Re: [avr-gcc-list] SIGNAL and INTERRUPT overhead

[Bernard Fouché]

Royce Pereira wrote:

> > That's a big problem with the usual way of building code, with 
> > functions  or set of related functions in
> > different files. When the compiler groks the files, it does not have 
> > all  the information that would help it build a much better optimized 
> > code.
>
>
> If a 'callee-saves" convention is followed universally, then the 
> calling  function need not know the registers used by the callee, right?
>
> --Royce.
Suppose you have:

f1()
{
 push rA
 push rB
 .. uses rA & rB then does not care of their value until the end of the 
function..
 f2()
 ...
 f2()
 ..
 f2()
 pop rB
 pop rA
}

f2()
{
 push rA
 push rB
 .. uses rA & rB..
 pop rB
 pop rA
}

If f2() is only called from f1(), then it is possible to skip the 
push/pops in f2().
If f2() is called only once in the final .elf then it could have been 
inline'd with
a sure win.

The point is that you don't need anymore "callee-saves" convention when you
don't need to call a function anymore, or if you perfectly know what the 
called
function does, you can optimize (and break!)  the convention locally.

If f1() and f2() are in two different files, the compiler can not know what
to do in a optimal way and has to stick to the convention.

 Bernard