nir: fix lower_memcpy

memcpy is divided into chunks that are vec4 sized max. The problem
here happens with a structure of 24 bytes :

  struct {
    float3 a;
    float3 b;
  }

If you memcpy that struct, the lowering will emit 2 load/store, one of
sized 8, next one sized 16. But both end up located at offset 0, so we
effectively drop 2 floats.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: a3177cca99 ("nir: Add a lowering pass to lower memcpy")
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15049>

src/compiler/nir/nir_lower_memcpy.c

@@ -111,11 +111,14 @@ lower_memcpy_impl(nir_function_impl *impl)
             uint64_t size = nir_src_as_uint(cpy->src[2]);
             uint64_t offset = 0;
             while (offset < size) {
-               uint64_t remaining = offset - size;
-               /* For our chunk size, we choose the largest power-of-two that
-                * divides size with a maximum of 16B (a vec4).
+               uint64_t remaining = size - offset;
+               /* Find the largest chunk size power-of-two (MSB in remaining)
+                * and limit our chunk to 16B (a vec4). It's important to do as
+                * many 16B chunks as possible first so that the index
+                * computation is correct for
+                * memcpy_(load|store)_deref_elem_imm.
                 */
-               unsigned copy_size = 1u << MIN2(ffsll(remaining) - 1, 4);
+               unsigned copy_size = 1u << MIN2(util_last_bit64(remaining) - 1, 4);
                const struct glsl_type *copy_type =
                   copy_type_for_byte_size(copy_size);