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

[Philip T. <ph...@za...>]

Gino van den Bergen wrote:
> Yes, I know. Call me reckless but I just do not believe that 
> strict-aliasing on union fields will be exploited by a C++ compiler any 
> time soon. It will break too much existing code (not only mine).

Exploiting strict aliasing for normal non-union variables breaks a lot 
of existing code too, but GCC does it by default anyway, presumably 
because the performance benefits are significant and because it can give 
adequate warnings for broken code.

It still seems unlikely that a compiler would break code using union, 
because the cost/benefit tradeoff isn't as worthwhile as for general 
pointer strict-aliasing (it probably wouldn't help performance in many 
programs, and it would probably be hard to emit warnings without lots of 
false positives or negatives). But what reasons are there to 
intentionally *not* write code that unambiguously respects the aliasing 
rules?

In the float-to-int case, instead of writing "i = *(int*)&f" you can 
simply write "memcpy(&i, &f, 4)", which (as far as I can tell) is 
perfectly valid in terms of aliasing, and usually just as efficient 
(sometimes a little more, sometimes a little less) as using *(int*)& or 
union, since the function call is entirely optimised away by common 
compilers. And then there's no need to continue arguing over the safety 
of union :-)

(And if you really care about performance in the float-to-int code then 
you'll use whatever compiler-specific tricks give the right result, and 
be careful to test its correctness and performance whenever you change 
compiler or compiler options, and none of the standards-related 
discussion matters.)

-- 
Philip Taylor
ph...@za...