On Wed, Feb 12, 2020 at 9:10 AM J.W. Jagersma <jwjagersma@xxxxxxxxx> wrote: > i686-gcc -march=i386: cmov (!) Did you check to see where the cmov instruction is coming from? The compiler won't generate it with -march=i386, but there may be a precompiled library like libgcc or libstdc++ that contains the cmov instruction because they were compiled for i686. In order to get the right effect with -march=i386, you would need to build multiple copies of the libraries, one of them with -march=i386. We call this multilibs. This is normally done with -m32/-m64 so a single compiler can generate both 32-bit and 64-bit code, but it is not normally done with i386 and i686. You would have to change the default configuration to build multilibs based on -march. The same thing happens in reverse when you configure for i386 and use -march=i686, the compiler generated code will be i686, but the precompiled libraries will be i386 code. > So it looks like gcc compiled with --target=i686-* is incapable of producing > i386 code, and --target=i386-* is the most versatile option for backwards > compatibility with older cpus. Are there any disadvantages (reduced > optimization, etc) to using an i386-gcc with -march=i686 when compiling for > "newer" systems? Besides the performance loss, i386 doesn't have cmpxchg which means atomic support will be a problem. Otherwise, it will probably work. Jim
