Re: [boost] [endian] Floating point reversal return by value concerns

Subject: Re: [boost] [endian] Floating point reversal return by value concerns
From: John Maddock (jz.maddock_at_[hidden])
Date: 2015-03-18 13:12:26

On 18/03/2015 11:36, Beman Dawes wrote:
> On Mon, Jan 26, 2015 at 11:25 PM, Cliff Green <cliffg_at_[hidden]> wrote:
>> ..., I do have concern about one aspect of the new functionality (or
>> maybe I didn't read deep enough in the docs) - floating point endian
>> reversal functions which return by value.
>>
>> Unless something has significantly changed in the last few years, swapping
>> and returning floating point values is likely to silently change bits for
>> certain bit patterns / values. In particular, returning by value will
>> normalize some values (I assume as part of loading or accessing the value
>> into floating point CPU registers). There may be other floating point
>> characteristics that silently change the bits dealing with NaN and infinity
>> values (but I'm far from an expert on floating point computations).
> Could you please provide code for one or more return-by-value test
> cases that fail because the value gets normalized?
>
> I'm really hesitant to change anything without having test cases to
> work with. That would also help with documenting the specifics of the
> problem.

I don't think this should *ever* happen - well with one possible 
exception - on Intel x87 hardware an 80-bit register value may get 
rounded to a 64-bit value at unpredictable times (ie only when it's 
written to memory).  I can't see that would be an issue here since Beman 
is already doing bit-fiddling on the value, it must by it's very nature 
be in memory and not a register.  I don't remember the specifics but 
return by value may actually be a return-by-register for some ABI's.... 
ah wait.... Beman are you returning by value a non-native 
representation?  If so I imagine that may be an issue as the value gets 
moved to and from a register: some bit values are not permitted I 
believe - it's anybodies guess what the hardware would do to those?   
You could actually test all possible bit-patterns in a float and see if 
this is a real issue?

John.