This facilitates "universal" builds, ones that target multiple
architectures, e.g. ARMv5 through ARMv7.
(Imported from upstream's c1669e1c205dc8e695fb0c10a655f434e758b9f7)
This is a change from a while ago which was a source of divergence between our
perlasm and upstream's. This change in upstream came with the following comment
in Configure:
Note that -march is not among compiler options in below linux-armv4
target line. Not specifying one is intentional to give you choice to:
a) rely on your compiler default by not specifying one;
b) specify your target platform explicitly for optimal performance,
e.g. -march=armv6 or -march=armv7-a;
c) build "universal" binary that targets *range* of platforms by
specifying minimum and maximum supported architecture;
As for c) option. It actually makes no sense to specify maximum to be
less than ARMv7, because it's the least requirement for run-time
switch between platform-specific code paths. And without run-time
switch performance would be equivalent to one for minimum. Secondly,
there are some natural limitations that you'd have to accept and
respect. Most notably you can *not* build "universal" binary for
big-endian platform. This is because ARMv7 processor always picks
instructions in little-endian order. Another similar limitation is
that -mthumb can't "cross" -march=armv6t2 boundary, because that's
where it became Thumb-2. Well, this limitation is a bit artificial,
because it's not really impossible, but it's deemed too tricky to
support. And of course you have to be sure that your binutils are
actually up to the task of handling maximum target platform.
Change-Id: Ie5f674d603393f0a1354a0d0973987484a4a650c
Reviewed-on: https://boringssl-review.googlesource.com/4488
Reviewed-by: Adam Langley <agl@google.com>
96ac819197