[PATCH] c++: fix broken copy elision with nested TARGET_EXPRs [PR105550]

[PATCH] c++: fix broken copy elision with nested TARGET_EXPRs [PR105550]

[Marek Polacek]

In this problem, we are failing to properly perform copy elision with
a conditional operator, so this:

  constexpr A a = true ? A{} : A{};

fails with:

  error: 'A{((const A*)(&<anonymous>))}' is not a constant expression

The whole initializer is

  TARGET_EXPR <D.2395, 1 ? TARGET_EXPR <D.2393, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}> : TARGET_EXPR <D.2394, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}>>

where the outermost TARGET_EXPR is elided, but not the nested ones.
Then we end up replacing the PLACEHOLDER_EXPRs with the temporaries the
TARGET_EXPRs represent, which is precisely what should *not* happen with
copy elision.

I've tried the approach of tweaking ctx->object, but I ran into gazillion
problems with that.  I thought that I would let cxx_eval_constant_expression
/TARGET_EXPR create a new object only when ctx->object was null, then
adjust setting of ctx->object in places like cxx_bind_parameters_in_call
and cxx_eval_component_reference but that failed completely.  Sometimes
ctx->object has to be reset, sometimes it cannot be reset, 'this' needed
special handling, etc.  I gave up.

But now that we have potential_prvalue_result_of, a simple but less
elegant solution is the following.  I thought about setting a flag on
a TARGET_EXPR to avoid adding ctx.full_expr, but a new flag would be
overkill and using TARGET_EXPR_DIRECT_INIT_P broke things.

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

	PR c++/105550

gcc/cp/ChangeLog:

	* constexpr.cc (struct constexpr_ctx): Add a tree member.
	(init_subob_ctx): Set it.
	(cxx_eval_constant_expression): Don't initialize a temporary object
	if potential_prvalue_result_of says true.
	(cxx_eval_outermost_constant_expr): Adjust the ctx initializer.  Set
	ctx.full_expr.
	* cp-tree.h (potential_prvalue_result_of): Declare.
	* typeck2.cc (potential_prvalue_result_of): No longer static.  Return
	if full_expr is null.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-elision1.C: New test.
---
 gcc/cp/constexpr.cc                           | 33 +++++++++---
 gcc/cp/cp-tree.h                              |  1 +
 gcc/cp/typeck2.cc                             |  4 +-
 .../g++.dg/cpp1y/constexpr-elision1.C         | 53 +++++++++++++++++++
 4 files changed, 82 insertions(+), 9 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 45208478c3f..73880fb089e 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -1129,6 +1129,9 @@ struct constexpr_ctx {
   tree ctor;
   /* The object we're building the CONSTRUCTOR for.  */
   tree object;
+  /* The whole initializer expression.  Currently only used when the whole
+     expression is a TARGET_EXPR.  */
+  tree full_expr;
   /* If inside SWITCH_EXPR.  */
   constexpr_switch_state *css_state;
   /* The aggregate initialization context inside which this one is nested.  This
@@ -4700,8 +4703,11 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
   new_ctx.ctor = elt;
 
   if (TREE_CODE (value) == TARGET_EXPR)
-    /* Avoid creating another CONSTRUCTOR when we expand the TARGET_EXPR.  */
-    value = TARGET_EXPR_INITIAL (value);
+    {
+      new_ctx.full_expr = value;
+      /* Avoid creating another CONSTRUCTOR when we expand the TARGET_EXPR.  */
+      value = TARGET_EXPR_INITIAL (value);
+    }
 }
 
 /* We're about to process an initializer for a class or array TYPE.  Make
@@ -6822,6 +6828,14 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    break;
 	  }
 	gcc_checking_assert (!TARGET_EXPR_DIRECT_INIT_P (t));
+	/* This TARGET_EXPR may be nested inside another TARGET_EXPR, but
+	   still serve as the initializer for the same object as the outer
+	   TARGET_EXPR, as in
+	     A a = true ? A{} : A{};
+	   so we can't materialize a temporary.  IOW, don't set ctx->object
+	   to the TARGET_EXPR's slot.  */
+	const bool prvalue = potential_prvalue_result_of (t, ctx->full_expr);
+	gcc_checking_assert (!prvalue || lval == vc_prvalue);
 	/* Avoid evaluating a TARGET_EXPR more than once.  */
 	tree slot = TARGET_EXPR_SLOT (t);
 	if (tree *p = ctx->global->values.get (slot))
@@ -6831,7 +6845,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    r = *p;
 	    break;
 	  }
-	if ((AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
+	if (!prvalue && (AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
 	  {
 	    /* We're being expanded without an explicit target, so start
 	       initializing a new object; expansion with an explicit target
@@ -7748,8 +7762,8 @@ 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,
+  constexpr_ctx ctx = { &global_ctx, nullptr, nullptr, NULL_TREE, NULL_TREE,
+			NULL_TREE, nullptr, nullptr, allow_non_constant, strict,
 			manifestly_const_eval || !allow_non_constant };
 
