On Wed, 6 May 2020, Patrick Palka wrote:
> On Wed, 6 May 2020, Patrick Palka wrote:
>
> > On Tue, 5 May 2020, Patrick Palka wrote:
> >
> > > On Tue, 5 May 2020, Patrick Palka wrote:
> > >
> > > > Unfortunately, the previous fix to PR94038 is fragile. When the
> > > > argument to fold_for_warn is a bare CALL_EXPR, then all is well: the
> > > > result of maybe_constant_value from fold_for_warn (with
> > > > uid_sensitive=true) is reused via the cv_cache in the subsequent call to
> > > > maybe_constant_value from cp_fold (with uid_sensitive=false), so we
> > > > avoid instantiating bar<int>.
> > > >
> > > > But when the argument to fold_for_warn is more complex, e.g. an
> > > > INDIRECT_REF of a CALL_EXPR, as in the testcase below (due to bar<int>()
> > > > returning const int& which we need to decay to int) then from
> > > > fold_for_warn we call maybe_constant_value on the INDIRECT_REF, and from
> > > > cp_fold we call it on the CALL_EXPR, so there is no reuse via the
> > > > cv_cache and we therefore end up instantiating bar<int>.
> > > >
> > > > So for a more robust solution to this general issue of warning flags
> > > > affecting code generation, it seems that we need a way to globally avoid
> > > > template instantiation during constexpr evaluation whenever we're
> > > > performing warning-dependent folding.
> > > >
> > > > To that end, this patch replaces the flag constexpr_ctx::uid_sensitive
> > > > with a global flag uid_sensitive_constexpr_evaluation_p, and enables it
> > > > during fold_for_warn using an RAII helper.
> > > >
> > > > The patch also adds a counter that keeps track of the number of times
> > > > uid_sensitive_constexpr_evaluation_p is called, and we use this to
> > > > determine whether the result of constexpr evaluation depended on the
> > > > flag's value. This lets us safely update the cv_cache and fold_cache
> > > > from fold_for_warn in the most common case where the flag's value was
> > > > irrelevant during constexpr evaluation.
Here's some statistics about about the fold cache and cv cache that were
collected when building the testsuite of range-v3 (with the default
build flags which include -O -Wall -Wextra, so warning-dependent folding
applies).
The "naive solution" below refers to the one in which we would refrain
from updating the caches at the end of cp_fold/maybe_constant_value
whenever we're in uid-sensitive constexpr evaluation mode (i.e. whenever
we're called from fold_for_warn), regardless of whether constexpr
evaluation was actually restricted.
FOLD CACHE | cache hit | cache miss | cache miss |
| | cache updated | cache not updated |
------------+-----------+---------------+-------------------+
naive sol'n | 5060779 | 11374667 | 12887790 |
------------------------------------------------------------+
this patch | 6665242 | 19688975 | 199137 |
------------+-----------+---------------+-------------------+
CV CACHE | cache hit | cache miss | cache miss |
| | cache updated | cache not updated |
------------+-----------+---------------+-------------------+
naive sol'n | 76214 | 3637218 | 6889213 |
------------------------------------------------------------+
this patch | 215980 | 9882310 | 405513 |
------------+-----------+---------------+-------------------+
-- >8 --
gcc/cp/ChangeLog:
PR c++/94038
* constexpr.c (constexpr_ctx::uid_sensitive): Remove field.
(uid_sensitive_constexpr_evaluation_value): Define.
(uid_sensitive_constexpr_evaluation_true_counter): Define.
(uid_sensitive_constexpr_evaluation_p): Define.
(uid_sensitive_constexpr_evaluation_sentinel): Define its
constructor.
(uid_sensitive_constexpr_evaluation_checker): Define its
constructor and its evaluation_restricted_p method.
(get_fundef_copy): Remove 'ctx' parameter. Use u_s_c_e_p
instead of constexpr_ctx::uid_sensitive.
(cxx_eval_call_expression): Use u_s_c_e_p instead, and test it
last. Adjust call to get_fundef_copy.
(instantiate_cx_fn_r): Test u_s_c_e_p so that we increment the
counter if necessary.
(cxx_eval_outermost_constant_expr): Remove 'uid_sensitive'
parameter. Adjust function body accordingly.
(maybe_constant_value): Remove 'uid_sensitive' parameter and
adjust function body accordingly. Set up a
uid_sensitive_constexpr_evaluation_checker, and use it to
conditionally update the cv_cache.
