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
Pointy-Haired Boss: I don't see anything that could stand in our way.
Dilbert: Sanity? Reality? The laws of physics?
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