aarch64: Fix invalid nested subregs [PR115464]

The testcase extracts one arm_neon.h vector from a pair (one subreg)
and then reinterprets the result as an SVE vector (another subreg).
Each subreg makes sense individually, but we can't fold them together
into a single subreg: it's 32 bytes -> 16 bytes -> 16*N bytes,
but the interpretation of 32 bytes -> 16*N bytes depends on
whether N==1 or N>1.

Since the second subreg makes sense individually, simplify_subreg
should bail out rather than ICE on it.  simplify_gen_subreg will
then do the same (because it already checks validate_subreg).
This leaves simplify_gen_subreg returning null, requiring the
caller to take appropriate action.

I think this is relatively likely to occur elsewhere, so the patch
adds a helper for forcing a subreg, allowing a temporary pseudo to
be created where necessary.

I'll follow up by using force_subreg in more places.  This patch
is intended to be a minimal backportable fix for the PR.

gcc/
	PR target/115464
	* simplify-rtx.cc (simplify_context::simplify_subreg): Don't try
	to fold two subregs together if their relationship isn't known
	at compile time.
	* explow.h (force_subreg): Declare.
	* explow.cc (force_subreg): New function.
	* config/aarch64/aarch64-sve-builtins-base.cc
	(svset_neonq_impl::expand): Use it instead of simplify_gen_subreg.

gcc/testsuite/
	PR target/115464
	* gcc.target/aarch64/sve/acle/general/pr115464.c: New test.

1 files changed, 15 insertions, 0 deletions

diff --git a/gcc/explow.cc b/gcc/explow.cc
index 8e5f6b8..f684339 100644
--- a/gcc/explow.cc
+++ b/gcc/explow.cc
@@ -745,6 +745,21 @@ force_reg (machine_mode mode, rtx x)
   return temp;
 }
 
+/* Like simplify_gen_subreg, but force OP into a new register if the
+   subreg cannot be formed directly.  */
+
+rtx
+force_subreg (machine_mode outermode, rtx op,
+	      machine_mode innermode, poly_uint64 byte)
+{
+  rtx x = simplify_gen_subreg (outermode, op, innermode, byte);
+  if (x)
+    return x;
+
+  op = copy_to_mode_reg (innermode, op);
+  return simplify_gen_subreg (outermode, op, innermode, byte);
+}
+
 /* If X is a memory ref, copy its contents to a new temp reg and return
    that reg.  Otherwise, return X.  */