Hi all,

In the testcase for this patch we try to vec_concat the lowpart and highpart of a vector, but the lowpart is expressed as a subreg.
simplify-rtx.cc does not recognise this and combine ends up trying to match:
Trying 7 -> 8:
    7: r93:V2SI=vec_select(r95:V4SI,parallel)
    8: r97:V4SI=vec_concat(r95:V4SI#0,r93:V2SI)
      REG_DEAD r95:V4SI
      REG_DEAD r93:V2SI
Failed to match this instruction:
(set (reg:V4SI 97)
    (vec_concat:V4SI (subreg:V2SI (reg/v:V4SI 95 [ a ]) 0)
        (vec_select:V2SI (reg/v:V4SI 95 [ a ])
            (parallel:V4SI [
                    (const_int 2 [0x2])
                    (const_int 3 [0x3])
                ]))))

This should be just (set (reg:V4SI 97) (reg:V4SI 95)). This patch adds such a simplification.
The testcase is a bit artificial, but I do have other aarch64-specific patterns that I want to optimise later
that rely on this simplification happening.

Without this patch for the testcase we generate:
foo:
        dup     d31, v0.d[1]
        ins     v0.d[1], v31.d[0]
        ret

whereas we should just not generate anything as the operation is ultimately a no-op.

Bootstrapped and tested on aarch64-none-linux-gnu and aarch64_be-none-elf.
Ok for trunk?
Thanks,
Kyrill

gcc/ChangeLog:

	* simplify-rtx.cc (simplify_context::simplify_binary_operation_1):
	Simplify vec_concat of lowpart subreg and high part vec_select.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/simd/low-high-combine_1.c: New test.