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Re: [PATCH] Add a new type attribute always_alias (PR79671)
On Fri, 7 Apr 2017, Jason Merrill wrote:
> On Fri, Apr 7, 2017 at 11:32 AM, Bernd Edlinger
> <bernd.edlinger@hotmail.de> wrote:
> > On 04/07/17 17:10, Richard Biener wrote:
> >> On April 7, 2017 3:37:30 PM GMT+02:00, Bernd Edlinger <bernd.edlinger@hotmail.de> wrote:
> >>> On 04/07/17 08:54, Richard Biener wrote:
> >>>> On Thu, 6 Apr 2017, Bernd Edlinger wrote:
> >>>>> I think get_alias_set(t) will return 0 for typeless_storage
> >>>>> types, and therefore has_zero_child will be set anyway.
> >>>>> I think both mean the same thing in the end, but it depends on
> >>>>> what typeless_storage should actually mean, and we have
> >>>>> not yet the same idea about it.
> >>>>
> >>>> But has_zero_child does not do what we like it to because otherwise
> >>>> in the PR using the char[] array member would have worked!
> >>>>
> >>>> has_zero_child doesn't do that on purpose of course, but this means
> >>>> returing alias-set zero for the typeless storage _member_ doesn't
> >>>> suffice.
> >>>>
> >>>
> >>> I see you have a certain idea how to solve the C++17 issue.
> >>> And yes, I apologize, if I tried to pee on your tree :)
> >>
> >> We do have the need to support this part of the C++ standard. For other user code may_alias suffices and I see no reason to haste inventing sth new without a single convincing testcase. GCC/Language extensions should not be added without a good reason.
> >>
> >> I didn't propose to expose the type flag to users at all.
> >>
> >> Richard.
> >>
> >
> > Well, actually you did:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671#c100
> >
> > But I won't argue if you prefer to do it as you like,
> > after all TBAA is your area. So that is fine for me.
> >
> > Attached is the latest version of my patch, I fixed
> > the issue with the type-flag conversion, and it is
> > already fully functional.
>
> I agree that we don't want a new attribute.
>
> This should not be limited to C++17 mode; std::aligned_storage is in
> C++11 and we might as well have the same behavior in C++98 mode.
So here's my variant of a fix. I constrained the new flag
TYPE_TYPELESS_STORAGE to arrays and thus hope my localized fix in
build_cplus_array_type will suffice (if I have time I'll play with
template instantiation).
I've created a nice small testcase that exposes the issue but it
needs a PTA fix to work (I'll commit the tree-ssa-structalias.c
fix separately). Some more obfuscation might remove the requirement
of it (don't use an asm but a noinline const function maybe).
Bootstrap and regtest is underway on x86_64-unknown-linux-gnu.
There may be some issues with LTO / cross-language support remaining,
I need to think more about this (TYPE_CANONICAL merging plus alias.c
punning down to TYPE_CANONICAL).
Richard.
2017-04-10 Richard Biener <rguenther@suse.de>
PR c++/79671
cp/
* tree.c (build_cplus_array_type): Set TYPE_TYPELESS_STORAGE
for arrays of unsigned char or std::byte.
* tree-core.h (tree_type_common): Add typeless_storage flag.
* tree.h (TYPE_TYPELESS_STORAGE): New macro.
* alias.c (alias_set_entry): Add has_typeless_storage member.
(alias_set_subset_of): Handle it.
(alias_sets_conflict_p): Likewise.
(init_alias_set_entry): Initialize it.
(get_alias_set): For ARRAY_TYPEs handle TYPE_TYPELESS_STORAGE.
(record_alias_subset): Likewise.
* lto-streamer-out.c (hash_tree): Hash TYPE_TYPELESS_STORAGE.
* tree-streamer-in.c (unpack_ts_type_common_value_fields): Stream
TYPE_TYPELESS_STORAGE.
* tree-streamer-out.c (pack_ts_type_common_value_fields): Likewise.
* tree-ssa-structalias.c (find_func_aliases): Properly handle
asm inputs.
lto/
* lto.c (compare_tree_sccs_1): Compare TYPE_TYPELESS_STORAGE.
* g++.dg/torture/pr79671.C: New testcase.
