Hi all,
OK fine I agree.
we have to use 'volatile' and all.
But why must it generate such horrid code...
(I reproduce the comparison again below to get the *real* issue into focus)
The compiler output with the 'correct' code ('volatile' used):
//---------------------------------------------------
void delay(unsigned int del_cnt)
{
2aa: cf 93 push r28
2ac: df 93 push r29
2ae: cd b7 in r28, 0x3d ; 61
2b0: de b7 in r29, 0x3e ; 62
2b2: 22 97 sbiw r28, 0x02 ; 2
2b4: 0f b6 in r0, 0x3f ; 63
2b6: f8 94 cli ;<-----disabling interrupts?
Why?
2b8: de bf out 0x3e, r29 ; 62
2ba: 0f be out 0x3f, r0 ; 63
2bc: cd bf out 0x3d, r28 ; 61
volatile unsigned int n = del_cnt;
2be: 9a 83 std Y+2, r25 ; 0x02
2c0: 89 83 std Y+1, r24 ; 0x01
while(n--);
2c2: 89 81 ldd r24, Y+1 ; 0x01
2c4: 9a 81 ldd r25, Y+2 ; 0x02
2c6: 01 97 sbiw r24, 0x01 ; 1
2c8: 9a 83 std Y+2, r25 ; 0x02
2ca: 89 83 std Y+1, r24 ; 0x01
2cc: 89 81 ldd r24, Y+1 ; 0x01
2ce: 9a 81 ldd r25, Y+2 ; 0x02
2d0: 8f 5f subi r24, 0xFF ; 255
2d2: 9f 4f sbci r25, 0xFF ; 255
2d4: b1 f7 brne .-20 ; 0x2c2 <delay+0x18>
2d6: 22 96 adiw r28, 0x02 ; 2
2d8: 0f b6 in r0, 0x3f ; 63
2da: f8 94 cli
2dc: de bf out 0x3e, r29 ; 62
2de: 0f be out 0x3f, r0 ; 63
2e0: cd bf out 0x3d, r28 ; 61
2e2: df 91 pop r29
2e4: cf 91 pop r28
2e6: 08 95 ret
return;
}
//=======================
Output of the older -WinAVR-20060421-version (with the alleged 'wrong' C code)
>> //==========================================
>> void delay(word cnt)
>> {
>> while(cnt--);
>> 286: 01 97 sbiw r24, 0x01 ; 1
>> 288: 2f ef ldi r18, 0xFF ; 255
>> 28a: 8f 3f cpi r24, 0xFF ; 255
>> 28c: 92 07 cpc r25, r18
>> 28e: d9 f7 brne .-10 ; 0x286 <delay>
>> 290: 08 95 ret
>> return;
>> }
>> //=======================================
>>
So which is the right output we want? Obviously the second.
Agreed not using 'volatile' optimises the code to a mere 'ret'. I'm ok with
that.
But should'nt the 'correct' code produce the same output (the shorter version
above)??
Thanks,
--Royce.
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