Re: [Qemu-devel] [PATCH 2/5] CODING_STYLE: add C type rules

[malc]

On Thu, 19 Aug 2010, Avi Kivity wrote:

>  On 08/19/2010 11:17 AM, Jes Sorensen wrote:
> > On 08/19/10 10:10, Avi Kivity wrote:
> > >   On 08/19/2010 10:58 AM, Jes Sorensen wrote:
> > > > > In C99, bool is a real type.
> > > > Kinda real, I would qualify it more as a pseudo type. It doesn't map to
> > > > any register size or even instruction actions.
> > > Neither do int, short, long, long long, or signed short long float.
> > Well most processors support 32 bit register ops, even when running in
> > 64 bit mode.
> 
> 8-bit processors have 8-bit register size and 16-bit ints.  Some processors
> have 36-bit ints.  If you only code for processors which have Linux ports, and
> for those ports, then everything will be the same.  Surprise!
> 
> > long is the maximum register size in pretty much any sane
> > setup,
> 
> Not on those 8-bit machines.
> 
> > which is why it is such a mess that M$ took the easy way out when
> > they picked for win64. Tons of user code will have portability problems
> > because of this.
> 
> They did that to avoid portability problems, presumably.

http://blogs.msdn.com/b/oldnewthing/archive/2005/01/31/363790.aspx

> 
> > > > Most processors, at least
> > > > the ones I have programmed, tend to treat it as zero == false,
> > > > everything else == true.
> > > Processors don't have _Bool.
> > No but something has to generate code to handle it.
> 
> gcc
> 
> > > > For structure packing it's ugly.
> > > Use uint*_t for externally visible structures.  use _Bool for internal
> > > booleans.
> > Structure packing is more than passing structures to external processes
> > or saving them, it's also cache line alignment. In your case we should
> > change pretty much any data structure in QEMU that is in a critical data
> > path to use *int*_t.
> 
> Not at all, _Bool is smaller or equal to int and thus can only improve
> packing.

That's not what standard says:

6.2.5#2

An object declared as type _Bool is large enough to store the values 0 and 

It later goes on to say (6.7.2.1 fn120):

While the number of bits in a _Bool object is at least CHAR_BIT, the width
(number of sign and value bits) of a _Bool may be just 1 bit.

> 
> If you want to align explicitly, use alignment attributes.
> 
> 

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