Re: [Qemu-devel] [PATCH v5 04/10] softfloat: Drop [u]int16 types in favor of [u]int_fast16_t

[Andreas Färber]

Am 07.03.2011 um 10:56 schrieb Aurelien Jarno:

> On Mon, Mar 07, 2011 at 01:34:07AM +0100, Andreas Färber wrote:
> > AIX already didn't use our definition, so let's start ripping out  
> > this one:
> > As pointed out by Peter Maydell, int16 is currently int on most
> > supported platforms, so let's replace it with int_fast16_t,
> > allowing the system to use a wider type if appropriate.
>
> Do you have a rationale about the use of the fast version of the  
> types?
> As you said it allows the compiler to use a wider type if appropriate,
> and this usually doesn't play very well when shifts are involved,  
> which
> is the case of the softfloat code. Have you verified that each
> conversion is correct? On the other hand I really doubt it has a
> measurable impact on speed.
>
> I would really go for uint16_t for now. This might be changed to the  
> fast
> version in a second step if we are sure everything is correct, but  
> let's
> don't mix the two steps for now.

SoftFloat distinguishes between [s]bits* and [u]int*. Wherever exact- 
width semantics were needed, [s]bits* was used (or should have been)  
and this is converted in 03/10. It was Peter's demand [1] that we  
retain this distinction and it was my suggestion to then use  
[u]int_fast*_t, which provides such semantics for POSIX, since the  
colliding use of [u]int* and its accidental uses outside SoftFloat are  
what this series seeks to address. The difference is that with POSIX  
the semantics are system-defined whereas right now they are QEMU- 
defined.

Most of the series was crafted in a semi-automatic fashion, doing a  
search and reviewing each hit before replacing. If we drop the type  
distinction and use uint16_t for both bits16 and uint16 then we cannot  
easily do the second step you propose.

My and, I figured, Peter's intent was to do this conversion without  
changing semantics as far as possible. Softfloat-native turned out  
difficult in that aspect, since it followed the definitions of the  
header rather than the types used in the SoftFloat implementation.  
There, the conversions need more careful review of whether they  
wrongly rely on fixed-width semantics.

But seeing the persistent rate of softfloat bug fixes, I would ask to  
address the license issue sooner than later. Doesn't need to be my  
patch, if someone feels tempted to do better.

Andreas

[1] http://lists.nongnu.org/archive/html/qemu-devel/2010-12/msg01552.html