C++ PATCH to fix rejects-valid with constexpr ctor in C++17 (PR c++/83692)

Message ID 20180125211639.GA2620@redhat.com
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  • C++ PATCH to fix rejects-valid with constexpr ctor in C++17 (PR c++/83692)
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Commit Message

Marek Polacek Jan. 25, 2018, 9:16 p.m.
This is a similar problem to 83116: we'd cached a constexpr call, but after a
store the result had become invalid, yet we used the wrong result again when
encountering the same call later.  This resulted in evaluating a THROW_EXPR
which doesn't work.  Details in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5

The fix for 83116 didn't work here, because when evaluating the body of the
ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do
cache.

It seems that we may no longer rely on the constexpr call table when we
do cxx_eval_store_expression, because that just rewrites *valp, i.e. the
value of an object.  Might be too big a hammer again, but I couldn't think
of how I could guard the caching of a constexpr call.

This doesn't manifest in C++14 because build_special_member_call in C++17 is
more aggressive with copy elisions (as required by P0135 which changed how we
view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so
expand_default_init calls maybe_constant_init, for which STRICT is false, so
we avoid caching as per 83116.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-01-25  Marek Polacek  <polacek@redhat.com>

	PR c++/83692
	* constexpr.c (cxx_eval_store_expression): Clear constexpr_call_table.

	* g++.dg/cpp1y/constexpr-83692.C: New test.


	Marek

Comments

Marek Polacek Jan. 25, 2018, 10:08 p.m. | #1
On Thu, Jan 25, 2018 at 10:16:39PM +0100, Marek Polacek wrote:
> This is a similar problem to 83116: we'd cached a constexpr call, but after a

> store the result had become invalid, yet we used the wrong result again when

> encountering the same call later.  This resulted in evaluating a THROW_EXPR

> which doesn't work.  Details in

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5

> 

> The fix for 83116 didn't work here, because when evaluating the body of the

> ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do

> cache.

> 

> It seems that we may no longer rely on the constexpr call table when we

> do cxx_eval_store_expression, because that just rewrites *valp, i.e. the

> value of an object.  Might be too big a hammer again, but I couldn't think

> of how I could guard the caching of a constexpr call.

> 

> This doesn't manifest in C++14 because build_special_member_call in C++17 is

> more aggressive with copy elisions (as required by P0135 which changed how we

> view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so

> expand_default_init calls maybe_constant_init, for which STRICT is false, so

> we avoid caching as per 83116.


...so the testcase should actually test c++17.  Consider that fixed.

	Marek
Jason Merrill Feb. 2, 2018, 7:11 p.m. | #2
On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@redhat.com> wrote:
> This is a similar problem to 83116: we'd cached a constexpr call, but after a

> store the result had become invalid, yet we used the wrong result again when

> encountering the same call later.  This resulted in evaluating a THROW_EXPR

> which doesn't work.  Details in

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5

>

> The fix for 83116 didn't work here, because when evaluating the body of the

> ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do

> cache.


> It seems that we may no longer rely on the constexpr call table when we

> do cxx_eval_store_expression, because that just rewrites *valp, i.e. the

> value of an object.  Might be too big a hammer again, but I couldn't think

> of how I could guard the caching of a constexpr call.


> This doesn't manifest in C++14 because build_special_member_call in C++17 is

> more aggressive with copy elisions (as required by P0135 which changed how we

> view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so

> expand_default_init calls maybe_constant_init, for which STRICT is false, so

> we avoid caching as per 83116.


So it sounds like the problem is using cxx_constant_value for the
diagnostic when it has different semantics from the
maybe_constant_init that follows right after.  I guess we want a
cxx_constant_init function that is a hybrid of the two.

Jason
Marek Polacek Feb. 5, 2018, 1:37 p.m. | #3
On Fri, Feb 02, 2018 at 02:11:27PM -0500, Jason Merrill wrote:
> On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@redhat.com> wrote:

> > This is a similar problem to 83116: we'd cached a constexpr call, but after a

> > store the result had become invalid, yet we used the wrong result again when

> > encountering the same call later.  This resulted in evaluating a THROW_EXPR

> > which doesn't work.  Details in

> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5

> >

> > The fix for 83116 didn't work here, because when evaluating the body of the

> > ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do

> > cache.

