How is -march different from -mtune?
compiler-construction, flags, gcc, optimization
Solution
If you use `-march` then GCC will be free to generate instructions that work on the specified CPU, but (typically) not on earlier CPUs in the architecture family.
If you just use `-mtune`, then the compiler will generate code that works on any of them, but will favour instruction sequences that run fastest on the specific CPU you indicated. e.g. setting loop-unrolling heuristics appropriately for that CPU.
`-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 your CPU if you want it to make good code.
Problem
I tried to scrub the GCC man page for this, but still don't get it, really. What's the difference between `-march` and `-mtune`? When does one use just `-march`, vs. both? Is it ever possible to just `-mtune`?