On Fri, May 18, 2018 at 9:43 AM Eric Botcazou <ebotca...@adacore.com> wrote:
> Hi,
> the compiler has had built-in support for nested functions for a long
time,
> which makes it possible to support them in GNU C (as an extension) and in
Ada;
> Fortran uses them too but marginally. The support was entirely rewritten
for
> GCC 4 and a few rough edges have remained since then, which are quite
visible
> in languages where they are first-class citizens like Ada.
> The first rough edge is the stack usage at -O0: it is twice as large as
needed
> for variables subject to up-level references. So you declare an array of
2KB
> bytes on the stack, factor out some code that reads it into a child
function
> and then you suddenly need 4KB of stack instead of 2KB. Even at -O0 this
can
> be annoying for very embedded platforms.
> The attached patch fixes it and also contains a tweak to
use_pointer_in_frame
> for the sake of consistency but with no functional changes in practice.
> Tested on x86-64/Linux, OK for the mainline?
+ /* If the next declaration is a PARM_DECL pointing to the
DECL,
+ we need to adjust its VALUE_EXPR directly, since chains of
+ VALUE_EXPRs run afoul of garbage collection. This occurs
+ in Ada for Out parameters that aren't copied in. */
+ if (next
+ && TREE_CODE (next) == PARM_DECL
+ && DECL_HAS_VALUE_EXPR_P (next)
+ && DECL_VALUE_EXPR (next) == decl)
+ SET_DECL_VALUE_EXPR (next, x);
maybe you can explain the GC issue a bit. I also think that you should
adjust
the next DECL_VALUE_EXRP before setting DECL_VALUE_EXPR on decl so
we can do sth like the following to verify we do not face this very
situation
(it's of course incomplete verification, we could search for a suitable
place
in some of the IL verifiers but I'm not sure there is one early enough and
catching all decls).
diff --git a/gcc/tree.c b/gcc/tree.c
index 68165f4deed..ded12782aea 100644
--- a/gcc/tree.c
+++ b/gcc/tree.c
@@ -6340,7 +6340,14 @@ decl_value_expr_lookup (tree from)
h = value_expr_for_decl->find_with_hash (&in, DECL_UID (from));
if (h)
- return h->to;
+ {
+ /* Chains of value-exprs run afoul of garbage collection, make
+ sure we never end up with those. */
+ gcc_checking_assert ((TREE_CODE (h->to) != PARM_DECL
+ && TREE_CODE (h->to) != VAR_DECL)
+ || !DECL_HAS_VALUE_EXPR_P (h->to));
+ return h->to;
+ }
return NULL_TREE;
}
@@ -6351,6 +6358,12 @@ decl_value_expr_insert (tree from, tree to)
{
struct tree_decl_map *h;
+ /* Chains of value-exprs run afoul of garbage collection, make
+ sure we never end up with those. */
+ gcc_checking_assert ((TREE_CODE (to) != PARM_DECL
+ && TREE_CODE (to) != VAR_DECL)
+ || !DECL_HAS_VALUE_EXPR_P (to));
+
h = ggc_alloc<tree_decl_map> ();
h->base.from = from;
h->to = to;
> 2018-05-18 Eric Botcazou <ebotca...@adacore.com>
> * tree-nested.c (use_pointer_in_frame): Tweak.
> (lookup_field_for_decl): Add assertion and declare the
transformation.
> 2018-05-18 Eric Botcazou <ebotca...@adacore.com>
> * gnat.dg/stack_usage5.adb: New test.
> --
> Eric Botcazou
