ali hagigat <hagigatali@xxxxxxxxx> writes:
> So why do we need -mtune? We can compile by -march from the beginning.
My understanding:
-march=FOO: specifies which instructions are OK to use: anything that
works on a FOO (or compatible) CPU
-mtune=BAR: specifies which CPU to optimize for when
choosing/scheduling instructions (taken from the set
which is OK for FOO)
So if FOO and BAR are _the same_, it's pretty obvious: it will require
that CPU, and be optimized for it.
If FOO and BAR are _different_, then you the result is guaranteed to run
on FOO, but will run best on BAR -- of course BAR should be a subset of
FOO!
A common case, I think is to be fairly conservative with FOO, so the
software will run on a wide range of CPUs, but try to choose BAR to
match the most common model in current use.
E.g., "-march=i386 -mtune=core2" will run on any i386 derivative, but
will work best on a core2.
For personal use, it's usually OK to just use -march (e.g.,
-march=native to run on the current machine), but somebody who ships
software may need more flexibility.
-Miles
--
"Most attacks seem to take place at night, during a rainstorm, uphill,
where four map sheets join." -- Anon. British Officer in WW I