> 

> > It seems that we may no longer rely on the constexpr call table when we

> > do cxx_eval_store_expression, because that just rewrites *valp, i.e. the

> > value of an object.  Might be too big a hammer again, but I couldn't think

> > of how I could guard the caching of a constexpr call.

> 

> > This doesn't manifest in C++14 because build_special_member_call in C++17 is

> > more aggressive with copy elisions (as required by P0135 which changed how we

> > view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so

> > expand_default_init calls maybe_constant_init, for which STRICT is false, so

> > we avoid caching as per 83116.

> 

> So it sounds like the problem is using cxx_constant_value for the

> diagnostic when it has different semantics from the

> maybe_constant_init that follows right after.  I guess we want a

> cxx_constant_init function that is a hybrid of the two.


So like the following?  Thanks,

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-02-04  Marek Polacek  <polacek@redhat.com>

	PR c++/83692
	* constexpr.c (cxx_constant_init): New function.
	* cp-tree.h (cxx_constant_init): Declare.
	* typeck2.c (store_init_value): Call cxx_constant_init instead of
	cxx_constant_value.

	* g++.dg/cpp1z/constexpr-83692.C: New test.

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 93dd8ae049c..f95aacf2580 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -4902,6 +4902,14 @@ cxx_constant_value (tree t, tree decl)
   return cxx_eval_outermost_constant_expr (t, false, true, decl);
 }
 
+/* Like cxx_constant_value, but non-strict mode.  */
+
+tree
+cxx_constant_init (tree t, tree decl)
+{
+  return cxx_eval_outermost_constant_expr (t, false, false, decl);
+}
+
 /* Helper routine for fold_simple function.  Either return simplified
    expression T, otherwise NULL_TREE.
    In contrast to cp_fully_fold, and to maybe_constant_value, we try to fold
diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
index a53f4fd9c03..9f973305fbb 100644
--- gcc/cp/cp-tree.h
+++ gcc/cp/cp-tree.h
@@ -7417,6 +7417,7 @@ extern bool require_potential_constant_expression (tree);
 extern bool require_constant_expression (tree);
 extern bool require_potential_rvalue_constant_expression (tree);
 extern tree cxx_constant_value			(tree, tree = NULL_TREE);
+extern tree cxx_constant_init			(tree, tree = NULL_TREE);
 extern tree maybe_constant_value		(tree, tree = NULL_TREE);
 extern tree maybe_constant_init			(tree, tree = NULL_TREE);
 extern tree fold_non_dependent_expr		(tree);
diff --git gcc/cp/typeck2.c gcc/cp/typeck2.c
index 899d60e8535..b4abc54f537 100644
--- gcc/cp/typeck2.c
+++ gcc/cp/typeck2.c
@@ -830,7 +830,7 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** cleanups, int flags)
 	  if (!require_constant_expression (value))
 	    value = error_mark_node;
 	  else
-	    value = cxx_constant_value (value, decl);
+	    value = cxx_constant_init (value, decl);
 	}
       value = maybe_constant_init (value, decl);
       if (TREE_CODE (value) == CONSTRUCTOR && cp_has_mutable_p (type))
diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
index e69de29bb2d..f6b61eeab85 100644
--- gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
@@ -0,0 +1,21 @@
+// PR c++/83692
+// { dg-options -std=c++17 }
+
+struct integer {
+  constexpr int value() const { return m_value;	}
+  int m_value;
+};
+
+struct outer {
+  integer m_x{0};
+  constexpr outer()
+    {
+      if (m_x.value() != 0)
+	throw 0;
+      m_x.m_value = integer{1}.value();
+      if (m_x.value() != 1)
+	throw 0;
+    }
+};
+
+constexpr outer o{};

