show size of stack needed by functions

[Senthil Kumar Selvaraj]

Eric Botcazou writes:

>> Output of -fstack-usage is not accurate
>> =======================================
>> 
>> This article mentions a "call cost":
>> https://mcuoneclipse.com/2015/08/21/gnu-static-stack-usage-analysis/
>> 
>> I checked for myself, by looking at the change of the stackpointer with a
>> debugger, and, yes, there seems to be a constant mismatch (2 bytes with
>> avr-gcc-5.3) between change of stack pointer and output of -fstack-usage.
>> In some rare cases there are more differences, which I didn't understand
>> yet.
>
> That's a bug, very likely in the AVR back-end, which must be fixed by someone 
> who knows the AVR architecture.

I'll take a look.

Regards
Senthil