MIPS Assembly Expert Needed!!! Architecture Question!

[asic_SoC_designer]

Hi

I have a question about the assembly code generated by this small patch of C code that I wrote. The code patch is shown below, basically what I am doing is reading floating point values in from a hardware updated register and performing the arctan2f function on those values.

#include "stdlib.h"
#include "math.h"
#include <stdarg.h>
#include "GLOBAL.H"

//Volatile Variables accessing hardware registers!!!
#define   rs232_uart2_fp    (*((volatile   float *) 0x8C080000))    
#define   quik_silva_reg_fp (*((volatile   float *) 0x8C040000))

int main (void) 
{  

float temp;
float ax, ay;

quik_silva_reg = 0;

while  ( 1 ) {

    ax = rs232_uart2_fp;
    ay = rs232_uart2_fp;

    quik_silva_reg = 0;
    quik_silva_reg_fp = atan2f(ay, ax);

}
  return 0;

}

OK so the code is not working and I have no idea why. I took a look at the disassemble list and I found something which looks very strange to me. The compiler places the two floating point variables x and y in floating point registers $f12, and $f13 and then calls the atan2f function as shown below.

while  ( 1 ) {

    ax = rs232_uart2_fp;
 24c:   c62d0002    lwc1    $f13,0(s1)
Trig_Tests/atan2_test.cpp:28
    ay = rs232_uart2_fp;
 250:   c62c0002    lwc1    $f12,0(s1)
Trig_Tests/atan2_test.cpp:29
    quik_silva_reg = 0;
 254:   ae000000    sw  zero,0(s0)
Trig_Tests/atan2_test.cpp:30
    quik_silva_reg_fp = atan2f(ay, ax);
 258:   0c00009b    jal 26c <atan2f>
 25c:   00000000    nop
 260:   e6000000    swc1    $f0,0(s0)
 264:   08000093    j   24c <main+0x20>
 268:   00000000    nop

Now here is the weird part, when I look through the atan2f code I see ABSOLUTELY NO mention of floating point register $f13. Am I missing something here??? I do see a reference for a mfc1 for floating point register $f14 but no mention of register $f13 which is the register which holds the x operand for the atan2f(y,x) function. I can attached the entire dissemble list for you to have a look for yourself.

Here are my command line and options.

mips-sde-elf-gcc -Os -msingle-float -fsingle-precision-constant -g -mno-memcpy -mno-branch-likely -mno-check-zero-division -mabi=32 -mfp32 -mno-paired-single -mips32r2 -mno-fp-exceptions -mtune=r3k -T elf32btsmip.xc startup.o atan2_test.cpp -o uPC0_Spr6.elf -lc -lm

Thanks in advance for your help!