Re: [avr-gcc-list] Shorter code?

[Paulo Marques]

Ruud Vlaming wrote:
> Hi
>
> Normally gcc generates well optimized code, but
> sometimes i wunder how gcc can do simple things
> so complicated.
>
> Here is an example, 
> [...]
>
> Is there a way  to write the methode defined above in C to make the 
> generate this assembly? Some special combine function maybe?

This missed optimization happens because the compiler promotes all 
operands to int before doing the operations and then it actually 
optimizes the "<< 8" using just mov's :P

One trick I use to help the compiler is to define something like:

typedef struct { uint8_t l, h; } BYTES;

typedef union {
  uint16_t w;
  BYTES b;
} WORD;

This way you can define:

WORD uxTickCount;

and then access the low byte using uxTickCount.b.l and the high byte 
using uxTickCount.b.h.

Then the function above would become:

uint16_t genGetTickCount(void)
{
  return uxTickCount.w;
}

At which point, it is probably not very useful anymore ;)

This trick could probably be even better by using anonymous structs, but 
I never bothered to try it.

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

"As far as we know, our computer has never had an undetected error."
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