[avr-gcc-list] Stack usage under heavy inlining

[Paulo Marques]

Hi all,

Yesterday I was tracking down a problem in a personal project of mine 
whose symptoms looked like a stack overflow.

As it turned out, it was the same problem with gcc that I had already 
read about on LKML: when gcc inlines a function it increases the stack 
usage of the caller by almost all the stack usage of the callee.

So, for instance, if function A calls function B, then function C and 
then function D, and functions B,C and D use 32 bytes of stack each, it 
means function A now uses 96 bytes of stack, whereas the "not-inlined" 
version only used 32 bytes.

Since I was compiling my project with "-combine -fwhole-program", the 
main function had almost the entire code and it started by pushing all 
call-saved registers and subtracting 126 bytes to the stack pointer :P

Marking the main function as __attribute__((noreturn)) took care of the 
push'es, but only -fno-inline reduced the stack usage.

This is not a avr specific problem. See, for instance, Linus complaining 
of the same problem on i386 (stack size is a problem for the kernel too):

http://lkml.org/lkml/2008/8/26/197

I tested this with several versions of gcc, including a gcc-4.4.0 (not a 
very recent one, though).

I can try with more recent versions, if someone suspects that this 
behavior is better in a more recent gcc, but I just wanted to warn other 
developers who might hit the same problem.

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

"To know recursion, you must first know recursion."