	Marek
Jason Merrill Feb. 5, 2018, 6:45 p.m. | #4
On Mon, Feb 5, 2018 at 8:37 AM, Marek Polacek <polacek@redhat.com> wrote:
> On Fri, Feb 02, 2018 at 02:11:27PM -0500, Jason Merrill wrote:

>> On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@redhat.com> wrote:

>> > This is a similar problem to 83116: we'd cached a constexpr call, but after a

>> > store the result had become invalid, yet we used the wrong result again when

>> > encountering the same call later.  This resulted in evaluating a THROW_EXPR

>> > which doesn't work.  Details in

>> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5

>> >

>> > The fix for 83116 didn't work here, because when evaluating the body of the

>> > ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do

>> > cache.

>>

>> > It seems that we may no longer rely on the constexpr call table when we

>> > do cxx_eval_store_expression, because that just rewrites *valp, i.e. the

>> > value of an object.  Might be too big a hammer again, but I couldn't think

>> > of how I could guard the caching of a constexpr call.

>>

>> > This doesn't manifest in C++14 because build_special_member_call in C++17 is

>> > more aggressive with copy elisions (as required by P0135 which changed how we

>> > view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so

>> > expand_default_init calls maybe_constant_init, for which STRICT is false, so

>> > we avoid caching as per 83116.

>>

>> So it sounds like the problem is using cxx_constant_value for the

>> diagnostic when it has different semantics from the

>> maybe_constant_init that follows right after.  I guess we want a

>> cxx_constant_init function that is a hybrid of the two.

>

> So like the following?  Thanks,

>

> Bootstrapped/regtested on x86_64-linux, ok for trunk?

>

> 2018-02-04  Marek Polacek  <polacek@redhat.com>

>

>         PR c++/83692

>         * constexpr.c (cxx_constant_init): New function.

>         * cp-tree.h (cxx_constant_init): Declare.

>         * typeck2.c (store_init_value): Call cxx_constant_init instead of

>         cxx_constant_value.

>

> +/* Like cxx_constant_value, but non-strict mode.  */

> +

> +tree

> +cxx_constant_init (tree t, tree decl)

> +{

> +  return cxx_eval_outermost_constant_expr (t, false, false, decl);

> +}


Hmm, that doesn't do the TARGET_EXPR stripping that
maybe_constant_init does.  I was thinking of a version of
maybe_constant_init that passes false to allow_non_constant.  Probably
by making "maybe_constant_init" and cxx_constant_init both call the
current function with an additional parameter.  And then the existing
call to maybe_constant_init can move under an 'else' to avoid
redundant constexpr evaluation.

Jason
Jason Merrill Feb. 16, 2018, 9:10 p.m. | #5
On Mon, Feb 5, 2018 at 1:45 PM, Jason Merrill <jason@redhat.com> wrote:
> On Mon, Feb 5, 2018 at 8:37 AM, Marek Polacek <polacek@redhat.com> wrote:

>> On Fri, Feb 02, 2018 at 02:11:27PM -0500, Jason Merrill wrote:

>>> On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@redhat.com> wrote:

>>> > This is a similar problem to 83116: we'd cached a constexpr call, but after a

>>> > store the result had become invalid, yet we used the wrong result again when

>>> > encountering the same call later.  This resulted in evaluating a THROW_EXPR

>>> > which doesn't work.  Details in

>>> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5

>>> >

>>> > The fix for 83116 didn't work here, because when evaluating the body of the

>>> > ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do

>>> > cache.

>>>

>>> > It seems that we may no longer rely on the constexpr call table when we

>>> > do cxx_eval_store_expression, because that just rewrites *valp, i.e. the

>>> > value of an object.  Might be too big a hammer again, but I couldn't think

>>> > of how I could guard the caching of a constexpr call.

>>>

>>> > This doesn't manifest in C++14 because build_special_member_call in C++17 is

>>> > more aggressive with copy elisions (as required by P0135 which changed how we

>>> > view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so

>>> > expand_default_init calls maybe_constant_init, for which STRICT is false, so

>>> > we avoid caching as per 83116.