Index: gcc/alias.c
===================================================================
--- gcc/alias.c (revision 246803)
+++ gcc/alias.c (working copy)
@@ -136,6 +136,9 @@ struct GTY(()) alias_set_entry {
bool is_pointer;
/* Nonzero if is_pointer or if one of childs have has_pointer set. */
bool has_pointer;
+ /* Nonzero if we have a child serving as typeless storage (or are
+ such storage ourselves). */
+ bool has_typeless_storage;
/* The children of the alias set. These are not just the immediate
children, but, in fact, all descendants. So, if we have:
@@ -419,7 +422,8 @@ alias_set_subset_of (alias_set_type set1
/* Check if set1 is a subset of set2. */
ase2 = get_alias_set_entry (set2);
if (ase2 != 0
- && (ase2->has_zero_child
+ && (ase2->has_typeless_storage
+ || ase2->has_zero_child
|| (ase2->children && ase2->children->get (set1))))
return true;
@@ -480,7 +484,8 @@ alias_sets_conflict_p (alias_set_type se
/* See if the first alias set is a subset of the second. */
ase1 = get_alias_set_entry (set1);
if (ase1 != 0
- && ase1->children && ase1->children->get (set2))
+ && (ase1->has_typeless_storage
+ || (ase1->children && ase1->children->get (set2))))
{
++alias_stats.num_dag;
return 1;
@@ -489,7 +494,8 @@ alias_sets_conflict_p (alias_set_type se
/* Now do the same, but with the alias sets reversed. */
ase2 = get_alias_set_entry (set2);
if (ase2 != 0
- && ase2->children && ase2->children->get (set1))
+ && (ase2->has_typeless_storage
+ || (ase2->children && ase2->children->get (set1))))
{
++alias_stats.num_dag;
return 1;
@@ -825,6 +831,7 @@ init_alias_set_entry (alias_set_type set
ase->has_zero_child = false;
ase->is_pointer = false;
ase->has_pointer = false;
+ ase->has_typeless_storage = false;
gcc_checking_assert (!get_alias_set_entry (set));
(*alias_sets)[set] = ase;
return ase;
@@ -955,6 +962,7 @@ get_alias_set (tree t)
Just be pragmatic here and make sure the array and its element
type get the same alias set assigned. */
else if (TREE_CODE (t) == ARRAY_TYPE
+ && ! TYPE_TYPELESS_STORAGE (t)
&& (!TYPE_NONALIASED_COMPONENT (t)
|| TYPE_STRUCTURAL_EQUALITY_P (t)))
set = get_alias_set (TREE_TYPE (t));
@@ -1094,6 +1102,15 @@ get_alias_set (tree t)
TYPE_ALIAS_SET (t) = set;
+ if (TREE_CODE (t) == ARRAY_TYPE
+ && TYPE_TYPELESS_STORAGE (t))
+ {
+ alias_set_entry *ase = get_alias_set_entry (set);
+ if (!ase)
+ ase = init_alias_set_entry (set);
+ ase->has_typeless_storage = true;
+ }
+
/* If this is an aggregate type or a complex type, we must record any
component aliasing information. */
if (AGGREGATE_TYPE_P (t) || TREE_CODE (t) == COMPLEX_TYPE)
@@ -1173,6 +1190,8 @@ record_alias_subset (alias_set_type supe
superset_entry->has_zero_child = true;
if (subset_entry->has_pointer)
superset_entry->has_pointer = true;
+ if (subset_entry->has_typeless_storage)
+ superset_entry->has_typeless_storage = true;
if (subset_entry->children)
{
Index: gcc/cp/tree.c
===================================================================
--- gcc/cp/tree.c (revision 246803)
+++ gcc/cp/tree.c (working copy)
@@ -938,6 +938,11 @@ build_cplus_array_type (tree elt_type, t
else
{
t = build_array_type (elt_type, index_type);
+ if (elt_type == unsigned_char_type_node
+ || (TREE_CODE (elt_type) == ENUMERAL_TYPE
+ && TYPE_CONTEXT (elt_type) == std_node
+ && !strcmp ("byte", TYPE_NAME_STRING (elt_type))))
+ TYPE_TYPELESS_STORAGE (t) = 1;
}
/* Now check whether we already have this array variant. */
Index: gcc/lto/lto.c
===================================================================
--- gcc/lto/lto.c (revision 246803)
+++ gcc/lto/lto.c (working copy)
@@ -1161,7 +1161,10 @@ compare_tree_sccs_1 (tree t1, tree t2, t
compare_values (TYPE_FINAL_P);
}
else if (code == ARRAY_TYPE)
- compare_values (TYPE_NONALIASED_COMPONENT);
+ {
+ compare_values (TYPE_NONALIASED_COMPONENT);
+ compare_values (TYPE_TYPELESS_STORAGE);
+ }
compare_values (TYPE_PACKED);
compare_values (TYPE_RESTRICT);
compare_values (TYPE_USER_ALIGN);
Index: gcc/lto-streamer-out.c
===================================================================
--- gcc/lto-streamer-out.c (revision 246803)
+++ gcc/lto-streamer-out.c (working copy)
