Re: [avr-gcc-list] Problem with delay loop

[David Brown]

Royce Pereira wrote:
> Hi,
>
> On Mon, 01 Oct 2007 13:02:32 +0530, David Brown
> <address@hidden> wrote:
> > Royce Pereira wrote:
> > > So I have to write more 'C' code :) to get the same stuff done,
> > > in the 'new & smarter' compiler! Interesting.....
> > >
> > > Doesn't seem right, some how.
> > >
> > > Regards, --Royce.
> > It might not seem right, but that's the way it is.  The compiler
> > only has to generate code that has the same effect as the source
> > you've written, and a simple "ret" has that effect.
>
> Why then was the empty 'ret' function retained?

If the function is globally visible (i.e., not static to the file), then
the compiler must generate a working function - "ret" is as small and
fast as it gets.

> I would think such a case would be the prime candidate for
> optimisation. The compiler should eliminate such a funtion, as well
> as all calls to that function. That would really make a difference in
> size/speed of the code.

The compiler will inline calls to the function in the same translation
unit, and thus eliminate the "ret".

> (Instead, the compiler destroys a perfectly good delay loop I've used
> for the last 2 years -yes, I'm still sore)
>
> > There is no way in C to express the idea of a time delay - the
> > language has no concept of time. Thus you have to go out of your
> > way to tell the compiler to be particularly stupid if you want this
> > sort of code to work.  There are other ways to get a delay, such as
> > the library routines or hardware timers, or by including something
> > like an "asm volatile ("nop")" in the loop, but using a volatile
> > loop counter is an easy way to get an approximate delay loop.
> Of course, there's no disputing that. But the delay loop is just an
> example, of how simple ,intuitive code can throw the compiler into a
> tizzy. I've used SDCC(for mcs51) where the compiler 'recognises' code
> patterns, and says "Oh, I know what this is - it's a delay loop! -
> Let it pass."(for example).

That sounds to me like a very bad idea, as you'll find out the day you 
write a slightly unusual delay loop which is not pattern matched, and 
your code gets eliminated.

> I've always maintained -  good software is one that does what you
> *want* it to do, *not* what you tell it to do. ;)

I prefer software to do as I tell it, but perhaps to warn me if that's 
not what it thinks I mean.  Thus gcc will accept code like "if (a = 1) 
...", but will warn that it's probably not what I meant.  Of course, it 
would all be a lot easier if the language in question had a way to 
express what I want!

> Regards, --Royce.