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ada: Fix suboptimal copy of discriminated record to local variable

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This happens for a discriminated record type with default discriminants, for
which GNAT allocates mutable objects with the maximum size, while trying not
to copy padding bits unnecessarily.  When the padded size is small enough to
be copied efficiently, it should nevertheless be profitable to copy them in
order to avoid a call to memcpy with a dynamic size.

gcc/ada/ChangeLog:

	* gcc-interface/trans.cc (gnat_to_gnu): For the LHS of an assignment
	or an actual parameter of a call, do not remove the padding even for
	a type of self-referential size when the padded size is small enough
	to be copied efficiently.
This commit is contained in:
Eric Botcazou
2025-12-16 12:30:14 +01:00
committed by Marc Poulhiès
parent 4e1468b9ff
commit 571f250c66
1 changed files with 13 additions and 9 deletions
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22
gcc/ada/gcc-interface/trans.cc
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@@ -8903,17 +8903,21 @@ gnat_to_gnu (Node_Id gnat_node)
&& !(TREE_CODE (gnu_result_type) == RECORD_TYPE
&& TYPE_JUSTIFIED_MODULAR_P (gnu_result_type))))
{
/* Remove padding only if the inner object is of self-referential
size: in that case it must be an object of unconstrained type
with a default discriminant and we want to avoid copying too
much data. But do not remove it if it is already too small. */
if (type_is_padding_self_referential (TREE_TYPE (gnu_result))
tree t = TREE_TYPE (gnu_result);
/* Remove the padding only if the inner object is of self-referential
size: in that case, it must be an object of an unconstrained type
with default discriminants and we want to avoid copying too much
data. But we would nevertheless rather copy up to MOVE_MAX_PIECES
bytes than invoke memcpy for such a small amount of data. In any
case, do not remove the padding if it is already too small. */
if (type_is_padding_self_referential (t)
&& (TREE_CODE (TYPE_SIZE_UNIT (t)) != INTEGER_CST
|| compare_tree_int (TYPE_SIZE_UNIT (t), MOVE_MAX_PIECES) > 0)
&& !(TREE_CODE (gnu_result) == COMPONENT_REF
&& DECL_BIT_FIELD (TREE_OPERAND (gnu_result, 1))
&& DECL_SIZE (TREE_OPERAND (gnu_result, 1))
!= TYPE_SIZE (TREE_TYPE (gnu_result))))
gnu_result = convert (TREE_TYPE (TYPE_FIELDS (TREE_TYPE (gnu_result))),
gnu_result);
&& DECL_SIZE (TREE_OPERAND (gnu_result, 1)) != TYPE_SIZE (t)))
gnu_result = convert (TREE_TYPE (TYPE_FIELDS (t)), gnu_result);
}
else if (TREE_CODE (gnu_result) == LABEL_DECL