[FFmpeg-devel] [PATCH 3/7] Add alias-safe union typedefs

[Michael Niedermayer]

On Sun, Jan 31, 2010 at 03:56:23PM -0500, Alexander Strange wrote:
> 
> On Jan 31, 2010, at 8:08 AM, Michael Niedermayer wrote:
> 
> > On Fri, Jan 29, 2010 at 11:06:15AM +0000, M?ns Rullg?rd wrote:
> >> Michael Niedermayer <michaelni at gmx.at> writes:
> >> 
> >>> On Fri, Jan 29, 2010 at 02:12:10AM +0000, Mans Rullgard wrote:
> >>>> ---
> >>>> libavutil/intreadwrite.h |   18 ++++++++++++++++++
> >>>> 1 files changed, 18 insertions(+), 0 deletions(-)
> >>> 
> >>> I belive these types would be usefull to projects using libavutil
> >> 
> >> Here we go again...
> >> 
> >> Those rely on the may_alias attribute being supported, which is tested
> >> by configure.  I guess it could be changed to check for gcc version
> >> instead, although that is less reliable with non-gcc compilers.
> > 
> > After looking in the c spec ..
> > why do we need may_alias at all? a union should be enough or am i missing
> > something?
> > 
> > That said, these unions only need a char array as a char access can alias
> > anything. Otherwise if iam wrong they need float & double too
> > 
> > And with gcc a much better solution is along the lines of
> > 
> > #define UD(v,access)\
> >    ((union {\
> >        access a;\
> >        typeof(v) b[sizeof(access)/sizeof(typeof(v))];\
> >    }*)(v))->a
> > 
> > this specifies only the effective type + the accesed type and thus reduces
> > the restrictions placed on the optimizer.
> > It also shows how ridiculous this is as the compiler clearly knows the types
> > already.
> 
> It's not legal to cast a pointer like that, so any type cast without extra attributes can't work with strict aliasing:
> 
> C99 6.3.2.3 p7:

> A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type.

fine


> If the resulting pointer is not correctly aligned for the pointed-to type, the behavior is undefined.

yes, of course, all accesses thrugh *intXY_t are aligned, or we use special
types marked with attributes that have no alignment requirements or plain char


> Otherwise, when converted back again, the result shall compare equal to the original pointer. 

not relevant to us here


>When a pointer to an object is
> converted to a pointer to a character type, the result points to the lowest addressed byte of the object.

not relevant to us here


> Successive increments of the result, up to the size of the object, yield pointers to the remaining bytes of the object.

not relevant to us here


> 
> There's no text about dereferencing a converted pointer, so it's undefined.

6.5.7 Expressions ISO/IEC 9899:TC2 Draft May 6 2005 (latest i have)
7 An object shall have its stored value accessed only by an lvalue expression that has one of
  the following types:74)
   a type compatible with the effective type of the object,
   a qualified version of a type compatible with the effective type of the object,
   a type that is the signed or unsigned type corresponding to the effective type of the
      object,
   a type that is the signed or unsigned type corresponding to a qualied version of the
      effective type of the object,
   an aggregate or union type that includes one of the aforementioned types among its
      members (including, recursively, a member of a subaggregate or contained union), or
   a character type.
74) The intent of this list is to specify those circumstances in which an object may or may not be aliased.


[...]

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