RE: [PATCH] C undefined behavior fix

[Bernard Dautrevaux]

> -----Original Message-----
> From: Laurent Guerby [mailto:[e-mail]]
> Sent: Sunday, January 06, 2002 2:41 PM
> To: [e-mail]
> Cc: [e-mail]; [e-mail]; [e-mail];
> [e-mail]; [e-mail]
> Subject: Re: [PATCH] C undefined behavior fix
> 
> 
> [e-mail] wrote:
> 
> > pragma Atomic in Ada (volatile gets close in C, but is not 
> close enough) will
> > ensure a byte store in practice, but may not ensure byte reads.
> 
> 
> I see no distinction between read and write in the text of 
> the Ada standard.
> 
> Also I think if you declare a byte array with Atomic_Component
> and Volatile_Component and that the compiler accepts it for its
> target architecture, then the compiler is required to generate
> a byte read and store for each occurence in the text source.
>  From C.6:
> 
> 15    For an atomic object (including an atomic component) 
> all reads and
> updates of the object as a whole are indivisible.
> 
> 16    For a volatile object all reads and updates of the object as a 
> whole are
> performed directly to memory.
> 
> 20    {external effect (volatile/atomic objects) [partial]} 
> The external
> effect of a program (see 1.1.3) is defined to include each read and 
> update of
> a volatile or atomic object. The implementation shall not 
> generate any 
> memory
> reads or updates of atomic or volatile objects other than 
> those specified by
> the program.
> 
> In my exemple byte array, if I say X := T (I) I don't see how 
> a conformant
> compiler accepting the declaration could generate anything other than
> one and exactly one byte read. Per 20 it has no right to read 
> T (I+1) or 
> T(I-1)
> since they are "other" objects (components to be pedantic).

It seems to me that you are right, but there is other cases; for example:
	X := T(0)*256 + T(1);
compiled on a big endian architecture may well generate just one 16-bit word
read... 

	Bernard

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Bernard Dautrevaux
Microprocess Ingenierie
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