@@ -1142,7 +1142,10 @@ hash_tree (struct streamer_tree_cache_d
hstate.add_flag (TYPE_FINAL_P (t));
}
else if (code == ARRAY_TYPE)
- hstate.add_flag (TYPE_NONALIASED_COMPONENT (t));
+ {
+ hstate.add_flag (TYPE_NONALIASED_COMPONENT (t));
+ hstate.add_flag (TYPE_TYPELESS_STORAGE (t));
+ }
hstate.commit_flag ();
hstate.add_int (TYPE_PRECISION (t));
hstate.add_int (TYPE_ALIGN (t));
Index: gcc/testsuite/g++.dg/torture/pr79671.C
===================================================================
--- gcc/testsuite/g++.dg/torture/pr79671.C (nonexistent)
+++ gcc/testsuite/g++.dg/torture/pr79671.C (working copy)
@@ -0,0 +1,25 @@
+// { dg-do run }
+
+void *operator new(__SIZE_TYPE__, void *p2) { return p2; }
+struct B { B(int i_) : i(i_) {} int i; };
+struct X
+{
+ unsigned char buf[sizeof (B)];
+};
+
+int __attribute__((noinline)) foo()
+{
+ X x, y;
+ new (&x) B (0);
+ y = x;
+ B *q = reinterpret_cast <B *>(&y);
+ asm volatile ("" : "=r" (q) : "r" (q));
+ return q->i;
+}
+
+int main()
+{
+ if (foo() != 0)
+ __builtin_abort ();
+ return 0;
+}
Index: gcc/tree-core.h
===================================================================
--- gcc/tree-core.h (revision 246803)
+++ gcc/tree-core.h (working copy)
@@ -1511,7 +1511,9 @@ struct GTY(()) tree_type_common {
so we need to store the value 32 (not 31, as we need the zero
as well), hence six bits. */
unsigned align : 6;
- unsigned spare : 25;
+ unsigned typeless_storage : 1;
+ unsigned spare : 24;
+
alias_set_type alias_set;
tree pointer_to;
tree reference_to;
Index: gcc/tree-ssa-structalias.c
===================================================================
--- gcc/tree-ssa-structalias.c (revision 246803)
+++ gcc/tree-ssa-structalias.c (working copy)
@@ -4944,14 +4944,14 @@ find_func_aliases (struct function *fn,
make_escape_constraint (build_fold_addr_expr (op));
/* The asm may read global memory, so outputs may point to
- any global memory. */
+ any global or escaped memory. */
if (op)
{
auto_vec<ce_s, 2> lhsc;
struct constraint_expr rhsc, *lhsp;
unsigned j;
get_constraint_for (op, &lhsc);
- rhsc.var = nonlocal_id;
+ rhsc.var = escaped_id;
rhsc.offset = 0;
rhsc.type = SCALAR;
FOR_EACH_VEC_ELT (lhsc, j, lhsp)
Index: gcc/tree-streamer-in.c
===================================================================
--- gcc/tree-streamer-in.c (revision 246803)
+++ gcc/tree-streamer-in.c (working copy)
@@ -375,7 +375,10 @@ unpack_ts_type_common_value_fields (stru
TYPE_FINAL_P (expr) = (unsigned) bp_unpack_value (bp, 1);
}
else if (TREE_CODE (expr) == ARRAY_TYPE)
- TYPE_NONALIASED_COMPONENT (expr) = (unsigned) bp_unpack_value (bp, 1);
+ {
+ TYPE_NONALIASED_COMPONENT (expr) = (unsigned) bp_unpack_value (bp, 1);
+ TYPE_TYPELESS_STORAGE (expr) = (unsigned) bp_unpack_value (bp, 1);
+ }
TYPE_PRECISION (expr) = bp_unpack_var_len_unsigned (bp);
SET_TYPE_ALIGN (expr, bp_unpack_var_len_unsigned (bp));
#ifdef ACCEL_COMPILER
Index: gcc/tree-streamer-out.c
===================================================================
--- gcc/tree-streamer-out.c (revision 246803)
+++ gcc/tree-streamer-out.c (working copy)
@@ -327,7 +327,10 @@ pack_ts_type_common_value_fields (struct
bp_pack_value (bp, TYPE_FINAL_P (expr), 1);
}
else if (TREE_CODE (expr) == ARRAY_TYPE)
- bp_pack_value (bp, TYPE_NONALIASED_COMPONENT (expr), 1);
+ {
+ bp_pack_value (bp, TYPE_NONALIASED_COMPONENT (expr), 1);
+ bp_pack_value (bp, TYPE_TYPELESS_STORAGE (expr), 1);
+ }
bp_pack_var_len_unsigned (bp, TYPE_PRECISION (expr));
bp_pack_var_len_unsigned (bp, TYPE_ALIGN (expr));
}
Index: gcc/tree.h
===================================================================
--- gcc/tree.h (revision 246803)
+++ gcc/tree.h (working copy)
@@ -2035,6 +2035,9 @@ extern machine_mode element_mode (const_
#define TYPE_NONALIASED_COMPONENT(NODE) \
(ARRAY_TYPE_CHECK (NODE)->type_common.transparent_aggr_flag)
+#define TYPE_TYPELESS_STORAGE(NODE) \
+ (ARRAY_TYPE_CHECK (NODE)->type_common.typeless_storage)
+
/* Indicated that objects of this type should be laid out in as
compact a way as possible. */
#define TYPE_PACKED(NODE) (TYPE_CHECK (NODE)->base.u.bits.packed_flag)