantee that MSVC will (in C++ mode), if they
ever start exploiting TSAA-based optimizations by default.  (they don't
support C99 yet either).

-charles

 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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