aarch64: Fix vector-to-vector vec_extract

The documentation says:

-------------------------------------------------------------------------
@cindex @code{vec_extract@var{m}@var{n}} instruction pattern
@item @samp{vec_extract@var{m}@var{n}}
Extract given field from the vector value.  [...]  The
@var{n} mode is the mode of the field or vector of fields that should be
extracted, [...]
If @var{n} is a vector mode, the index is counted in units of that mode.
-------------------------------------------------------------------------

However, Robin pointed out that, in practice, the index is counted
in whole multiples of @var{n}.  These are the semantics that x86
and target-independent code follow.

This patch updates the aarch64 pattern to match, which also removes
the FAIL.  I think Robin has patches that update the documentation
and make more use of the de facto semantics.

I haven't found an existing testcase that shows the difference.
We do now use the pattern for:

union u { int32x4_t x; int32x2_t y[2]; };
int32x2_t f(int32x4_t x) { union u u = { x }; return u.y[1]; }

but we were already generating perfect code for it.  Because of that,
it didn't really seem worth adding a specific dump test.

gcc/
	* config/aarch64/aarch64-simd.md (vec_extract<mode><Vhalf>): Expect
	the index to be 0 or 1.

1 files changed, 2 insertions, 2 deletions

diff --git a/gcc/config/aarch64/aarch64-simd.md b/gcc/config/aarch64/aarch64-simd.md
index f1687d9..d953941 100644
--- a/gcc/config/aarch64/aarch64-simd.md
+++ b/gcc/config/aarch64/aarch64-simd.md
@@ -8755,8 +8755,8 @@
   "TARGET_SIMD"
 {
   int start = INTVAL (operands[2]);
-  if (start != 0 && start != <nunits> / 2)
-    FAIL;
+  gcc_assert (start == 0 || start == 1);
+  start *= <nunits> / 2;
   rtx sel = aarch64_gen_stepped_int_parallel (<nunits> / 2, start, 1);
   emit_insn (gen_aarch64_get_half<mode> (operands[0], operands[1], sel));
   DONE;