>>>

>>> So it sounds like the problem is using cxx_constant_value for the

>>> diagnostic when it has different semantics from the

>>> maybe_constant_init that follows right after.  I guess we want a

>>> cxx_constant_init function that is a hybrid of the two.

>>

>> So like the following?  Thanks,

>>

>> Bootstrapped/regtested on x86_64-linux, ok for trunk?

>>

>> 2018-02-04  Marek Polacek  <polacek@redhat.com>

>>

>>         PR c++/83692

>>         * constexpr.c (cxx_constant_init): New function.

>>         * cp-tree.h (cxx_constant_init): Declare.

>>         * typeck2.c (store_init_value): Call cxx_constant_init instead of

>>         cxx_constant_value.

>>

>> +/* Like cxx_constant_value, but non-strict mode.  */

>> +

>> +tree

>> +cxx_constant_init (tree t, tree decl)

>> +{

>> +  return cxx_eval_outermost_constant_expr (t, false, false, decl);

>> +}

>

> Hmm, that doesn't do the TARGET_EXPR stripping that

> maybe_constant_init does.  I was thinking of a version of

> maybe_constant_init that passes false to allow_non_constant.  Probably

> by making "maybe_constant_init" and cxx_constant_init both call the

> current function with an additional parameter.  And then the existing

> call to maybe_constant_init can move under an 'else' to avoid

> redundant constexpr evaluation.


Want me to take this over?

Jason
Marek Polacek Feb. 23, 2018, 2:29 p.m. | #6
On Fri, Feb 16, 2018 at 04:10:20PM -0500, Jason Merrill wrote:
> On Mon, Feb 5, 2018 at 1:45 PM, Jason Merrill <jason@redhat.com> wrote:

> > On Mon, Feb 5, 2018 at 8:37 AM, Marek Polacek <polacek@redhat.com> wrote:

> >> On Fri, Feb 02, 2018 at 02:11:27PM -0500, Jason Merrill wrote:

> >>> On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@redhat.com> wrote:

> >>> > This is a similar problem to 83116: we'd cached a constexpr call, but after a

> >>> > store the result had become invalid, yet we used the wrong result again when

> >>> > encountering the same call later.  This resulted in evaluating a THROW_EXPR

> >>> > which doesn't work.  Details in

> >>> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5

> >>> >

> >>> > The fix for 83116 didn't work here, because when evaluating the body of the

> >>> > ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do

> >>> > cache.

> >>>

> >>> > It seems that we may no longer rely on the constexpr call table when we

> >>> > do cxx_eval_store_expression, because that just rewrites *valp, i.e. the

> >>> > value of an object.  Might be too big a hammer again, but I couldn't think

> >>> > of how I could guard the caching of a constexpr call.

> >>>

> >>> > This doesn't manifest in C++14 because build_special_member_call in C++17 is

> >>> > more aggressive with copy elisions (as required by P0135 which changed how we

> >>> > view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so

> >>> > expand_default_init calls maybe_constant_init, for which STRICT is false, so

> >>> > we avoid caching as per 83116.

> >>>

> >>> So it sounds like the problem is using cxx_constant_value for the

> >>> diagnostic when it has different semantics from the

> >>> maybe_constant_init that follows right after.  I guess we want a

> >>> cxx_constant_init function that is a hybrid of the two.

> >>

> >> So like the following?  Thanks,

> >>

> >> Bootstrapped/regtested on x86_64-linux, ok for trunk?

> >>

> >> 2018-02-04  Marek Polacek  <polacek@redhat.com>

> >>

> >>         PR c++/83692

> >>         * constexpr.c (cxx_constant_init): New function.

> >>         * cp-tree.h (cxx_constant_init): Declare.

> >>         * typeck2.c (store_init_value): Call cxx_constant_init instead of

> >>         cxx_constant_value.

