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From: Martin Bonner (Martin.Bonner_at_[hidden])
Date: 2008-02-26 10:34:04

----Original Message----
From: boost-bounces_at_[hidden]
[mailto:boost-bounces_at_[hidden]] On Behalf Of Johan Råde Sent: 26
February 2008 15:08 To: boost_at_[hidden]
Subject: Re: [boost] Ann: Floating Point Utilities Review starts today

> 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:
>>> 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
> 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.

I'm not sure I'm a pedantic language lawyer, but the standard says "In a union, at most one of the data members can be active at any time". It is quite clear that experts in CLC++M regard John Maddock's version of setbits and getbits as undefined behaviour. In general, I would regard using undefined behaviour as unacceptable for a boost library. The floating point utilities are different, they pretty much /have/ to use undefined behaviour. They also seem to be taking the correct approach of using #ifdef, and testing on as many platforms as possible (I haven't reviewed them yet).

On the other hand, even if floating point utilities have to go through a minefield, there doesn't seem any point in wandering around more than necessary. Particularly if the tested compilers don't actually generate better code for the undefined behaviour version!

Martin Bonner
Senior Software Engineer/Team Leader
Telephone: +44 1223 441434 / 203894 (direct)
Fax: +44 1223 203999
Email: martin.bonner_at_[hidden]

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