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

[Paulo Marques]

Eric Weddington wrote:
> [...]
> I can't say why you are seeing what you describe above. But I know that 
> the vast majority of new development work (especially in the AVR port) 
> is being done in 4.x and specifically in the GCC SVN HEAD which will 
> become GCC 4.1 upon release.
>
> Björn Hasse could probably better answer your question about GCC internals.

I had a somewhat similar problem with gcc 4.0.2.

The following code:

> unsigned char a, b;
> unsigned short w;
>
> w = a * b;

correctly translates into a single "mul" instruction under gcc 3.4.3 
(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.

Compiling with -mint8 and changing the "unsigned short" to "unsigned 
long" is even worse as the multiply is done with an 8 bit result, 
explicitly zeroing the upper byte.

I did checkout the latest gcc, using subversion, but since I know little 
about gcc internals, I got lost before reaching any useful conclusion.

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? :)

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

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