Re: [avr-chat] Missed Optimisation ?

[Michael Hennebry]

On Tue, 1 Mar 2011, Graham Davies wrote:

> Michael Hennebry wrote:
>
> > We haven't been told how or if "result" is defined.
>
> Yes, we have.

On further examination, I did find a "volatile uint32_t result;".
In context, I would guess that it was a complete
statement in the same file as the ISR.
Note the absence of attributes.
How could result not be in internal SRAM?

> > ISRs can be called like ordinary functions.
>
> Only if you're nuts, but it's irrelevant, anyway.

The compiler doesn't know whether the programmer is nuts.

> > The compiler could know [whether Bob is right or wrong to
> > declare 'result' volatile].  If the definition is in the same file
> > as the ISR, the compiler could know that "result" need not
> > be volatile, though not in this case.
>
> This is completely wrong-headed.  The whole point of volatile is to tell the
> compiler something about a variable that it cannot see in the source code of
> the translation unit.

That is not the same as blinding it to
things it can see in the source code.

> > In this case, if the definition is in the same file as the ISR,
> > the compiler would know that "result" is in internal SRAM.
>
> No, it wouldn't.  The C language supports only a single address space and
> assumes that all variables that are not declared or are implicitly automatic
> are in "memory".  We have to jump through hoops to make the C language work
> with the Harvard architecture of many microcontrollers.  This does not
> further the discussion.
>
> > From this, the compiler could correctly conclude that
> > neither fetches nor stores have side-effects.
>
> No, it can't.  Or, it isn't useful to conclude that there are not side
> effects because there are other reasons to use the storage qualifier
> volatile that require the same behavior from the compiler.

Can you give an example that cannot be
eliminated by looking at the source code?

PORTB |= _BV(3);
is usually compiled as an SBI instruction.
Following the volatile rules blindly
would require at least two accesses.

--
Michael   address@hidden
"Pessimist: The glass is half empty.
Optimist:   The glass is half full.
Engineer:   The glass is twice as big as it needs to be."