nop optimized away

Log in or register to post comments
Go To Last Post
6 posts / 0 new
Author
Message
darsie
darsie's picture
Level: Rookie
Joined: Fri. May 11, 2007
Posts: 43 View posts
#1
Posted by darsie: Mon. Jul 12, 2010 - 11:53 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

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
Top
Koshchi
Koshchi's picture
Level: 10k+ Postman
Joined: Wed. Nov 17, 2004
Posts: 15674 View posts
Location: Vancouver, BC
Posted by Koshchi: Tue. Jul 13, 2010 - 01:59 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

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.

Top
EW
EW's picture
Level: Raving Lunatic
Joined: Thu. Mar 1, 2001
Posts: 4934 View posts
Location: Rocky Mountains
Posted by EW: Tue. Jul 13, 2010 - 05:09 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

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/

Top
meslomp
meslomp's picture
Level: Raving Lunatic
Joined: Wed. May 2, 2007
Posts: 5750 View posts
Location: Nieuwegein, Netherlands
Posted by meslomp: Tue. Jul 13, 2010 - 05:19 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

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

Top
alank2
alank2's picture
Level: Raving Lunatic
Joined: Thu. Jul 16, 2009
Posts: 2783 View posts
Posted by alank2: Tue. Jul 13, 2010 - 12:56 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi,

Or use my library here:

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

Thanks,

Alan

Top
alank2
alank2's picture
Level: Raving Lunatic
Joined: Thu. Jul 16, 2009
Posts: 2783 View posts
Posted by alank2: Tue. Jul 13, 2010 - 12:59 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

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

Top