Re: [avr-gcc-list] Avr-gcc Produces Incorrect Code with -Os

[Dave N6NZ]

John Regehr wrote:
> > > # define sei()  __asm__ __volatile__ ("sei" ::: "memory")
> > > # define cli()  __asm__ __volatile__ ("cli" ::: "memory")
<snip>
> Adding these clobbers is an extremely good idea because they make 
> cli/sei act like proper locks, in two senses.  First, they prevent 
> reordering of computations as has been discussed here.  Second, they 
> force data cached in registers to be written to RAM when the interrupt 
> lock is released. Both of these are important.
>
> Here's a subtle but important point: If you consistently protect shared 
> data with clobbering locks, you don't need to make data volatile when it 
> is shared between interrupts and main().  Of course you still need 
> volatile for accessing hardware registers.

Well, isn't the net effect of volatile simply a more fine-grained 
clobbering lock?

<snip>
> > The downside is that a memory clobber like that drops all 
> > optimizations of global variables that the compiler might be 
> > temporarily storing in registers. So, it actually hurts performance 
> > for code unrelated to the critical section.
>
> We studied the impact of adding clobbers to critical sections in TinyOS 
> and found that both code size and CPU usage actually decreased slightly 
> due to the clobbers.  No idea why.  Details here:
>
>   http://www.cs.utah.edu/~regehr/papers/plos06b.pdf
>
> TinyOS currently clobbers memory on every cli/sei.

That's a counter-intuitive result. The "No idea why." part makes me a 
little squinty-eyed.  It certainly *could* be a generalizable result, 
but then again it might be an artifact of your code structure.

I could see including both clobbering and non-clobbering flavors of 
sei()/cli() in avrlibc -- then we can have a whole new and entertaining 
argument about whether the old names sei()/cli() should have the new or 
old behavior :)  :)

-dave

> John Regehr