How to use the -fstack-limit-register option?

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

Hello,

GCC has a -fstack-limit-register option

https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#index-fstack-limit-register

If the ABI has no dedicated register for the stack limit, I guess this 
needs to be combined for example with -ffixed-reg. For example on ARM, 
we get this code for

int g(void);

int f(void)
{
	return g() + g();
}

gcc -O2 -fstack-limit-register=r4 -ffixed-r4 -S -o - test.c

f:
         push    {r5, lr}
         bl      g
         mov     r5, r0
         bl      g
         add     r0, r5, r0
         pop     {r5, lr}
         bx      lr

The -ffixed-r4 seems to work, but I don't see that the stack limit is 
checked. The documentation says: "Generate code to ensure that the stack 
does not grow beyond a certain value, either the value of a register or 
the address of a symbol."

How is this option supposed to work?

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