* cp-gimplify.h (cp_fold): Set up a
uid_sensitive_constexpr_evaluation_checker, and use it to
conditionally update the fold_cache.
* cp-tree.h (maybe_constant_value): Update declaration.
(struct uid_sensitive_constexpr_evaluation_sentinel): Define.
(struct sensitive_constexpr_evaluation_checker): Define.
* expr.c (fold_for_warn): Set up a
uid_sensitive_constexpr_evaluation_sentinel before calling
the folding subroutines. Drop all but the first argument to
maybe_constant_value.
gcc/testsuite/ChangeLog:
PR c++/94038
* g++.dg/warn/pr94038-2.C: New test.
---
gcc/cp/constexpr.c | 92 ++++++++++++++++++++-------
gcc/cp/cp-gimplify.c | 13 ++--
gcc/cp/cp-tree.h | 23 ++++++-
gcc/cp/expr.c | 4 +-
gcc/testsuite/g++.dg/warn/pr94038-2.C | 28 ++++++++
5 files changed, 130 insertions(+), 30 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/pr94038-2.C
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 706d8a13d8e..d2b75ddaa19 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1089,11 +1089,59 @@ struct constexpr_ctx {
bool strict;
/* Whether __builtin_is_constant_evaluated () should be true. */
bool manifestly_const_eval;
- /* Whether we want to avoid doing anything that will cause extra DECL_UID
- generation. */
- bool uid_sensitive;
};
+/* This internal flag controls whether we should avoid doing anything during
+ constexpr evaluation that would cause extra DECL_UID generation, such as
+ template instantiation and function copying. */
+
+static bool uid_sensitive_constexpr_evaluation_value;
+
+/* An internal counter that keeps track of the number of times
+ uid_sensitive_constexpr_evaluation_p returned true. */
+
+static unsigned uid_sensitive_constexpr_evaluation_true_counter;
+
+/* The accessor for uid_sensitive_constexpr_evaluation_value which also
+ increments the corresponding counter. */
+
+static bool
+uid_sensitive_constexpr_evaluation_p ()
+{
+ if (uid_sensitive_constexpr_evaluation_value)
+ {
+ ++uid_sensitive_constexpr_evaluation_true_counter;
+ return true;
+ }
+ else
+ return false;
+}
+
+uid_sensitive_constexpr_evaluation_sentinel
+::uid_sensitive_constexpr_evaluation_sentinel ()
+ : ovr (uid_sensitive_constexpr_evaluation_value, true)
+{
+}
+
+uid_sensitive_constexpr_evaluation_checker
+::uid_sensitive_constexpr_evaluation_checker ()
+ : saved_counter (uid_sensitive_constexpr_evaluation_true_counter)
+{
+}
+
+/* Returns true iff uid_sensitive_constexpr_evaluation_p is true,
+ and some constexpr evaluation was restricted due to u_s_c_e_p
+ being called and returning true after this checker object was
+ constructed. */
+
+bool
+uid_sensitive_constexpr_evaluation_checker::evaluation_restricted_p () const
+{
+ return (uid_sensitive_constexpr_evaluation_value
+ && saved_counter != uid_sensitive_constexpr_evaluation_true_counter);
+}
+
+
/* A table of all constexpr calls that have been evaluated by the
compiler in this translation unit. */
@@ -1156,7 +1204,7 @@ static GTY(()) hash_map<tree, tree> *fundef_copies_table;
is parms, TYPE is result. */
static tree
-get_fundef_copy (const constexpr_ctx *ctx, constexpr_fundef *fundef)
+get_fundef_copy (constexpr_fundef *fundef)
{
tree copy;
bool existed;
@@ -1173,7 +1221,7 @@ get_fundef_copy (const constexpr_ctx *ctx,
constexpr_fundef *fundef)
}
else if (*slot == NULL_TREE)
{
- if (ctx->uid_sensitive)
+ if (uid_sensitive_constexpr_evaluation_p ())
return NULL_TREE;
/* We've already used the function itself, so make a copy. */
@@ -2292,8 +2340,8 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree
t,
/* We can't defer instantiating the function any longer. */
