Re: AW: [avr-libc-dev] eeprom_read_word causes problems for the compiler

[Wouter van Gulik]

Haase Bjoern (PT/EMM1) schreef:
> > Yes I will try to put together a patch with the minimal changes. I only
> need
> > to know one thing; can I assume that using this construction:
> >
> > /** \ingroup avr_eeprom
> >    Read one 16-bit word (little endian) from EEPROM address \c addr.
> */
> > uint16_t
> > eeprom_read_word (const uint16_t *addr)
> > {
> >  register uint16_t result asm("r24"); //I can specify r24 only
> >
> >  __asm__ __volatile__ (
> >        XCALL " __eeprom_read_word_" _REG_LOCATION_SUFFIX CR_TAB
> >       : "+x" (addr),
> >         "=w" (result) //Use w restrict to adiw capable registers
> >       : );
> >  return result;
> > }
>
> I fear that this will not be guaranteed to be correct! The compiler
> might reorder code and might be tempted to optimize away your result
> variable so that it could be placed in another register. The dangerous
> thing is, that your code might work sometimes!

I found even more "prove":

http://gcc.gnu.org/onlinedocs/gcc-4.2.2/gcc/Extended-Asm.html#Extended-Asm
"Sometimes you need to make an asm operand be a specific register, but 
there's no matching constraint letter for that register by itself. To 
force the operand into that register, use a local variable for the 
operand and specify the register in the variable declaration. See 
Explicit Reg Vars. Then for the asm operand, use any register constraint 
letter that matches the register:
     register int *p1 asm ("r0") = ...;
     register int *p2 asm ("r1") = ...;
     register int *result asm ("r0");
     asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));"

So I think it's perfectly legal to do what I wanted to do.

I checked the documentation, this feature is documented back to at least 
2.9.5. Did AVR even existed in GCC back then? If not it will not causes 
a problem for users building avr-libc with an older version of GCC
The 2.9.5 documentation:
http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC96

HTH,

Wouter