> >>

> >> +/* Like cxx_constant_value, but non-strict mode.  */

> >> +

> >> +tree

> >> +cxx_constant_init (tree t, tree decl)

> >> +{

> >> +  return cxx_eval_outermost_constant_expr (t, false, false, decl);

> >> +}

> >

> > Hmm, that doesn't do the TARGET_EXPR stripping that

> > maybe_constant_init does.  I was thinking of a version of

> > maybe_constant_init that passes false to allow_non_constant.  Probably

> > by making "maybe_constant_init" and cxx_constant_init both call the

> > current function with an additional parameter.  And then the existing

> > call to maybe_constant_init can move under an 'else' to avoid

> > redundant constexpr evaluation.

> 

> Want me to take this over?


Sorry again for the delay.

I tried to do what you suggested.  There was one twist: it regressed
constexpr-nullptr-2.C, in particular we lost diagnostics for

constexpr int* pj0 = &((S*)0)->j;       // { dg-error "not a constant expression" }
constexpr int* pj1 = &((S*)nullptr)->j;  // { dg-error "not a constant expression" }

because when maybe_constant_init_1 saw a constant:

5142   else if (CONSTANT_CLASS_P (t))
5143     /* No evaluation needed.  */;

so it didn't call cxx_eval_outermost_constant_expr which is supposed to give
the error.  I fixed it by adding "&& allow_non_constant" so now it gives the
proper diagnostics.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-02-23  Marek Polacek  <polacek@redhat.com>

	PR c++/83692
	* constexpr.c (maybe_constant_init_1): New function.
	(maybe_constant_init): Make it a wrapper around maybe_constant_init_1.
	(cxx_constant_init): New function.
	* cp-tree.h (cxx_constant_init): Declare.
	* typeck2.c (store_init_value): Call cxx_constant_init instead of
	cxx_constant_value.  Move the maybe_constant_init call under an 'else'.

	* g++.dg/cpp1z/constexpr-83692.C: New test.

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 47ff90cb055..26d0d099a05 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -5123,8 +5123,8 @@ fold_non_dependent_expr (tree t)
 /* Like maybe_constant_value, but returns a CONSTRUCTOR directly, rather
    than wrapped in a TARGET_EXPR.  */
 
-tree
-maybe_constant_init (tree t, tree decl)
+static tree
+maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant)
 {
   if (!t)
     return t;
@@ -5139,10 +5139,10 @@ maybe_constant_init (tree t, tree decl)
     t = TARGET_EXPR_INITIAL (t);
   if (!is_nondependent_static_init_expression (t))
     /* Don't try to evaluate it.  */;
-  else if (CONSTANT_CLASS_P (t))
+  else if (CONSTANT_CLASS_P (t) && allow_non_constant)
     /* No evaluation needed.  */;
   else
-    t = cxx_eval_outermost_constant_expr (t, true, false, decl);
+    t = cxx_eval_outermost_constant_expr (t, allow_non_constant, false, decl);
   if (TREE_CODE (t) == TARGET_EXPR)
     {
       tree init = TARGET_EXPR_INITIAL (t);
@@ -5152,6 +5152,22 @@ maybe_constant_init (tree t, tree decl)
   return t;
 }
 
