gcc generates binary of size 0 with -Os - c

Question

I have a simple FreeRTOS application that just toggles an LED in the main loop.
When I compile it with -Os, the resulting binary has size 0. Without -Os everything works as expected. What is happening here?
My CFLAGS are:
CPUFLAG = -mthumb -mcpu=cortex-m4
FPUFLAG = -mfpu=fpv4-sp-d16 -mfloat-abi=hard
WFLAG = -Wall -Wextra -Werror -Wstrict-prototypes
CFLAGS += -std=gnu99 $(WFLAG) $(CPUFLAG) $(FPUFLAG) -mlittle-endian -mthumb -nostartfiles
CFLAGS += -ffunction-sections -fdata-sections -fno-builtin
LDFLAGS += -nostdlib --gc-sections -static
main is basically TI's blinky demo:
int main(void)
{
volatile uint32_t ui32Loop;
//
// Enable the GPIO port that is used for the on-board LED.
//
SYSCTL_RCGC2_R = SYSCTL_RCGC2_GPIOF;
//
// Do a dummy read to insert a few cycles after enabling the peripheral.
//
ui32Loop = SYSCTL_RCGC2_R;
//
// Enable the GPIO pin for the LED (PF3). Set the direction as output, and
// enable the GPIO pin for digital function.
//
GPIO_PORTF_DIR_R = 0x08;
GPIO_PORTF_DEN_R = 0x08;
//
// Loop forever.
//
while(1)
{
//
// Turn on the LED.
//
GPIO_PORTF_DATA_R |= 0x08;
//
// Delay for a bit.
//
for(ui32Loop = 0; ui32Loop < 200000; ui32Loop++)
{
}
//
// Turn off the LED.
//
GPIO_PORTF_DATA_R &= ~(0x08);
//
// Delay for a bit.
//
for(ui32Loop = 0; ui32Loop < 200000; ui32Loop++)
{
}
}
return 0;
}
With -Os this generates
text data bss dec hex filename
0 0 0 0 0 image.elf
otherwise
text data bss dec hex filename
2012 4 728 2744 ab8 image.elf
edit: differences in .map files

Answer 1

Since you specified -nostartfiles, the standard startup and its entrypoint is not used, so there is no reference to your main function, and --gc-sections discards the whole section as unused.
To fix, try to either name your function start or add -e _main to the ld flags.