March 3rd Sign

Using -march will also allow you more possibilities to use 3rd party closed source as well. You should be able to link -mcpu=cortex-r5 with -march=armv7-r code; well it is fine in one directions, so the tools may complain.

What are the differences and tradeoffs between -march=haswell, -march=core-avx2, and -mavx2 for compiling avx2 intrinsics? I know that -mavx2 is a flag and -march=haswell/core-avx2 are architectures which just translate to a bunch of flags. So -mavx2 is a subset of the other two. But beyond that, how do I choose the right one for my application?

-march=foo implies -mtune=foo unless you also specify a different -mtune. This is one reason why using -march is better than just enabling options like -mavx without doing anything about tuning. Caveat: -march=native on a CPU that GCC doesn't specifically recognize will still enable new instruction sets that GCC can detect, but will leave -mtune=generic. Use a new enough GCC that knows about ...

For -O0, whether -march=native or -march= is the default still specifies the same family, so both are perfectly compatibly with -O0; and whenever another optimization level is specified, -march=native is beneficial to performance. So, for me, the fact that -O0 is the default doesn't matter for -march 's default.

As I understand it, -march=native will detect the ISA and extensions to use from cpuid (which include model, family and stepping information). -march=xxx will use a baseline set of extensions and a baseline ISA. There are a lot of possible combinations of extensions, so only the most relevant were chosen (e.g. skylake-avx512 was added to reflect an important extension of some skylakes). -march ...