+/* Wrapper for maybe_constant_init_1 which permits non constants.  */
+
+tree
+maybe_constant_init (tree t, tree decl)
+{
+  return maybe_constant_init_1 (t, decl, true);
+}
+
+/* Wrapper for maybe_constant_init_1 which does not permit non constants.  */
+
+tree
+cxx_constant_init (tree t, tree decl)
+{
+  return maybe_constant_init_1 (t, decl, false);
+}
+
 #if 0
 /* FIXME see ADDR_EXPR section in potential_constant_expression_1.  */
 /* Return true if the object referred to by REF has automatic or thread
diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
index 9038d677b2d..04c7b7ce3a9 100644
--- gcc/cp/cp-tree.h
+++ gcc/cp/cp-tree.h
@@ -7411,6 +7411,7 @@ extern bool require_potential_constant_expression (tree);
 extern bool require_constant_expression (tree);
 extern bool require_potential_rvalue_constant_expression (tree);
 extern tree cxx_constant_value			(tree, tree = NULL_TREE);
+extern tree cxx_constant_init			(tree, tree = NULL_TREE);
 extern tree maybe_constant_value		(tree, tree = NULL_TREE);
 extern tree maybe_constant_init			(tree, tree = NULL_TREE);
 extern tree fold_non_dependent_expr		(tree);
diff --git gcc/cp/typeck2.c gcc/cp/typeck2.c
index 899d60e8535..153b46cca77 100644
--- gcc/cp/typeck2.c
+++ gcc/cp/typeck2.c
@@ -830,9 +830,10 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** cleanups, int flags)
 	  if (!require_constant_expression (value))
 	    value = error_mark_node;
 	  else
-	    value = cxx_constant_value (value, decl);
+	    value = cxx_constant_init (value, decl);
 	}
-      value = maybe_constant_init (value, decl);
+      else
+	value = maybe_constant_init (value, decl);
       if (TREE_CODE (value) == CONSTRUCTOR && cp_has_mutable_p (type))
 	/* Poison this CONSTRUCTOR so it can't be copied to another
 	   constexpr variable.  */
diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
index e69de29bb2d..f6b61eeab85 100644
--- gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
@@ -0,0 +1,21 @@
+// PR c++/83692
+// { dg-options -std=c++17 }
+
+struct integer {
+  constexpr int value() const { return m_value;	}
+  int m_value;
+};
+
+struct outer {
+  integer m_x{0};
+  constexpr outer()
+    {
+      if (m_x.value() != 0)
+	throw 0;
+      m_x.m_value = integer{1}.value();
+      if (m_x.value() != 1)
+	throw 0;
+    }
+};
+
+constexpr outer o{};

	Marek
Jason Merrill Feb. 24, 2018, 1:55 a.m. | #7
OK, thanks.

On Fri, Feb 23, 2018 at 9:29 AM, Marek Polacek <polacek@redhat.com> wrote:
> On Fri, Feb 16, 2018 at 04:10:20PM -0500, Jason Merrill wrote:

>> On Mon, Feb 5, 2018 at 1:45 PM, Jason Merrill <jason@redhat.com> wrote:

>> > On Mon, Feb 5, 2018 at 8:37 AM, Marek Polacek <polacek@redhat.com> wrote:

>> >> On Fri, Feb 02, 2018 at 02:11:27PM -0500, Jason Merrill wrote:

>> >>> On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@redhat.com> wrote:

>> >>> > This is a similar problem to 83116: we'd cached a constexpr call, but after a

>> >>> > store the result had become invalid, yet we used the wrong result again when

>> >>> > encountering the same call later.  This resulted in evaluating a THROW_EXPR

>> >>> > which doesn't work.  Details in

>> >>> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5

>> >>> >

>> >>> > The fix for 83116 didn't work here, because when evaluating the body of the

>> >>> > ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do

>> >>> > cache.

>> >>>

>> >>> > It seems that we may no longer rely on the constexpr call table when we

>> >>> > do cxx_eval_store_expression, because that just rewrites *valp, i.e. the

>> >>> > value of an object.  Might be too big a hammer again, but I couldn't think

>> >>> > of how I could guard the caching of a constexpr call.

>> >>>

>> >>> > This doesn't manifest in C++14 because build_special_member_call in C++17 is

>> >>> > more aggressive with copy elisions (as required by P0135 which changed how we

>> >>> > view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so

>> >>> > expand_default_init calls maybe_constant_init, for which STRICT is false, so

>> >>> > we avoid caching as per 83116.

>> >>>

>> >>> So it sounds like the problem is using cxx_constant_value for the

>> >>> diagnostic when it has different semantics from the

>> >>> maybe_constant_init that follows right after.  I guess we want a

>> >>> cxx_constant_init function that is a hybrid of the two.

>> >>

>> >> So like the following?  Thanks,

>> >>

>> >> Bootstrapped/regtested on x86_64-linux, ok for trunk?

>> >>

>> >> 2018-02-04  Marek Polacek  <polacek@redhat.com>

>> >>

>> >>         PR c++/83692

>> >>         * constexpr.c (cxx_constant_init): New function.

