Hi,
gimplify_modify_expr_rhs has an optimization whereby the assignment to an
aggregate variable from a read-only object with a DECL_INITIAL is optimized
into the direct assignment of the DECL_INITIAL, provided that no temporary is
created in the process.
The optimization is blocked if the read-only object is volatile, which is OK
as per the semantics of volatile, but also if the target variable is volatile,
on the grounds that the modified assignment might end up being done on a per
field basis, which is also OK. But this latter restriction is enforced a
priori and there are cases where the modified assignment would be OK, for
example if there is only one field or the DECL_INITIAL is equivalent to the
empty CONSTRUCTOR, i.e. all zeros.
So, in the latter case, the patch changes gimplify_modify_expr_rhs to ask
gimplify_init_constructor whether the assignment would be done as a block,
which is easy because gimplify_init_constructor knows that it must create a
temporary if the LHS is volatile and this would not be the case, so it's just
a matter of completing the NOTIFY_TEMP_CREATION mechanism for this case.
Tested on x86-64/Linux, OK for the mainline?
2020-06-11 Eric Botcazou <ebotca...@adacore.com>
* gimplify.c (gimplify_init_constructor) <AGGREGATE_TYPE>: Declare new
ENSURE_SINGLE_ACCESS constant and move variables down. If it is true
and all elements are zero, then always clear. Return GS_ERROR if a
temporary would be created for it and NOTIFY_TEMP_CREATION is set.
(gimplify_modify_expr_rhs) <VAR_DECL>: If the target is volatile but
the type is aggregate non-addressable, ask gimplify_init_constructor
whether it can generate a single access to the target.
2020-06-11 Eric Botcazou <ebotca...@adacore.com>
* gnat.dg/aggr30.adb[sb]: New test.
--
Eric Botcazouwith Interfaces;
package Aggr30 is
type Data_Type is array (1 .. 4) of Interfaces.Unsigned_8;
type Padding_Type is array (5 .. 4096) of Interfaces.Unsigned_8;
type Rec is record
Data : Data_Type;
Padding : Padding_Type;
end record;
procedure Init;
procedure Init_Volatile;
private
Instance : Rec;
Instance_Volatile : Rec;
pragma Volatile (Instance_Volatile);
end Aggr30;
-- { dg-do compile }
-- { dg-options "-fdump-tree-gimple" }
package body Aggr30 is
Null_Constant : constant Rec := (Data => (others => 0),
Padding => (others => 0));
procedure Init is
begin
Instance := Null_Constant;
end;
procedure Init_Volatile is
begin
Instance_Volatile := Null_Constant;
end;
end Aggr30;
-- { dg-final { scan-tree-dump-times "= {}" 2 "gimple"} }
diff --git a/gcc/gimplify.c b/gcc/gimplify.c
index e14932fafaf..ff2be7b4fc4 100644
--- a/gcc/gimplify.c
+++ b/gcc/gimplify.c
@@ -4865,10 +4865,6 @@ gimplify_init_constructor (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
case QUAL_UNION_TYPE:
case ARRAY_TYPE:
{
- struct gimplify_init_ctor_preeval_data preeval_data;
- HOST_WIDE_INT num_ctor_elements, num_nonzero_elements;
- HOST_WIDE_INT num_unique_nonzero_elements;
- bool cleared, complete_p, valid_const_initializer;
/* Use readonly data for initializers of this or smaller size
regardless of the num_nonzero_elements / num_unique_nonzero_elements
ratio. */
@@ -4876,6 +4872,17 @@ gimplify_init_constructor (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
/* If num_nonzero_elements / num_unique_nonzero_elements ratio
is smaller than this, use readonly data. */
const int unique_nonzero_ratio = 8;
+ /* True if a single access of the object must be ensured. This is the
+ case if the target is volatile, the type is non-addressable and more
+ than one field need to be assigned. */
+ const bool ensure_single_access
+ = TREE_THIS_VOLATILE (object)
+ && !TREE_ADDRESSABLE (type)
+ && vec_safe_length (elts) > 1;
+ struct gimplify_init_ctor_preeval_data preeval_data;
+ HOST_WIDE_INT num_ctor_elements, num_nonzero_elements;
+ HOST_WIDE_INT num_unique_nonzero_elements;
+ bool cleared, complete_p, valid_const_initializer;
/* Aggregate types must lower constructors to initialization of
individual elements. The exception is that a CONSTRUCTOR node
@@ -4914,6 +4921,7 @@ gimplify_init_constructor (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
{
if (notify_temp_creation)
return GS_ERROR;
+
DECL_INITIAL (object) = ctor;
TREE_STATIC (object) = 1;
if (!DECL_NAME (object))
@@ -4961,6 +4969,10 @@ gimplify_init_constructor (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
/* If there are "lots" of zeros, it's more efficient to clear
the memory and then set the nonzero elements. */
cleared = true;
+ else if (ensure_single_access && num_nonzero_elements == 0)
+ /* If a single access to the target must be ensured and all elements
+ are zero, then it's optimal to clear whatever their number. */
+ cleared = true;
else
cleared = false;
@@ -5029,13 +5041,14 @@ gimplify_init_constructor (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
}
}
- /* If the target is volatile, we have non-zero elements and more than
- one field to assign, initialize the target from a temporary. */
- if (TREE_THIS_VOLATILE (object)
- && !TREE_ADDRESSABLE (type)
- && (num_nonzero_elements > 0 || !cleared)
- && vec_safe_length (elts) > 1)
+ /* If a single access to the target must be ensured and there are
+ nonzero elements or the zero elements are not assigned en masse,
+ initialize the target from a temporary. */
+ if (ensure_single_access && (num_nonzero_elements > 0 || !cleared))
{
+ if (notify_temp_creation)
+ return GS_ERROR;
+
tree temp = create_tmp_var (TYPE_MAIN_VARIANT (type));
TREE_OPERAND (*expr_p, 0) = temp;
*expr_p = build2 (COMPOUND_EXPR, TREE_TYPE (*expr_p),
@@ -5243,14 +5256,20 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p,
{
case VAR_DECL:
/* If we're assigning from a read-only variable initialized with
- a constructor, do the direct assignment from the constructor,
- but only if neither source nor target are volatile since this
- latter assignment might end up being done on a per-field basis. */
- if (DECL_INITIAL (*from_p)
- && TREE_READONLY (*from_p)
+ a constructor and not volatile, do the direct assignment from
+ the constructor, but only if the target is not volatile either
+ since this latter assignment might end up being done on a per
+ field basis. However, if the target is volatile and the type
+ is aggregate and non-addressable, gimplify_init_constructor
+ knows that it needs to ensure a single access to the target
+ and it will return GS_OK only in this case. */
+ if (TREE_READONLY (*from_p)
+ && DECL_INITIAL (*from_p)
+ && TREE_CODE (DECL_INITIAL (*from_p)) == CONSTRUCTOR
&& !TREE_THIS_VOLATILE (*from_p)
- && !TREE_THIS_VOLATILE (*to_p)
- && TREE_CODE (DECL_INITIAL (*from_p)) == CONSTRUCTOR)
+ && (!TREE_THIS_VOLATILE (*to_p)
+ || (AGGREGATE_TYPE_P (TREE_TYPE (*to_p))
+ && !TREE_ADDRESSABLE (TREE_TYPE (*to_p)))))
{
tree old_from = *from_p;
enum gimplify_status subret;
