[gcc r13-4619] AArch64: Fix vector re-interpretation between partial SIMD modes

[Tamar Christina <tnfchris@gcc.gnu.org>]

https://gcc.gnu.org/g:594264e9bcb592b8edc4b50b5d9be5eb34c1d6d7

commit r13-4619-g594264e9bcb592b8edc4b50b5d9be5eb34c1d6d7
Author: Tamar Christina <tamar.christina@arm.com>
Date:   Mon Dec 12 15:20:30 2022 +0000

    AArch64: Fix vector re-interpretation between partial SIMD modes
    
    While writing a patch series I started getting incorrect codegen out from
    VEC_PERM on partial struct types.
    
    It turns out that this was happening because the TARGET_CAN_CHANGE_MODE_CLASS
    implementation has a slight bug in it.  The hook only checked for SIMD to
    Partial but never Partial to SIMD.   This resulted in incorrect subregs to be
    generated from the fallback code in VEC_PERM_EXPR expansions.
    
    I have unfortunately not been able to trigger it using a standalone testcase as
    the mid-end optimizes away the permute every time I try to describe a permute
    that would result in the bug.
    
    The patch now rejects any conversion of partial SIMD struct types, unless they
    are both partial structures of the same number of registers or one is a SIMD
    type who's size is less than 8 bytes.
    
    gcc/ChangeLog:
    
            * config/aarch64/aarch64.cc (aarch64_can_change_mode_class): Restrict
            conversions between partial struct types properly.

Diff:
---
 gcc/config/aarch64/aarch64.cc | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index fd92212f96a..523d49a1a42 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -26731,9 +26731,10 @@ aarch64_can_change_mode_class (machine_mode from,
   bool from_pred_p = (from_flags & VEC_SVE_PRED);
   bool to_pred_p = (to_flags & VEC_SVE_PRED);
 
-  bool from_full_advsimd_struct_p = (from_flags == (VEC_ADVSIMD | VEC_STRUCT));
   bool to_partial_advsimd_struct_p = (to_flags == (VEC_ADVSIMD | VEC_STRUCT
 						   | VEC_PARTIAL));
+  bool from_partial_advsimd_struct_p = (from_flags == (VEC_ADVSIMD | VEC_STRUCT
+						   | VEC_PARTIAL));
 
   /* Don't allow changes between predicate modes and other modes.
      Only predicate registers can hold predicate modes and only
@@ -26755,9 +26756,10 @@ aarch64_can_change_mode_class (machine_mode from,
 	  || GET_MODE_UNIT_SIZE (from) != GET_MODE_UNIT_SIZE (to)))
     return false;
 
-  /* Don't allow changes between partial and full Advanced SIMD structure
-     modes.  */
-  if (from_full_advsimd_struct_p && to_partial_advsimd_struct_p)
+  /* Don't allow changes between partial and other registers only if
+     one is a normal SIMD register, allow only if not larger than 64-bit.  */
+  if ((to_partial_advsimd_struct_p ^ from_partial_advsimd_struct_p)
+      && (known_gt (GET_MODE_SIZE (to), 8) || known_gt (GET_MODE_SIZE (to), 8)))
     return false;
 
   if (maybe_ne (BITS_PER_SVE_VECTOR, 128u))