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, 2 insertions, 0 deletions

diff --git a/gcc/explow.h b/gcc/explow.h
index 16aa02c..cbd1fcb 100644
--- a/gcc/explow.h
+++ b/gcc/explow.h
@@ -42,6 +42,8 @@ extern rtx copy_to_suggested_reg (rtx, rtx, machine_mode);
    Args are mode (in case value is a constant) and the value.  */
 extern rtx force_reg (machine_mode, rtx);
 
+extern rtx force_subreg (machine_mode, rtx, machine_mode, poly_uint64);
+
 /* Return given rtx, copied into a new temp reg if it was in memory.  */
 extern rtx force_not_mem (rtx);