Compiler/TMDSEVM6678: Is there any limit on recursive function call for TMDSEVM6678?

Mike ITGeek

Part Number: TMDSEVM6678

Tool/software: TI C/C++ Compiler

I have a very simple openmp application which needs to call a very simple recursive function ... unfortunately it only works for 2K recursion although I have allocated a large stack size (256KB) see the code below and example project attached.

Any clue is much appreciated ....

#include <ti/runtime/openmp/omp.h>
#include <stdio.h>

#define REC_LIMIT (2*1024)   // works
#define REC_LIMIT (25*1024)  // not working

double bad_recursion(double x){
    if (x > REC_LIMIT){
        return 1;
    } else {
        return 1+bad_recursion(1+x);
    }
}

int main (int argc, char *argv[]) {

  int nthreads, tid;

  double retval = bad_recursion(1);
  printf("retval = %f \n", retval);
  return 0;
}

recursion_limit.zip

over 5 years ago

0 Victor Kazmirenko over 5 years ago

Guru 13202 points

Hi Mike,

Your guess is right - stack is the limiting factor of your recursion depth. Stack size of 256KB - yes, it is large enough comparing to ubiquitous use cases, but might be not that much for your case. Obvious stack usage is argument, return value, and return address, which make at least 20B in your example. However, caller must save certain registers to stack before calling child function. So number of data being placed on stack increases. In addition, there might be alignment requirements for new page of stack. So 256KB/2K=128B, which is just 16 doubles seems not huge stack.

There is certain instrumentation in ROV to monitor stack usage. In your situation I would try to monitor B15 register value before function call and within callee to see how much stack was used. B15 is by convention a Stack Pointer.

Also you may want to skim through 7.4. Function Structure and Calling Conventions clause of C6000 compiler user guide.

On the forum there is nice hint on how to monitor stack usage at e2e.ti.com/.../2131849

0 Mike ITGeek over 5 years ago in reply to Victor Kazmirenko

Expert 1320 points

Thanks for the confirm ...

Apparently, I could use __entry_hook and __exit_hook to identify the stack overflow by comparing B15 and the stack start address;

however I need a sample code to accomplish the task. I mean I need to know how to get B15 and stack start address in C code?

0 ToddMullanix over 5 years ago in reply to Mike ITGeek

TI__Guru* 96960 points

Mike,

Are you using SYS/BIOS? If you are, you can enable both Hwi (system) and Task stack checking on context switches. Take a look at this for more details: https://training.ti.com/debugging-common-application-issues-ti-rtos

Again if you are using SYS/BIOS, plugging your own task or Hwi hooks look at the following:

Todd

0 Mike ITGeek over 5 years ago in reply to ToddMullanix

Expert 1320 points

I believe we are not using Sys/BIOS because we are not using any tasks ... everything is done inside the main function ... so I believe we don't use the kernel.

I think I just need the address for B15 and stacks start/end address (cuz stack grows inversely from end address to start address ....). I don't seem to need anything else ...

If you have the answer please let me know ....

Regards and Thanks so much,

0 lding over 5 years ago in reply to Mike ITGeek

TI__Guru* 95265 points

Hi,

See those topics:

https://e2e.ti.com/support/processors/f/791/t/186068

https://e2e.ti.com/support/microcontrollers/c2000/f/171/t/21107

Regards, Eric

0 Mike ITGeek over 5 years ago in reply to lding

Expert 1320 points

Hi Eric,

I have come up with the bellow assembly function returning B15 and tested it with an infinite recursive function ... the numbers looks promising.

Could you please comment on that for us just to make sure?

Thanks

Mike

	.global asmfunc
asmfunc:
	MV b15,a4
	B b3
	NOP 5

extern "C" {
    extern int asmfunc(int a); /* declare external asm function */
    int gvar = 10; /* define global variable */
}

int I;

void sample_rec(double a){
    double test = a + 4; double b = a / 4;
    printf("SP-OFFSET: %d %f\n", I - asmfunc(0), test+b);
    sample_rec(0);
}


void main()
{
    I = asmfunc(0); /* call function normally */
    sample_rec(0);
    printf("I = %d \n", I);
}

0 Mike ITGeek over 5 years ago in reply to Mike ITGeek

Expert 1320 points

I tested with a better example and looks find ... see below :

Processors

Processors forum

Compiler/TMDSEVM6678: Is there any limit on recursive function call for TMDSEVM6678?