if (!DECL_INITIAL (fun)
- && !ctx->uid_sensitive
- && DECL_TEMPLOID_INSTANTIATION (fun))
+ && DECL_TEMPLOID_INSTANTIATION (fun)
+ && !uid_sensitive_constexpr_evaluation_p ())
{
location_t save_loc = input_location;
input_location = loc;
@@ -2454,7 +2502,7 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree
t,
gcc_assert (at_eof >= 2 && ctx->quiet);
*non_constant_p = true;
}
- else if (tree copy = get_fundef_copy (ctx, new_call.fundef))
+ else if (tree copy = get_fundef_copy (new_call.fundef))
{
tree body, parms, res;
releasing_vec ctors;
@@ -6485,7 +6533,8 @@ instantiate_cx_fn_r (tree *tp, int *walk_subtrees, void
*/*data*/)
&& DECL_DECLARED_CONSTEXPR_P (*tp)
&& !DECL_INITIAL (*tp)
&& !trivial_fn_p (*tp)
- && DECL_TEMPLOID_INSTANTIATION (*tp))
+ && DECL_TEMPLOID_INSTANTIATION (*tp)
+ && !uid_sensitive_constexpr_evaluation_p ())
{
++function_depth;
instantiate_decl (*tp, /*defer_ok*/false, /*expl_inst*/false);
@@ -6552,8 +6601,7 @@ cxx_eval_outermost_constant_expr (tree t, bool
allow_non_constant,
bool strict = true,
bool manifestly_const_eval = false,
bool constexpr_dtor = false,
- tree object = NULL_TREE,
- bool uid_sensitive = false)
+ tree object = NULL_TREE)
{
auto_timevar time (TV_CONSTEXPR);
@@ -6569,8 +6617,7 @@ cxx_eval_outermost_constant_expr (tree t, bool
allow_non_constant,
constexpr_global_ctx global_ctx;
constexpr_ctx ctx = { &global_ctx, NULL, NULL, NULL, NULL, NULL, NULL,
allow_non_constant, strict,
- manifestly_const_eval || !allow_non_constant,
- uid_sensitive };
+ manifestly_const_eval || !allow_non_constant };
tree type = initialized_type (t);
tree r = t;
@@ -6660,8 +6707,7 @@ cxx_eval_outermost_constant_expr (tree t, bool
allow_non_constant,
auto_vec<tree, 16> cleanups;
global_ctx.cleanups = &cleanups;
- if (!uid_sensitive)
- instantiate_constexpr_fns (r);
+ instantiate_constexpr_fns (r);
r = cxx_eval_constant_expression (&ctx, r,
false, &non_constant_p, &overflow_p);
@@ -6909,14 +6955,12 @@ fold_simple (tree t)
Otherwise, if T does not have TREE_CONSTANT set, returns T.
Otherwise, returns a version of T without TREE_CONSTANT.
MANIFESTLY_CONST_EVAL is true if T is manifestly const-evaluated
- as per P0595. UID_SENSITIVE is true if we can't do anything that
- would affect DECL_UID ordering. */
+ as per P0595. */
static GTY((deletable)) hash_map<tree, tree> *cv_cache;
tree
-maybe_constant_value (tree t, tree decl, bool manifestly_const_eval,
- bool uid_sensitive)
+maybe_constant_value (tree t, tree decl, bool manifestly_const_eval)
{
tree r;
@@ -6934,8 +6978,7 @@ maybe_constant_value (tree t, tree decl, bool
manifestly_const_eval,
return t;
if (manifestly_const_eval)
- return cxx_eval_outermost_constant_expr (t, true, true, true, false,
- decl, uid_sensitive);
+ return cxx_eval_outermost_constant_expr (t, true, true, true, false, decl);
if (cv_cache == NULL)
cv_cache = hash_map<tree, tree>::create_ggc (101);
@@ -6950,14 +6993,15 @@ maybe_constant_value (tree t, tree decl, bool
manifestly_const_eval,
return r;
}
- r = cxx_eval_outermost_constant_expr (t, true, true, false, false,
- decl, uid_sensitive);
+ uid_sensitive_constexpr_evaluation_checker c;
+ r = cxx_eval_outermost_constant_expr (t, true, true, false, false, decl);
gcc_checking_assert (r == t
|| CONVERT_EXPR_P (t)
|| TREE_CODE (t) == VIEW_CONVERT_EXPR
|| (TREE_CONSTANT (t) && !TREE_CONSTANT (r))
|| !cp_tree_equal (r, t));
- cv_cache->put (t, r);
+ if (!c.evaluation_restricted_p ())
+ cv_cache->put (t, r);
return r;
}
diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index fc26a85f43a..f298fa7cec1 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -2443,6 +2443,8 @@ cp_fold (tree x)
