Hello, coming from an x86 C programming background, I've recently found myself working alot with atmel's microprocessors and the the avr-gcc toolchain.
I'm writing code for the attiny13 which is severely space-limited, and thus
have a question/suggestion about how avr-gcc generates code in case of reading
and writing large data types from/to memory.
Consider this piece of code:
// Global variable curacc: Current accumulated value
uint32_t curacc;
static void check_adc(void)
{
if((ADCSRA & _BV(ADIF)))
{
ADCSRA |= _BV(ADIF);
curacc += ADC;
}
}
I've found that the compiler translates the line:
curacc += ADC;
into:
0x0000025c <fh_timeout+52>: in r18, 0x04 ; 4
0x0000025e <fh_timeout+54>: in r19, 0x05 ; 5
0x00000260 <fh_timeout+56>: lds r24, 0x006E
0x00000264 <fh_timeout+60>: lds r25, 0x006F
0x00000268 <fh_timeout+64>: lds r26, 0x0070
0x0000026c <fh_timeout+68>: lds r27, 0x0071
0x00000270 <fh_timeout+72>: add r24, r18
0x00000272 <fh_timeout+74>: adc r25, r19
0x00000274 <fh_timeout+76>: adc r26, r1
0x00000276 <fh_timeout+78>: adc r27, r1
0x00000278 <fh_timeout+80>: sts 0x006E, r24
0x0000027c <fh_timeout+84>: sts 0x006F, r25
0x00000280 <fh_timeout+88>: sts 0x0070, r26
0x00000284 <fh_timeout+92>: sts 0x0071, r27
is there a reason why it has to use 4-byte long direct data space operations?
It might be more beneficial to set up a pointer and use indirect addressing:
in r18, 0x04 ; 4
in r19, 0x05 ; 5
clr r31
ldi r30, 0x6E
ld r24, Z+
ld r25, Z+
ld r26, Z+
ld r27, Z
add r24, r18
adc r25, r19
adc r26, r1
adc r27, r1
st Z, r27
st -Z, r26
st -Z, r25
st -Z, r24
which would save us 6 words in program space. The benefit would even be greater
for larger data types.
It comes with a 2-cycle performance penalty though, maybe higher if Z register
needs to be saved first.
I'm not much into compiler building and have no idea how hard it would be to
do. I'm just asking out of curiosity.
--
Best regards,
Thilo Schulz
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