Provide a x86_64_v3 microarchitecture level port

Signed-off-by: Allan McRae allan@archlinux.org

rfcs/0002-march.rst

+=====================================
+Use x86-64-v2 microarchitecture level
+=====================================
+
+- Date proposed: 2020-03-02
+- RFC MR: https://gitlab.archlinux.org/archlinux/rfcs/-/merge_requests/0002
+
+Summary
+-------
+
+Adjust our buildflags to build with -march=x86-64-v2.
+
+Motivation
+----------
+
+Arch used to pride itself in providing optimised binaries out of the box.
+However, the days where i686, and even x86_64, showed improvements over
+other distributions are long behind us.
+
+Recently, AMD, Intel, Red Hat, and SUSE collaborated to define three
+x86-64 microarchitecture levels on top of the x86-64 baseline. The three
+microarchitectures group together CPU features roughly based on hardware
+release dates.
+
+The first of these microarchitecture levels, x86-64-v2, assumes the
+following on top of base level x86_64 instructions:
+
+``CMPXCHG16B, LAHF-SAHF, POPCNT, SSE3, SSE4.1, SSE4.2, SSSE3``.
+
+This basically raises the processor feature level requirement to around
+Intel Nehalem, and supports any x86_64 processor made in the last decade.
+
+This microarchitecture became available in GCC version 11 and LLVM
+version 12, and supported in glibc-2.33 and binutils-2.36.
+
+Assuming SSE4 (and others) while compiling will provide greater 
+out-of-the-box performance in Arch Linux.
+
+Alternatives Considered
+-----------------------
+
+Using x86-64-v3 requires the following instruction sets:
+
+``AVX, AVX2, BMI1, BMI2, F16C, FMA, LZCNT, MOVBE, XSAVE``.
+
+That is close to a Haswell processor.  Unfortunately, some recent low
+end Intel CPU do not have AVX support, making this not a viable option
+for the distribution baseline level architecture.  We can potentially
+provide x86_64-v3 as a secondary architecture when automated 
+database/package signing has been implemented.
+
+Specification
+-------------
+
+We will change the distributed makepkg.conf to include the following:
+::
+
+ CFLAGS="-march=x86-64-v2 -mtune=generic ...
+ CXXFLAGS="$CFLAGS" 
+
+Drawbacks
+---------
+
+People with original AMD64 CPUs will no longer be able to use Arch.
+All two of them will be sorely missed. 
+
+Unresolved Questions
+--------------------
+
+None.