Merge #1087

1087: Use loop to initialize repeat arrays r=philberty a=dafaust

This PR changes how we compile initializers for arrays of repeating elements. I use the same approach outlined in the comments of the linked issue, with some tweaks. It is very similar to how the D language front-end compiles, the new function `Gcc_backend::array_initializer` is heavily inspired by the D front-end's `build_array_set`

This fixes the issue where the compiler tries to allocate a vec containing all elements of the array to be constructed, and therefore explodes on huge constructions (e.g. `let x = [0u8; 4 * 1024 * 1024 * 1024 * 1024]`)

However, we can only initialize non-const arrays in this way. For arrays in const contexts we must initialize them at compile time, and therefore continue using the old method.

Fixes: #1068 


Co-authored-by: David Faust <david.faust@oracle.com>

1 files changed, 73 insertions, 0 deletions

diff --git a/gcc/rust/rust-gcc.cc b/gcc/rust/rust-gcc.cc
index 6ea9ef3..334e9e5 100644
--- a/gcc/rust/rust-gcc.cc
+++ b/gcc/rust/rust-gcc.cc
@@ -246,6 +246,8 @@ public:
   tree array_constructor_expression (tree, const std::vector<unsigned long> &,
 				     const std::vector<tree> &, Location);
 
+  tree array_initializer (tree, tree, tree, tree, tree, tree *, Location);
+
   tree array_index_expression (tree array, tree index, Location);
 
   tree call_expression (tree caller, tree fn, const std::vector<tree> &args,
@@ -1694,6 +1696,77 @@ Gcc_backend::array_constructor_expression (
   return ret;
 }
 
+// Build insns to create an array, initialize all elements of the array to
+// value, and return it
+tree
+Gcc_backend::array_initializer (tree fndecl, tree block, tree array_type,
+				tree length, tree value, tree *tmp,
+				Location locus)
+{
+  std::vector<tree> stmts;
+
+  // Temporary array we initialize with the desired value.
+  tree t = NULL_TREE;
+  Bvariable *tmp_array = this->temporary_variable (fndecl, block, array_type,
+						   NULL_TREE, true, locus, &t);
+  tree arr = tmp_array->get_tree (locus);
+  stmts.push_back (t);
+
+  // Temporary for the array length used for initialization loop guard.
+  Bvariable *tmp_len = this->temporary_variable (fndecl, block, size_type_node,
+						 length, true, locus, &t);
+  tree len = tmp_len->get_tree (locus);
+  stmts.push_back (t);
+
+  // Temporary variable for pointer used to initialize elements.
+  tree ptr_type = this->pointer_type (TREE_TYPE (array_type));
+  tree ptr_init
+    = build1_loc (locus.gcc_location (), ADDR_EXPR, ptr_type,
+		  this->array_index_expression (arr, integer_zero_node, locus));
+  Bvariable *tmp_ptr = this->temporary_variable (fndecl, block, ptr_type,
+						 ptr_init, false, locus, &t);
+  tree ptr = tmp_ptr->get_tree (locus);
+  stmts.push_back (t);
+
+  // push statement list for the loop
+  std::vector<tree> loop_stmts;
+
+  // Loop exit condition:
+  //   if (length == 0) break;
+  t = this->comparison_expression (ComparisonOperator::EQUAL, len,
+				   this->zero_expression (TREE_TYPE (len)),
+				   locus);
+
+  t = this->exit_expression (t, locus);
+  loop_stmts.push_back (t);
+
+  // Assign value to the current pointer position
+  //   *ptr = value;
+  t = this->assignment_statement (build_fold_indirect_ref (ptr), value, locus);
+  loop_stmts.push_back (t);
+
+  // Move pointer to next element
+  //   ptr++;
+  tree size = TYPE_SIZE_UNIT (TREE_TYPE (ptr_type));
+  t = build2 (POSTINCREMENT_EXPR, ptr_type, ptr, convert (ptr_type, size));
+  loop_stmts.push_back (t);
+
+  // Decrement loop counter.
+  //   length--;
+  t = build2 (POSTDECREMENT_EXPR, TREE_TYPE (len), len,
+	      convert (TREE_TYPE (len), integer_one_node));
+  loop_stmts.push_back (t);
+
+  // pop statments and finish loop
+  tree loop_body = this->statement_list (loop_stmts);
+  stmts.push_back (this->loop_expression (loop_body, locus));
+
+  // Return the temporary in the provided pointer and the statement list which
+  // initializes it.
+  *tmp = tmp_array->get_tree (locus);
+  return this->statement_list (stmts);
+}
+
 // Return an expression representing ARRAY[INDEX]
 
 tree