[PATCH v2][RFC] vect: Verify that GET_MODE_NUNITS is greater than one for vect_grouped_store_supported

[Kevin Lee <kevinl@rivosinc.com>]

This patch is a proper fix to the previous patch 
https://gcc.gnu.org/pipermail/gcc-patches/2023-March/614463.html 
vect_grouped_store_supported checks if the count is a power of 2, but
doesn't check the size of the GET_MODE_NUNITS.
This should handle the riscv case where the mode is VNx1DI since the
nelt would be {1, 1}. 
It was tested on RISCV and x86_64-linux-gnu. Would this be correct 
for the vectors with size smaller than 2?

---
 gcc/tree-vect-data-refs.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/gcc/tree-vect-data-refs.cc b/gcc/tree-vect-data-refs.cc
index 8daf7bd7dd3..04ad12f7d04 100644
--- a/gcc/tree-vect-data-refs.cc
+++ b/gcc/tree-vect-data-refs.cc
@@ -5399,6 +5399,8 @@ vect_grouped_store_supported (tree vectype, unsigned HOST_WIDE_INT count)
 	  poly_uint64 nelt = GET_MODE_NUNITS (mode);
 
 	  /* The encoding has 2 interleaved stepped patterns.  */
+    if(!nelt.is_constant() && maybe_lt(nelt, (unsigned int) 2))
+      return false;
 	  vec_perm_builder sel (nelt, 2, 3);
 	  sel.quick_grow (6);
 	  for (i = 0; i < 3; i++)
-- 
2.25.1