posted September 18, 2006 14:07            

This would also happen when calling a function with more parameters than will fit in registers, forcing usage of the stack to pass the remaining parameters.

Here's an example of why this is a problem:

With O2, or -Os optimization, the following code breaks:

/* Display the line1 */
/* -------- Prior to call, r28,r29 are set as frame pointer -------- */
twi_lcd_printf(&twi_lcd_msg, 1, 1, "%-16s", "Set Offset");
OS_WaitBinSem(TWI_LCD_BINSEM_P, OSNO_TIMEOUT, LBL_SAO_5);
/* -------- r28, r29 no longer valid! OSDispatch() version of r28,r29 is restored in call to OSCtxSw() -------- */

twi_lcd_goto_xy(&twi_lcd_msg, 0, 1);
OS_WaitBinSem(TWI_LCD_BINSEM_P, OSNO_TIMEOUT, LBL_SAO_6);

/* Display Line 2
*/
/* -------- Error, assumes r28,r29 has not been modified since the last call to twi_lcd_printf() -------- */
twi_lcd_printf(&twi_lcd_msg, 1, 1, "%-16d", offset);
OS_WaitBinSem(TWI_LCD_BINSEM_P, OSNO_TIMEOUT, LBL_SAO_7);

Adding the avr-gcc option -fno-gcse will fix this. The -fgcse option is included with the -O2 optimization level and is also included in the -Os optimization level. So, -O2 and -Os optimization levels require the -fno-gcse option when compiling a source module with Salvo tasks.

-Tom