[Dprglist] gcc ARM Cross Compiler help

[David Anderson]

For any who have worked with the STMicros STM32401RE board using the 
arm-none-eabi-gcc cross compiler under linux.  The LMX RTOS code is now 
running on the board and I can flash leds and write output to a UART.

But there is a problem with the math generated by the compiler. 
Basically the math function "divide"  (/)  for single precision integers 
works, but crashes for double precision and for floats, doubles etc.

Suspicious that it was something I'm doing, I stripped out everything, 
told the loader to give it a lot of stack space,  wrote this simple 
test, and turned off the -O2 optimizer:

#include "main.h"
#include "leds.h"

/* 
----------------------------------------------------------------------------*/
/* main */
/* 
----------------------------------------------------------------------------*/

int a,b,c;
// long a,b,c;
// long long a,b,c;
// float  a,b,c;
// double a,b,c;

int main(void)
{
         /* initialize */
         leds_init();
         LED2_ON;

         a = 65535;
         b = 10;
         c = a / b;

         LED3_ON;
         while (1) {};

         /* should never get here */
         return 0;
}

/* ------------------------------------------------------------ */

The code turns on one LED, runs the math test, and turns on the second 
LED.   This works with ints and longs, but fails (2nd LED never lights) 
with long longs and floats and doubles.  I added casts for the 
appropriate constants but it didn't make any difference.

I dumped the assembly code to see what was generated, and it appears 
that the ints and longs are divided by the machine instruction "sdiv" 
while the long longs call the function "__aeabi_ldivmod" and the floats 
call the function "__aeabi_fdiv".  But neither one of those calls appear 
to work.

And why is it calling a function?  I thought this ARM Cortex-M4 
processor included a hardware single precision FPU (FPv4-SP)?   Did I 
leave out some flag when building the compiler, newlib, or the 
application?  I think they were all built with "-mcpu=cortex-m4". Is 
something else required?  I tried "-mfpu=fpv4-sp" but the compiler said 
"no such thing" and offered fpv4-sp-d16 as the closest option.  It did 
not help.

So... help!  Anyone seem this before?  Any of you compiler guru's have 
any suggestions?

much thanks,
dpa