Re: [boost] Ann: Floating Point Utilities Review starts today

From: Johan Råde (rade_at_[hidden])
Date: 2008-02-26 10:07:33

John Maddock wrote:

> Sorry to bring this up again (!), but I've been trying to get my head around 
> the aliasing rules and what is and isn't allowed, in particular by following 
> the rather useful article here: 
> http://www.cellperformance.com/mike_acton/2006/06/understanding_strict_aliasing.html
> 
>>From this it appears that your current code is perfectly fine, however I 
> still can't help feeling that we could do better than call an external 
> function (memcpy) to move 4 or 8 bytes.  

When you compile on Visual Studio, with the compiler option /Oi (or /O2),
then the compiler treats memcpy as a compiler intrinsic (i.e. essentially as a key word)
and not as an external function; see http://msdn2.microsoft.com/en-us/library/tzkfha43.aspx
I looked at assembler output a year ago, and, if I remember right,
a memcpy of four bytes was compiled to a single move instruction.

The suggestion in the article is to
> use a union to move from one type to another:
> 
> union float_to_int32
> {
>    float f;
>    boost::uint32_t i;
> };
> 
> boost::uint32_t get_bits(float f)
> {
>    float_to_int32 u;
>    u.f = f;
>    return u.i;
> }
> 
> I can't claim to have tested exhaustively, but modifying get_bits and 
> set_bits to use this method seems to work OK for VC8&9 and Minwg GCC.
> 

Interesting, but I wouldn't do this without the approval of a pedantic langauge lawyer.
I have been bitten by this sort of things in the past.
And I'm not sure it would make things faster on Visual Studio anyway.

--Johan