Re: [avr-chat] Missed Optimisation ?

[Graham Davies]

Michael Hennebry wrote:

> 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?

You may know that 'result' is going to be in internal SRAM, but you know 
things that the compiler doesn't.  The compiler just puts the variable in 
memory.

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

... and is not required to make the program behave sensibly when the 
programmer does something nutty like calling an ISR as an ordinary function.

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

The compiler is not allowed to infer that just because a variable is only 
accessed from one place in the source code it can see, that it is not also 
accessed from source code it cannot see during the translation.

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

No, but that's not the point.  The compiler can't see all the source code of 
the project at the same time.  If a variable is accessed by two different 
threads of execution that appear in two different translation units, the 
compiler cannot know that it must avoid certain optimizations unless the 
programmer "tells it" using the volatile storage qualifier.

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

Strictly speaking, this AVR-specific optimization violates the language 
specification, given that PORTB is volatile.  The compiler should generate 
code that reads the PORTB register, performs the OR operation and then 
writes the result back to the PORTB register.  Actually, I would hazard a 
guess that this is what the compiler actually does and it is some 
post-compilation step that embodies specific knowledge about the AVR 
architecture that swaps out the read-modify-write for the set-bit-immediate.

Graham.

P.S.  We may be descending into a personal argument.  I would be happy at 
this point to say "we're both right, from out different points of view, but 
are perhaps not explaining it well to each other".  OK?