Re: [avr-gcc-list] Missing compiler optimizations?

[Paulo Marques]

Erik Walthinsen wrote:
> [...]
> In other cases I've seen all kinds of relatively common idioms get 
> translated very poorly.  A classic one is constructing a word from two 
> bytes, which in my case happens as TWI interrupts feed them to me one at 
> a time:
>
>   w = b | (c<<8);
>   68:   99 27           eor     r25, r25
>   6a:   33 27           eor     r19, r19
>   6c:   32 2f           mov     r19, r18
>   6e:   22 27           eor     r18, r18
>   70:   82 2b           or      r24, r18
>   72:   93 2b           or      r25, r19
>
> w is 16 bits, b and c are both 8.  This should be doable with just two 
> mov's, but instead it takes 6 instructions.  I understand that it's 
> promoting the variables, but it's still inefficient.

I can not help you with the other stuff, but for this case I usually 
define a structure like this:

typedef struct { uint8_t l, h} BYTES;

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

Then, you can just do:

        var.b.l = b;
        var.b.h = c;

and access the 16 bit value with "var.w";

Not only gcc gets this right, but it even makes the code more readble :)

I hope this helps,

--
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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