Hi,

Modeling the zero-high-half semantics of the XTN narrowing
instruction in RTL indicates to the compiler that this is a totally
destructive operation. This enables more RTL simplifications and also
prevents some register allocation issues.

Regression tested and bootstrapped on aarch64-none-linux-gnu - no
issues.

Ok for master?

Thanks,
Jonathan

---

gcc/ChangeLog:

2021-06-11  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd.md (aarch64_xtn<mode>_insn_le):
	Define - modeling zero-high-half semantics.
	(aarch64_xtn<mode>): Change to an expander that emits the
	appropriate instruction depending on endianness.
	(aarch64_xtn<mode>_insn_be): Define - modeling zero-high-half
	semantics.
	(aarch64_xtn2<mode>_le): Rename to...
	(aarch64_xtn2<mode>_insn_le): This.
	(aarch64_xtn2<mode>_be): Rename to...
	(aarch64_xtn2<mode>_insn_be): This.
	(vec_pack_trunc_<mode>): Emit truncation instruction instead
	of aarch64_xtn.
	* config/aarch64/iterators.md (Vnarrowd): Add Vnarrowd mode
	attribute iterator.