>> >>         * cp-tree.h (cxx_constant_init): Declare.

>> >>         * typeck2.c (store_init_value): Call cxx_constant_init instead of

>> >>         cxx_constant_value.

>> >>

>> >> +/* Like cxx_constant_value, but non-strict mode.  */

>> >> +

>> >> +tree

>> >> +cxx_constant_init (tree t, tree decl)

>> >> +{

>> >> +  return cxx_eval_outermost_constant_expr (t, false, false, decl);

>> >> +}

>> >

>> > Hmm, that doesn't do the TARGET_EXPR stripping that

>> > maybe_constant_init does.  I was thinking of a version of

>> > maybe_constant_init that passes false to allow_non_constant.  Probably

>> > by making "maybe_constant_init" and cxx_constant_init both call the

>> > current function with an additional parameter.  And then the existing

>> > call to maybe_constant_init can move under an 'else' to avoid

>> > redundant constexpr evaluation.

>>

>> Want me to take this over?

>

> Sorry again for the delay.

>

> I tried to do what you suggested.  There was one twist: it regressed

> constexpr-nullptr-2.C, in particular we lost diagnostics for

>

> constexpr int* pj0 = &((S*)0)->j;       // { dg-error "not a constant expression" }

> constexpr int* pj1 = &((S*)nullptr)->j;  // { dg-error "not a constant expression" }

>

> because when maybe_constant_init_1 saw a constant:

>

> 5142   else if (CONSTANT_CLASS_P (t))

> 5143     /* No evaluation needed.  */;

>

> so it didn't call cxx_eval_outermost_constant_expr which is supposed to give

> the error.  I fixed it by adding "&& allow_non_constant" so now it gives the

> proper diagnostics.

>

> Bootstrapped/regtested on x86_64-linux, ok for trunk?

>

> 2018-02-23  Marek Polacek  <polacek@redhat.com>

>

>         PR c++/83692

>         * constexpr.c (maybe_constant_init_1): New function.

>         (maybe_constant_init): Make it a wrapper around maybe_constant_init_1.

>         (cxx_constant_init): New function.

>         * cp-tree.h (cxx_constant_init): Declare.

>         * typeck2.c (store_init_value): Call cxx_constant_init instead of

>         cxx_constant_value.  Move the maybe_constant_init call under an 'else'.

>

>         * g++.dg/cpp1z/constexpr-83692.C: New test.

>

> diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c

> index 47ff90cb055..26d0d099a05 100644

> --- gcc/cp/constexpr.c

> +++ gcc/cp/constexpr.c

> @@ -5123,8 +5123,8 @@ fold_non_dependent_expr (tree t)

>  /* Like maybe_constant_value, but returns a CONSTRUCTOR directly, rather

>     than wrapped in a TARGET_EXPR.  */

>

> -tree

> -maybe_constant_init (tree t, tree decl)

> +static tree

> +maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant)

>  {

>    if (!t)

>      return t;

> @@ -5139,10 +5139,10 @@ maybe_constant_init (tree t, tree decl)

>      t = TARGET_EXPR_INITIAL (t);

>    if (!is_nondependent_static_init_expression (t))

>      /* Don't try to evaluate it.  */;

> -  else if (CONSTANT_CLASS_P (t))

> +  else if (CONSTANT_CLASS_P (t) && allow_non_constant)

>      /* No evaluation needed.  */;

>    else

> -    t = cxx_eval_outermost_constant_expr (t, true, false, decl);

> +    t = cxx_eval_outermost_constant_expr (t, allow_non_constant, false, decl);

>    if (TREE_CODE (t) == TARGET_EXPR)

>      {

>        tree init = TARGET_EXPR_INITIAL (t);

> @@ -5152,6 +5152,22 @@ maybe_constant_init (tree t, tree decl)

>    return t;

>  }

>

> +/* Wrapper for maybe_constant_init_1 which permits non constants.  */

> +

> +tree

> +maybe_constant_init (tree t, tree decl)