if (tree *cached = fold_cache->get (x))
return *cached;
+ uid_sensitive_constexpr_evaluation_checker c;
+
code = TREE_CODE (x);
switch (code)
{
@@ -2925,10 +2927,13 @@ cp_fold (tree x)
return org_x;
}
- fold_cache->put (org_x, x);
- /* Prevent that we try to fold an already folded result again. */
- if (x != org_x)
- fold_cache->put (x, x);
+ if (!c.evaluation_restricted_p ())
+ {
+ fold_cache->put (org_x, x);
+ /* Prevent that we try to fold an already folded result again. */
+ if (x != org_x)
+ fold_cache->put (x, x);
+ }
return x;
}
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index deb000babc1..c2511196de4 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7949,7 +7949,7 @@ extern bool require_potential_rvalue_constant_expression
(tree);
extern tree cxx_constant_value (tree, tree = NULL_TREE);
extern void cxx_constant_dtor (tree, tree);
extern tree cxx_constant_init (tree, tree = NULL_TREE);
-extern tree maybe_constant_value (tree, tree = NULL_TREE, bool =
false, bool = false);
+extern tree maybe_constant_value (tree, tree = NULL_TREE, bool =
false);
extern tree maybe_constant_init (tree, tree =
NULL_TREE, bool = false);
extern tree fold_non_dependent_expr (tree,
tsubst_flags_t =
tf_warning_or_error,
@@ -7968,6 +7968,27 @@ extern tree fold_sizeof_expr (tree);
extern void clear_cv_and_fold_caches (bool = true);
extern tree unshare_constructor (tree
CXX_MEM_STAT_INFO);
+/* An RAII sentinel used to restrict constexpr evaluation so that it
+ doesn't do anything that causes extra DECL_UID generation. */
+
+struct uid_sensitive_constexpr_evaluation_sentinel
+{
+ temp_override<bool> ovr;
+ uid_sensitive_constexpr_evaluation_sentinel ();
+};
+
+/* Used to determine whether uid_sensitive_constexpr_evaluation_p was
+ called and returned true, indicating that we've restricted constexpr
+ evaluation in order to avoid UID generation. We use this to control
+ updates to the fold_cache and cv_cache. */
+
+struct uid_sensitive_constexpr_evaluation_checker
+{
+ const unsigned saved_counter;
+ uid_sensitive_constexpr_evaluation_checker ();
+ bool evaluation_restricted_p () const;
+};
+
/* In cp-ubsan.c */
extern void cp_ubsan_maybe_instrument_member_call (tree);
extern void cp_ubsan_instrument_member_accesses (tree *);
diff --git a/gcc/cp/expr.c b/gcc/cp/expr.c
index 9b535708c57..0d68a738f8f 100644
--- a/gcc/cp/expr.c
+++ b/gcc/cp/expr.c
@@ -399,6 +399,8 @@ fold_for_warn (tree x)
{
/* C++ implementation. */
+ uid_sensitive_constexpr_evaluation_sentinel s;
+
/* It's not generally safe to fully fold inside of a template, so
call fold_non_dependent_expr instead. */
if (processing_template_decl)
@@ -410,7 +412,7 @@ fold_for_warn (tree x)
return f;
}
else if (cxx_dialect >= cxx11)
- x = maybe_constant_value (x, NULL_TREE, false, true);
+ x = maybe_constant_value (x);
return c_fully_fold (x, /*for_init*/false, /*maybe_constp*/NULL);
}
diff --git a/gcc/testsuite/g++.dg/warn/pr94038-2.C
b/gcc/testsuite/g++.dg/warn/pr94038-2.C
new file mode 100644
index 00000000000..a468cc055eb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/pr94038-2.C
@@ -0,0 +1,28 @@
+// PR c++/94038
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-O -Wall" }
+
+static constexpr int x = 0;
+
+template<typename T>
+constexpr const int&
+foo()
+{
+ static_assert(T(1) == 0, "");
+ return x;
+}
+
+template<typename T>
+constexpr const int&
+bar()
+{
+ return foo<T>();
+}
+
+constexpr int
+baz(int a)
+{
+ return a;
+}
+
+static_assert(decltype(baz(bar<int>())){} == 0, "");
--
2.26.2.561.g07d8ea56f2
