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

Question

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.

Answer 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" );