On 2017-06-11 09:29 +0200, Toby Douglass wrote: > However, when I look in /usr/lib, I find this... > > /usr/lib/i386-linux-gnu This is just Debian multiarch hierarchy specification. Irrelevant with your issue. > 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 In the first case, the architecture is decided by "--with-arch=" when GCC was configured. In the second case, the architecture is guessed by GCC driver. Use "gcc [-march=native] -E -x c /dev/null -v". And see the flag "-march=xxx" passed to `cc1` by `gcc` to figure out the real architecture GCC used. > 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. No. I don't think so. GCC is guessing the architecture for "-march=native" using "cpuid" instruction on x86. GCC doesn't look at OS. See the source code of gcc_source/gcc/config/i386/driver-i386.c: host_detect_local_cpu. > I want to use "-march=native" because it means the makefile selects > the correct target for whomever is building. I doubt it. '__i386__' is defined for all 32-bit x86 architectures, but '__i686__' is only for "-march=i686", not its 'subsets'. For example, on my laptop (x86-64, multilib), I can use "gcc -m32 -march=native". But then GCC guessed out "- march=ivybridge" and the macro "__corei7_avx__" was defined, instead of "__i686__". > Any advice, insight, corrections, words of wisdom, etc? I think it's better to use configuration script (maybe generated by GNU autoconf) to get the architecture from several tests. -- Xi Ruoyao <ryxi@xxxxxxxxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