> +{

> +  return maybe_constant_init_1 (t, decl, true);

> +}

> +

> +/* Wrapper for maybe_constant_init_1 which does not permit non constants.  */

> +

> +tree

> +cxx_constant_init (tree t, tree decl)

> +{

> +  return maybe_constant_init_1 (t, decl, false);

> +}

> +

>  #if 0

>  /* FIXME see ADDR_EXPR section in potential_constant_expression_1.  */

>  /* Return true if the object referred to by REF has automatic or thread

> diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h

> index 9038d677b2d..04c7b7ce3a9 100644

> --- gcc/cp/cp-tree.h

> +++ gcc/cp/cp-tree.h

> @@ -7411,6 +7411,7 @@ extern bool require_potential_constant_expression (tree);

>  extern bool require_constant_expression (tree);

>  extern bool require_potential_rvalue_constant_expression (tree);

>  extern tree cxx_constant_value                 (tree, tree = NULL_TREE);

> +extern tree cxx_constant_init                  (tree, tree = NULL_TREE);

>  extern tree maybe_constant_value               (tree, tree = NULL_TREE);

>  extern tree maybe_constant_init                        (tree, tree = NULL_TREE);

>  extern tree fold_non_dependent_expr            (tree);

> diff --git gcc/cp/typeck2.c gcc/cp/typeck2.c

> index 899d60e8535..153b46cca77 100644

> --- gcc/cp/typeck2.c

> +++ gcc/cp/typeck2.c

> @@ -830,9 +830,10 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** cleanups, int flags)

>           if (!require_constant_expression (value))

>             value = error_mark_node;

>           else

> -           value = cxx_constant_value (value, decl);

> +           value = cxx_constant_init (value, decl);

>         }

> -      value = maybe_constant_init (value, decl);

> +      else

> +       value = maybe_constant_init (value, decl);

>        if (TREE_CODE (value) == CONSTRUCTOR && cp_has_mutable_p (type))

>         /* Poison this CONSTRUCTOR so it can't be copied to another

>            constexpr variable.  */

> diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C

> index e69de29bb2d..f6b61eeab85 100644

> --- gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C

> +++ gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C

> @@ -0,0 +1,21 @@

> +// PR c++/83692

> +// { dg-options -std=c++17 }

> +

> +struct integer {

> +  constexpr int value() const { return m_value;        }

> +  int m_value;

> +};

> +

> +struct outer {

> +  integer m_x{0};

> +  constexpr outer()

> +    {

> +      if (m_x.value() != 0)

> +       throw 0;

> +      m_x.m_value = integer{1}.value();

> +      if (m_x.value() != 1)

> +       throw 0;

> +    }

> +};

> +

> +constexpr outer o{};

>

>         Marek

Patch

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 4d2ee4a28fc..0202d22f320 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -3663,6 +3663,10 @@  cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
   else
     *valp = init;
 
+  /* We've rewritten a value of a temporary in this constexpr
+     context which might invalide a cached call.  */
+  constexpr_call_table = NULL;
+
   /* Update TREE_CONSTANT and TREE_SIDE_EFFECTS on enclosing
      CONSTRUCTORs, if any.  */
   tree elt;
diff --git gcc/testsuite/g++.dg/cpp1y/constexpr-83692.C gcc/testsuite/g++.dg/cpp1y/constexpr-83692.C
index e69de29bb2d..292ba7c22e9 100644
--- gcc/testsuite/g++.dg/cpp1y/constexpr-83692.C
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-83692.C
@@ -0,0 +1,21 @@ 
+// PR c++/83692
+// { dg-do compile { target c++14 } }
+
+struct integer {
+  constexpr int value() const { return m_value;	}
+  int m_value;
+};
+
+struct outer {
+  integer m_x{0};
+  constexpr outer()
+    {
+      if (m_x.value() != 0)
+	throw 0;
+      m_x.m_value = integer{1}.value();
+      if (m_x.value() != 1)
+	throw 0;
+    }
+};
+
+constexpr outer o{};