aarch64: Fix invalid subregs for BE svread/write_za

Multi-register svread_za and svwrite_za are implemented using one
pattern per register count, with the register contents being bitcast
on entry (for writes) or return (for reads).  Previously we relied
on subregs for this, with the subreg for reads being handled by
target-independent code.  But using subregs isn't correct for many
big-endian cases, where following subreg rules often requires actual
instructions.  The semantics are instead supposed to be those of
svreinterpret.

gcc/
	PR target/112931
	PR target/112933
	* config/aarch64/aarch64-protos.h (aarch64_sve_reinterpret): Declare.
	* config/aarch64/aarch64.cc (aarch64_sve_reinterpret): New function.
	* config/aarch64/aarch64-sve-builtins-sme.cc (svread_za_impl::expand)
	(svwrite_za_impl::expand): Use it to cast the SVE register to the
	right mode.

3 files changed, 26 insertions, 2 deletions

diff --git a/gcc/config/aarch64/aarch64-protos.h b/gcc/config/aarch64/aarch64-protos.h
index d1af7f4..eaf74a7 100644
--- a/gcc/config/aarch64/aarch64-protos.h
+++ b/gcc/config/aarch64/aarch64-protos.h
@@ -789,6 +789,7 @@ bool aarch64_mask_and_shift_for_ubfiz_p (scalar_int_mode, rtx, rtx);
 bool aarch64_masks_and_shift_for_bfi_p (scalar_int_mode, unsigned HOST_WIDE_INT,
 					unsigned HOST_WIDE_INT,
 					unsigned HOST_WIDE_INT);
+rtx aarch64_sve_reinterpret (machine_mode, rtx);
 bool aarch64_zero_extend_const_eq (machine_mode, rtx, machine_mode, rtx);
 bool aarch64_move_imm (unsigned HOST_WIDE_INT, machine_mode);
 machine_mode aarch64_sve_int_mode (machine_mode);
diff --git a/gcc/config/aarch64/aarch64-sve-builtins-sme.cc b/gcc/config/aarch64/aarch64-sve-builtins-sme.cc
index 8d06a72..047a333 100644
--- a/gcc/config/aarch64/aarch64-sve-builtins-sme.cc
+++ b/gcc/config/aarch64/aarch64-sve-builtins-sme.cc
@@ -365,7 +365,8 @@ public:
   expand (function_expander &e) const override
   {
     machine_mode mode = e.vectors_per_tuple () == 4 ? VNx8DImode : VNx4DImode;
-    return e.use_exact_insn (code_for_aarch64_sme_read (mode));
+    rtx res = e.use_exact_insn (code_for_aarch64_sme_read (mode));
+    return aarch64_sve_reinterpret (e.result_mode (), res);
   }
 };
 
@@ -457,7 +458,7 @@ public:
   expand (function_expander &e) const override
   {
     machine_mode mode = e.vectors_per_tuple () == 4 ? VNx8DImode : VNx4DImode;
-    e.args[1] = lowpart_subreg (mode, e.args[1], e.tuple_mode (1));
+    e.args[1] = aarch64_sve_reinterpret (mode, e.args[1]);
     return e.use_exact_insn (code_for_aarch64_sme_write (mode));
   }
 };
diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index 2a64053..0889ceb 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -3226,6 +3226,28 @@ aarch64_split_simd_move (rtx dst, rtx src)
     }
 }
 
+/* Return a register that contains SVE value X reinterpreted as SVE mode MODE.
+   The semantics of those of svreinterpret rather than those of subregs;
+   see the comment at the head of aarch64-sve.md for details about the
+   difference.  */
+
+rtx
+aarch64_sve_reinterpret (machine_mode mode, rtx x)
+{
+  if (GET_MODE (x) == mode)
+    return x;
+
+  /* can_change_mode_class must only return true if subregs and svreinterprets
+     have the same semantics.  */
+  if (targetm.can_change_mode_class (GET_MODE (x), mode, FP_REGS))
+    return lowpart_subreg (mode, x, GET_MODE (x));
+
+  rtx res = gen_reg_rtx (mode);
+  x = force_reg (GET_MODE (x), x);
+  emit_insn (gen_aarch64_sve_reinterpret (mode, res, x));
+  return res;
+}
+
 bool
 aarch64_zero_extend_const_eq (machine_mode xmode, rtx x,
 			      machine_mode ymode, rtx y)