aarch64: Improve generic SVE tuning defaults

This patch adds the recently-added tweak to split some SVE VL-based scalar
operations [1] to the generic tuning used for SVE, as enabled by adding +sve
to the -march flag, for example -march=armv8.2-a+sve.

The recommendation for best performance on a particular CPU remains unchanged:
use the -mcpu option for that CPU, where possible. -mcpu=native makes this
straightforward for native compilation.

The tweak to split out SVE VL-based scalar operations is a consistent win for
the Neoverse V1 CPU and should be neutral for the Fujitsu A64FX. A run of
SPEC2017 on A64FX with this tweak on didn't show any non-noise differences.
It is also expected to be neutral on SVE2 implementations.

Therefore, the patch enables the tweak for generic +sve tuning e.g.
-march=armv8.2-a+sve. No SVE2 CPUs are expected to benefit from it,
therefore the tweak is disabled for generic tuning when +sve2 is in
-march e.g. -march=armv8.2-a+sve2.

The implementation of this approach requires a bit of custom logic in
aarch64_override_options_internal to handle these kinds of
architecture-dependent decisions, but we do believe the user-facing principle
here is important to implement.

In general, for the generic target we're using a decision framework that looks
like:

* If all cores that are known to benefit from an optimization
are of architecture X, and all other cores that implement X or above
are not impacted, or have a very slight impact, we will consider it for
generic tuning for architecture X.
* We will not enable that optimisation for generic tuning for architecture X+1
if no known cores of architecture X+1 or above will benefit.

This framework allows us to improve generic tuning for CPUs of generation X
while avoiding accumulating tweaks for future CPUs of generation X+1, X+2...
that do not need them, and thus avoid even the slight negative effects of
these optimisations if the user is willing to tell us the desired architecture
accurately.

X above can mean either annual architecture updates (Armv8.2-a, Armv8.3-a etc)
or optional architecture extensions (like SVE, SVE2).

[1] http://gcc.gnu.org/g:a65b9ad863c5fc0aea12db58557f4d286a1974d7

gcc/ChangeLog:

	* config/aarch64/aarch64.c (aarch64_adjust_generic_arch_tuning): Define.
	(aarch64_override_options_internal): Use it.
	(generic_tunings): Add AARCH64_EXTRA_TUNE_CSE_SVE_VL_CONSTANTS to
	tune_flags.

gcc/testsuite/ChangeLog:

	* g++.target/aarch64/sve/aarch64-sve.exp: Add -moverride=tune=none to
	sve_flags.
	* g++.target/aarch64/sve/acle/aarch64-sve-acle-asm.exp: Likewise.
	* g++.target/aarch64/sve/acle/aarch64-sve-acle.exp: Likewise.
	* gcc.target/aarch64/sve/aarch64-sve.exp: Likewise.
	* gcc.target/aarch64/sve/acle/aarch64-sve-acle-asm.exp: Likewise.
	* gcc.target/aarch64/sve/acle/aarch64-sve-acle.exp: Likewise.

0 files changed, 0 insertions, 0 deletions