Re: [avr-chat] Missed Optimisation ?

[Graham Davies]

Pertti Kellomäki wrote

> If you think of avr-gcc primarily as a tool that maps C programs to
> the specific chips manufactured by Atmel, then the compiler indeed has
> complete freedom to do whatever it wants as long as the external
> behavior of the chip (i.e. the physical pins) is in accordance with
> the source program.

That might be going a bit too far.  When debugging a program, I like to be 
able to follow along the logic I expressed in the source code, so I would 
not be happy if it had changed beyond all recognition.  I prefer the 
compiler to follow the standard except where there is no alternative but to 
deviate and maybe a few well-documented exceptions, such as the SBI thing.

> As noted, the behavior wrt. sbi probably violates the ANSI standard,
> yet nobody gets upset because of the convenience value offered.

Something else to note here is that, as far as I remember, SBI is only used 
on special function registers that have no read side effect.  So, the same 
knowledge of the AVR architecture that makes the use of SBI possible also 
ensures that it is safe, even with the target object declared volatile.  To 
my mind, this is just something that should be clearly documented with one 
of those warning icons in the margin.  In other words, I have no objection 
to this; I was just using it as an example of how subtle the semantics of 
volatile can be.

Graham.