nop optimized away

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darsie
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#1
Posted by darsie: Mon. Jul 12, 2010 - 11:53 PM
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Hi!

I have an unresponsive LPC flash BIOS chip which I want to program via an ATmega168. Since adding nops has resolved a timing problem before, I tried to add blocks of 4 nops, but they got optimized away, I guess.

For instance the example from
http://www.nongnu.org/avr-libc/u... produces no nop instead of 4:

void main(void) {

asm volatile("nop\n\t"
             "nop\n\t"
             "nop\n\t"
             "nop\n\t"
             ::);

}

Same thing with more than 3

asm volatile("nop"::);

It seems that any sequence of 4 or more nops produces no nops if it is an even number and one nop if it is odd.

This now explains how avr-gcc magically only included the original nops as it was then the 5th with the 4 nop block added.

Tags:

Tools, Compilers and General Programming, ATmega16, ATmega168
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Koshchi
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Posted by Koshchi: Tue. Jul 13, 2010 - 01:59 AM
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I tried it on all optimization levels (-O0, -O1, -O2, -O3, -Os), and it produced 4 NOPs each time.

Regards,
Steve A.

The Board helps those that help themselves.

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EW
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Posted by EW: Tue. Jul 13, 2010 - 05:09 AM
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You need to use the new builtin function for delaying a specific number of cycles:

extern void __builtin_avr_delay_cycles(unsigned long __n);

Specify the exact number of cycles that you need as the parameter. The compiler will generate the do-nothing code for you, and it won't be optimized away.

Eric Weddington
www.linkedin.com/in/ericweddington/

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meslomp
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Posted by meslomp: Tue. Jul 13, 2010 - 05:19 AM
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what versions software do you use?

what happens if you do:

asm volatile("nop\n\t"::); 
asm volatile("nop\n\t"::); 
asm volatile("nop\n\t"::); 
asm volatile("nop\n\t"::);

regards

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alank2
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Posted by alank2: Tue. Jul 13, 2010 - 12:56 PM
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Hi,

Or use my library here:

https://www.avrfreaks.net/index.p...

Thanks,

Alan

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alank2
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Posted by alank2: Tue. Jul 13, 2010 - 12:59 PM
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Hi,

Or you can use the builtin function that EW mentions. Here are some defines that help with that:

#define Delay_ns(__ns) \
    if((unsigned long) (F_CPU/1000000000.0 * __ns) != F_CPU/1000000000.0 * __ns)\
          __builtin_avr_delay_cycles((unsigned long) ( F_CPU/1000000000.0 * __ns)+1);\
    else __builtin_avr_delay_cycles((unsigned long) ( F_CPU/1000000000.0 * __ns))
#define Delay_us(__us) \
    if((unsigned long) (F_CPU/1000000.0 * __us) != F_CPU/1000000.0 * __us)\
          __builtin_avr_delay_cycles((unsigned long) ( F_CPU/1000000.0 * __us)+1);\
    else __builtin_avr_delay_cycles((unsigned long) ( F_CPU/1000000.0 * __us))
#define Delay_ms(__ms) \
    if((unsigned long) (F_CPU/1000.0 * __ms) != F_CPU/1000.0 * __ms)\
          __builtin_avr_delay_cycles((unsigned long) ( F_CPU/1000.0 * __ms)+1);\
    else __builtin_avr_delay_cycles((unsigned long) ( F_CPU/1000.0 * __ms))
#define Delay_s(__s) \
    if((unsigned long) (F_CPU/1.0 * __s) != F_CPU/1.0 * __s)\
          __builtin_avr_delay_cycles((unsigned long) ( F_CPU/1.0 * __s)+1);\
    else __builtin_avr_delay_cycles((unsigned long) ( F_CPU/1.0 * __s))

Good luck,

Alan

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