test app

Zoltán Kócsi zoltan at bendor.com.au
Tue Mar 15 07:47:28 EDT 2011

> If I use a division operator or a modulo division operator in a
> static function, all compiles well. If I change the static function
> into a global function, I get following errors : Undefined reference
> to '__aeabi_idiv' Undefined reference to '__aeabi_idivmod'

Those are the division routines in gcc's libgcc.a library. Your problem
is this:

> CFLAGS		= -Wall -Os -nostdlib -Wl,-Ttext=0xA0000000

-nostdlib tells the compiler not to link against the standard
libraries, which include libgcc.a.

> %.elf : $(SRCS)
> 	$(CC) $(CFLAGS) $^ -o $@

If you add a -lgcc to the line above, it should compile.

> I never encountered this problem before in other projects and I don't
> see why this is working in static functions and not in global
> functions.... 

Possibly because if you have static functions, then the compiler can
inline them. If after the inlining it can work out that you are
dividing with a constant, then it won't pull in the division routines,
but substitute division with a multiplication, two shifts and an
addition (the cost of calculating the constant to multiply with and
the shift amounts involves, among other things, a 64-bit division, but
that all happens in compile time - at runtime it's just the mul, shift,
add, which is orders of magnitue faster as well as smaller than a real
division). If you have a global function, gcc must assume that it can
be called with arbitrary arguments from other C files, therefore it must
do real (i.e. runtime) division.


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