Re: [avr-gcc-list] multiply by constant always expands to int

[Paulo Marques]

Eric Weddington wrote:
> Paulo Marques wrote:
> > [...]
> > (modulo the register moves) but promotes "a" and "b" to 2-byte 
> > integers under gcc 4.0.2, generating a bunch of mul's and add's.
>
> Could you provide a complete testcase, and not just a snippet? I'm 
> having a hard time reproducing what you are describing with 4.0.2.

So did I!

I thought this would just happen with any multiply operation, and tried 
to build a simple test case, and to my surprise everything was just fine.

I tried to make the simple case more and more like my real program, to 
no avail. I finally gave up and started the other way around, peeling of 
code from my real program until I could get the simplest possible 
program that also showed the problem.

After a lot of peeling, this is it:

> void test(void)
> {
>         unsigned short data;
>         unsigned char a, b;
>
>         a = PORTD;
>         b = PORTC;
>         
>         data = b * a;
>
>         OCR1A = data;
>
>         b = PORTC;
>         
>         data = b * a;
>
>         OCR1B = data;
> }

I read/write values to ports just to make sure the compiler doesn't 
optimize the values away.

The weird thing is that, if you comment out the _second_ multiply, the 
_first_ one becomes properly compiled as a single "mul" instruction. If 
you leave it in, then _both_ become 2 byte wide!

I compiled this with today's gcc from subversion and the result is the 
same. This was compiled with -Os, but I believe it happens with other 
otpimization levels too.

> > Is there any documentation (good or bad) about the gcc internals that 
> > I could use as a starting point? Or is this a "use the source, Luke" 
> > kind of scenario? :)
>
> That's a question for the gcc mailing list. 

Oops, sorry.

> The best thing to do is to 
> start looking around on the GCC website and looking at all of the 
> available documentation. 

Will do.

> Yes, they have an internals document. However, 
> from what I've heard from others, it's not exactly easy to get through; 
> it takes a lot of perserverance to learn GCC internals.

Ok, thanks for all the help :)

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

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