Simple assembly example : set inputs and get output - right syntax

问题

I try to do a simple example to insert, into a C code, a piece of Sparc assembly 32 bits; this little code performs an incrementation on the variable "sum".

The code is :

#include <stdio.h>
#include <sys/time.h>
#include <unistd.h>

int n;
int sum;

int main ()
{
  n = 100;
  sum = 0;

  struct timeval tv1, tv2;
  long long diff;

  gettimeofday (&tv1, NULL);

  asm volatile ("set sum, %g1\n\t" \
                "set n, %g3\n" \
                "loop:\n\t" \
                "add %g1, 1, %g2\n\t" \
                "sub %g3, 1, %g4\n\t" \
                "bne loop\n\t" \
                "nop\n\t" \
                : "=r" (sum)
                : "r" (n)
                );

  gettimeofday (&tv2, NULL);

  diff = (tv2.tv_sec - tv1.tv_sec) * 1000000L + (tv2.tv_usec - tv1.tv_usec);
  printf ("Elapsed time = %d usec\n", diff);

  printf ("Sum = %d\n", sum);

  return 0;

}

Unfortunately, compilation with gcc4.1.2 produces the following errors :

loop_dev_for-assembly_code.c: In function #main#:
loop_dev_for-assembly_code.c:18: error: invalid 'asm': invalid operand output code
loop_dev_for-assembly_code.c:18: error: invalid 'asm': operand number out of range
loop_dev_for-assembly_code.c:18: error: invalid 'asm': invalid operand output code
loop_dev_for-assembly_code.c:18: error: invalid 'asm': operand number out of range
loop_dev_for-assembly_code.c:18: error: invalid 'asm': operand number out of range
loop_dev_for-assembly_code.c:18: error: invalid 'asm': operand number out of range

It seems the line 18 corresponds to "asm volatile ("set sum, %g1\n\t" \ ...".

But I don't know how to circumvent these errors. It may come from the variable sum which is set to %g1 register.

About the links between variable belonging to C code and variable localted in Assembly code part. I have also seen, for inputs and outputs parameters, the syntax "=g" (output paramter ??), "g" (input parameter) : I think that it corresponds to different registers between the 2 syntax.

if someone could give to me some clues to understand this link and debug my little code which does a simple loop to increment variable sum.

Thanks for your help, regards.

回答1:

As somebody else said, there are many errors and misconceptions in your inline assembly code. Here are just a few things. First, in extended asm syntax, you must escape all the '%' symbols with another '%', so for example you need to put '%%g1' instead of '%g1' and do this for all the registers you access. Second, you can't use 'set' for either of the variables n or sum, since they are both stack variables, not globals. You have already declared these variables as positional parameters in your asm statement, so sum is parameter %0 and n is %1. Your add instruction puts the result in %g2, which is never initialized or used anywhere.

I think the entire sequence could be rendered much more simply like this (not tested):

 asm volatile ("clr %%g1\n" \
            "loop:\n\t" \
            "add %%g1, 1, %%g1\n\t" \
            "subcc %1, 1, %1\n\t" \
            "bne loop\n\t" \
            "nop\n\t" \
            "mov %%g1, %0\n" \
            : "=r" (sum)
            : "r" (n)
            : "g1" );

来源：https://stackoverflow.com/questions/44058989/simple-assembly-example-set-inputs-and-get-output-right-syntax