Do not change GC pace when creating sub-arrays of bigarrays

Fixes: ocaml#12491

runtime/bigarray.c

@@ -277,6 +277,34 @@ CAMLexport value caml_ba_alloc_dims(int flags, int num_dims, void * data, ...)
   return res;
 }
 
+/* Same as caml_ba_alloc, but the data is shared with another bigarray,
+   so the size of the data must not be counted again, otherwise the
+   GC works too hard. */
+
+static value
+caml_ba_sub_alloc(int flags, int num_dims, void * data, intnat * dim)
+{
+  uintnat asize;
+  int i;
+  value res;
+  struct caml_ba_array * b;
+  intnat dimcopy[CAML_BA_MAX_NUM_DIMS];
+
+  CAMLassert(num_dims >= 0 && num_dims <= CAML_BA_MAX_NUM_DIMS);
+  CAMLassert((flags & CAML_BA_KIND_MASK) < CAML_BA_FIRST_UNIMPLEMENTED_KIND);
+  CAMLassert(data != NULL);
+  for (i = 0; i < num_dims; i++) dimcopy[i] = dim[i];
+  asize = SIZEOF_BA_ARRAY + num_dims * sizeof(intnat);
+  res = caml_alloc_custom_mem(&caml_ba_ops, asize, 0);
+  b = Caml_ba_array_val(res);
+  b->data = data;
+  b->num_dims = num_dims;
+  b->flags = flags;
+  b->proxy = NULL;
+  for (i = 0; i < num_dims; i++) b->dim[i] = dimcopy[i];
+  return res;
+}
+
 /* Finalization of a big array */
 
 CAMLexport void caml_ba_finalize(value v)
@@ -1133,7 +1161,7 @@ CAMLprim value caml_ba_slice(value vb, value vind)
     (char *) b->data +
     offset * caml_ba_element_size[b->flags & CAML_BA_KIND_MASK];
   /* Allocate an OCaml bigarray to hold the result */
-  res = caml_ba_alloc(b->flags, b->num_dims - num_inds, sub_data, sub_dims);
+  res = caml_ba_sub_alloc(b->flags, b->num_dims - num_inds, sub_data, sub_dims);
   /* Copy the finalization function from the original array (PR#8568) */
   Custom_ops_val(res) = Custom_ops_val(vb);
   /* Create or update proxy in case of managed bigarray */
@@ -1160,7 +1188,7 @@ CAMLprim value caml_ba_change_layout(value vb, value vlayout)
     intnat new_dim[CAML_BA_MAX_NUM_DIMS];
     unsigned int i;
     for(i = 0; i < b->num_dims; i++) new_dim[i] = b->dim[b->num_dims - i - 1];
-    res = caml_ba_alloc(flags, b->num_dims, b->data, new_dim);
+    res = caml_ba_sub_alloc(flags, b->num_dims, b->data, new_dim);
     /* Copy the finalization function from the original array (PR#8568) */
     Custom_ops_val(res) = Custom_ops_val(vb);
     caml_ba_update_proxy(b, Caml_ba_array_val(res));
@@ -1205,7 +1233,7 @@ CAMLprim value caml_ba_sub(value vb, value vofs, value vlen)
     (char *) b->data +
     ofs * mul * caml_ba_element_size[b->flags & CAML_BA_KIND_MASK];
   /* Allocate an OCaml bigarray to hold the result */
-  res = caml_ba_alloc(b->flags, b->num_dims, sub_data, b->dim);
+  res = caml_ba_sub_alloc(b->flags, b->num_dims, sub_data, b->dim);
   /* Copy the finalization function from the original array (PR#8568) */
   Custom_ops_val(res) = Custom_ops_val(vb);
   /* Doctor the changed dimension */
@@ -1388,7 +1416,7 @@ CAMLprim value caml_ba_reshape(value vb, value vdim)
   if (num_elts != caml_ba_num_elts(b))
     caml_invalid_argument("Bigarray.reshape: size mismatch");
   /* Create bigarray with same data and new dimensions */
-  res = caml_ba_alloc(b->flags, num_dims, b->data, dim);
+  res = caml_ba_sub_alloc(b->flags, num_dims, b->data, dim);
   /* Copy the finalization function from the original array (PR#8568) */
   Custom_ops_val(res) = Custom_ops_val(vb);
   /* Create or update proxy in case of managed bigarray */