Hi all. I have a potentially slightly fiendish problem, relating to GCCs "-march=native" functionality, and I would appreciate any words of wisdom which might be forthcoming. I have recently acquired an i686 (actual genuine 32-bit Intel processor) dev board, which I am using to support a library I write. The board has no particular OS support provided, e.g. like Raspbian; rather, you just install a normal Linux distro to it. I've installed Debian, as I have Debian on all of the other dev boards I own. A year or two ago, Debian depricated i386. I may be completely wrong in what I am about to say, but from what I've read, Googling, it seems that i386 is now an alias for i686. (I486 support had already accidentally been dropped, and so they ditched i586 as well, and went to i686 as the minimum). The OS image I stalled from was "firmware-8.8.0-i386-netinst", an unofficial build of the normal i386 net-install image, which contains non-free firmware (as needed by the dev board I have). Looking at my installed OS, I see this uname; "Linux minnow 3.16.0-4-686-pae #1 SMP Debian 3.16.43-2 (2017-04-30) i686 GNU/Linux" So it is indeed the case that the installer picked the correct kernel. However, when I look in /usr/lib, I find this... /usr/lib/i386-linux-gnu And now we come to the crux of the matter; If I run; "gcc -dM -E - < /dev/null | grep 86" I get this (re-ordered for clarity); #define __i586 1 #define __i586__ 1 #define __i386 1 #define __i386__ 1 #define i386 1 The compiler thinks it's on a 586, rather than a 686, but that's okay. But when I use "-march=native", like so; "gcc -march=native -dM -E - < /dev/null | grep 86" I only get this! #define __i386 1 #define __i386__ 1 #define i386 1 This is a problem for me. I use these compiler defined macros to control a porting abstraction layer, which in turn uses atomic intrinsics. The i386 has no atomic operations, the i486 has everything except double-word CAS, and i586 and higher has the lot. When I use "-march=native", building fails because the porting abstraction layer throws a #error (due to it being on an unsupportable target). Now, I may be completely wrong, but it looks to me where the OS is pretending to be i386, it's actually misleading the mechanism behind "-march=native". There *is* no actual i686 Debian build, so I can't just use that instead of the i386 Debian. I want to use "-march=native" because it means the makefile selects the correct target for whomever is building. Any advice, insight, corrections, words of wisdom, etc?
