C/C++ macro to repeat code

Question

is there any way to repeat a C code N times with a macro? Also N is a macro.
For example if I have this macros:

#define N 5  
#define COODE "nop\n\t"
#define REPEAT [...]

When I call repeat the preprocessor writes CODE N times, so

 __asm__(REPEAT);

would became

__asm__("nop\n\t""nop\n\t""nop\n\t""nop\n\t""nop\n\t");

I have an Arduino that have to wait for an exact (and small, about 10-15) number of clock. Each "nop" (no operation) takes exactly 1 clock cycle to be executed, and it does nothing. I can't just do a cycle, because each cycle is executed in more than one operation (initialize the counter, increment the counter, check if reached end), so instead of writing manually "nop\n\t" I'd like to have a macro. This way I can also simply change N to modify the program without rewriting it.

Thank you in advance

Answer 1

If you want to do it without including a whole library or using define, you can use a simple recursive template:

//By Christopher Andrews, released under MIT licence.

template< unsigned N > struct Nops{
  static void generate() __attribute__((always_inline)){
    __asm__ volatile ("nop");
    Nops< N - 1 >::generate();
  }
};
template<> struct Nops<0>{ static inline void generate(){} };

void setup() {
  Nops<10>::generate();
}

void loop(){}

This will generate the exact number of nop's required.

0000010a setup:
10a: 00 00 nop
10c: 00 00 nop
10e: 00 00 nop
110: 00 00 nop
112: 00 00 nop
114: 00 00 nop
116: 00 00 nop
118: 00 00 nop
11a: 00 00 nop
11c: 00 00 nop
11e: 08 95 ret

I have used this method in a TFT driver for Arduino.

EDIT:

There is another way to easily do this on an AVR compiled with avr-gcc. I'm assuming this may not be available on older toolchains.

In order to delay execution for a specific number of cycles, GCC implements

void __builtin_avr_delay_cycles (unsigned long ticks) ticks is the number of ticks to delay execution. Note that this built-in does not take into account the effect of interrupts which might increase delay time. ticks must be a compile time integer constant; delays with a variable number of cycles are not supported

From here: https://gcc.gnu.org/onlinedocs/gcc-4.7.1/gcc/AVR-Built_002din-Functions.html

Answer 2

The following code works with GNU C,

#define NOP __asm__("nop")

#define ten(a)     a;a;a;a;a;a;a;a;a;a
#define handred(a) ten(ten(a))


int
main()
{
    handred(NOP);
    return 0;
}

Compile and debug:

code@lab:~/debug$ gcc -g -o debug_NOP debug_NOP.c
code@lab:~/debug$ gdb -q --nh debug_NOP
Reading symbols from debug_NOP...done.
(gdb) set disassembly-flavor intel
(gdb) start
Temporary breakpoint 1 at 0x664: file debug_NOP.c, line 10.
Starting program: /home/code/debug/debug_NOP 

Temporary breakpoint 1, main () at debug_NOP.c:10
10      handred(NOP);
(gdb) disassemble 
Dump of assembler code for function main:
   0x0000555555554660 <+0>: push   rbp
   0x0000555555554661 <+1>: mov    rbp,rsp
=> 0x0000555555554664 <+4>: nop
   0x0000555555554665 <+5>: nop
   0x0000555555554666 <+6>: nop
   0x0000555555554667 <+7>: nop
   0x0000555555554668 <+8>: nop
   0x0000555555554669 <+9>: nop
   ....
   0x00005555555546c6 <+102>:   nop
   0x00005555555546c7 <+103>:   nop
   0x00005555555546c8 <+104>:   mov    eax,0x0
   0x00005555555546cd <+109>:   pop    rbp
   0x00005555555546ce <+110>:   ret    
End of assembler dump.

Answer 3

Boost has Boost.Preprocessor which does this among other things. From:

http://www.boost.org/doc/libs/1_57_0/libs/preprocessor/doc/index.html

BOOST_PP_REPEAT

For your code:

#include <boost/preprocessor/repetition/repeat.hpp>

#define OP(z, n, text) text
...
__asm__( BOOST_PP_REPEAT(5, OP, "noop\n"\t);