Hello!
Here is an extremely simple test case:
-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --
int a __attribute__ ((section("section")));
int b __attribute__ ((section("section")));
int c __attribute__ ((section("section")));
-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --
If I compile this file with GCC 8, with no optimizations and dump the
object file I get the variables in the exact order as in the source
file:
-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --
$ gcc -c order.c
$ nm -n order.o
0000000000000000 D a
0000000000000004 D b
0000000000000008 D c
-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --
However if I enable the optimization (any level), the variables are
reversed in memory:
-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --
$ gcc -O -c order.c
$ nm -n order.o
0000000000000000 D c
0000000000000004 D b
0000000000000008 D a
-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --
As I need to keep consistent addresses of various variables in specific
memory section for different versions of the program, I decided to put
all of these variables in the same source file. This is for an embedded
project and the section of memory I'm using has backup battery power,
so its contents are not lost when the device is powered down. However I
also need the addresses of "old" objects to remain constant and I don't
want them to change when I add some "new" objects.
So I'm wondering whether this particular behavior of GCC is something
which I could control with any compiler flag (other than disabling
optimizations [; ) and whether this is something consistent across
different versions of GCC? For example can I assume that in GCC 12, in
year 2022, the order of the variables in the memory will still be
reverse of the order in source file, or maybe someday the order will be
different than today with GCC 8?
Thanks in advance!
Regards,
FCh
