Hi all,

This patch adds codegen for FEAT_CSSC from the 2022 Architecture extensions.
It fits various existing optabs in GCC quite well.
There are instructions for scalar signed/unsigned min/max, abs, ctz, popcount.
We have expanders for these already, so they are wired up to emit single-insn
patterns for the new TARGET_CSSC.

These instructions are enabled by the +cssc command-line extension.
This version of the patch follows Andrew's suggestion for the expanders.
Pushing to trunk.
Thanks,
Kyrill

gcc/ChangeLog:

	* config/aarch64/aarch64-option-extensions.def (cssc): Define.
	* config/aarch64/aarch64.h (AARCH64_ISA_CSSC): Define.
	(TARGET_CSSC): Likewise.
	* config/aarch64/aarch64.md (*aarch64_abs<mode>2_cssc_ins): New define_insn.
	(abs<mode>2): Adjust for the above.
	(aarch64_umax<mode>3_insn): New define_insn.
	(umax<mode>3): Adjust for the above.
	(*aarch64_popcount<mode>2_cssc_insn): New define_insn.
	(popcount<mode>2): Adjust for the above.
	(<optab><mode>3): New define_insn.
	* config/aarch64/constraints.md (Usm): Define.
	(Uum): Likewise.
	* doc/invoke.texi (AArch64 options): Document +cssc.
	* config/aarch64/iterators.md (MAXMIN_NOUMAX): New code iterator.
	* config/aarch64/predicates.md (aarch64_sminmax_immediate): Define.
	(aarch64_sminmax_operand): Likewise.
	(aarch64_uminmax_immediate): Likewise.
	(aarch64_uminmax_operand): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/cssc_1.c: New test.
	* gcc.target/aarch64/cssc_2.c: New test.
	* gcc.target/aarch64/cssc_3.c: New test.
	* gcc.target/aarch64/cssc_4.c: New test.
	* gcc.target/aarch64/cssc_5.c: New test.