   /* Turn off -frounding-math for manifestly constant evaluation.  */
@@ -7834,9 +7848,12 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
       if (object && DECL_P (object))
 	global_ctx.values.put (object, ctx.ctor);
       if (TREE_CODE (r) == TARGET_EXPR)
-	/* Avoid creating another CONSTRUCTOR when we expand the
-	   TARGET_EXPR.  */
-	r = TARGET_EXPR_INITIAL (r);
+	{
+	  ctx.full_expr = r;
+	  /* Avoid creating another CONSTRUCTOR when we expand the
+	     TARGET_EXPR.  */
+	  r = TARGET_EXPR_INITIAL (r);
+	}
     }
 
   auto_vec<tree, 16> cleanups;
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index d77fd1eb8a9..c3c1f5889a3 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8160,6 +8160,7 @@ extern tree build_functional_cast		(location_t, tree, tree,
 						 tsubst_flags_t);
 extern tree add_exception_specifier		(tree, tree, tsubst_flags_t);
 extern tree merge_exception_specifiers		(tree, tree);
+extern bool potential_prvalue_result_of		(tree, tree);
 
 /* in mangle.cc */
 extern void init_mangle				(void);
diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 1a96be3d412..6578d5ed6d2 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -1383,9 +1383,11 @@ digest_init_flags (tree type, tree init, int flags, tsubst_flags_t complain)
 
    FULL_EXPR is the whole expression, SUBOB is its TARGET_EXPR subobject.  */
 
-static bool
+bool
 potential_prvalue_result_of (tree subob, tree full_expr)
 {
+  if (!full_expr)
+    return false;
   if (subob == full_expr)
     return true;
   else if (TREE_CODE (full_expr) == TARGET_EXPR)
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
new file mode 100644
index 00000000000..b225ae29cde
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
@@ -0,0 +1,53 @@
+// PR c++/105550
+// { dg-do compile { target c++14 } }
+
+struct A {
+  const A *p = this;
+};
+
+struct B {
+  const B *p = this;
+  constexpr operator A() const { return {}; }
+};
+
+constexpr A
+bar (A)
+{
+  return {};
+}
+
+constexpr A baz() { return {}; }
+
+struct E {
+  A a1 = true ? A{} : A{};
+  A a2 = true ? A{} : B{};
+  A a3 = false ? A{} : B{};
+  A a4 = false ? B{} : B{};
+  A a5 = A{};
+  A a6 = B{};
+  A a7 = false ? B{} : (true ? A{} : A{});
+  A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
+  A a9 = (A{});
+  A a10 = (true, A{});
+  A a11 = bar (A{});
+  A a12 = baz ();
+  A a13 = (A{}, A{});
+};
+
+constexpr E e{};
+
+constexpr A a1 = true ? A{} : A{};
+constexpr A a2 = true ? A{} : B{};
+constexpr A a3 = false ? A{} : B{};
+constexpr A a4 = false ? B{} : B{};
+constexpr A a5 = A{};
+constexpr A a6 = B{};
+constexpr A a7 = false ? B{} : (true ? A{} : A{});
+constexpr A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
+constexpr A a9 = (A{});
+constexpr A a10 = (true, A{});
+constexpr A a11 = bar (A{});
+//static_assert(a10.p == &a10, ""); // bug, 105619
+constexpr A a12 = baz ();
+//static_assert(a11.p == &a11, ""); // bug, 105619
+constexpr A a13 = (A{}, A{});

base-commit: 97dc78d705a90c1ae83c78a7f2e24942cc3a6257
-- 
2.36.1

Re: [PATCH] c++: fix broken copy elision with nested TARGET_EXPRs [PR105550]

[Jason Merrill]

On 5/26/22 11:01, Marek Polacek wrote:
> In this problem, we are failing to properly perform copy elision with
> a conditional operator, so this:
> 
>    constexpr A a = true ? A{} : A{};
> 
> fails with:
> 
>    error: 'A{((const A*)(&<anonymous>))}' is not a constant expression
> 
> The whole initializer is
> 
>    TARGET_EXPR <D.2395, 1 ? TARGET_EXPR <D.2393, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}> : TARGET_EXPR <D.2394, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}>>
> 
> where the outermost TARGET_EXPR is elided, but not the nested ones.
> Then we end up replacing the PLACEHOLDER_EXPRs with the temporaries the
> TARGET_EXPRs represent, which is precisely what should *not* happen with
> copy elision.
> 
> I've tried the approach of tweaking ctx->object, but I ran into gazillion
> problems with that.  I thought that I would let cxx_eval_constant_expression
> /TARGET_EXPR create a new object only when ctx->object was null, then
> adjust setting of ctx->object in places like cxx_bind_parameters_in_call
> and cxx_eval_component_reference but that failed completely.  Sometimes
> ctx->object has to be reset, sometimes it cannot be reset, 'this' needed
> special handling, etc.  I gave up.
> > But now that we have potential_prvalue_result_of, a simple but less
> elegant solution is the following.  I thought about setting a flag on
> a TARGET_EXPR to avoid adding ctx.full_expr, but a new flag would be
> overkill and using TARGET_EXPR_DIRECT_INIT_P broke things.

This doesn't seem like a general solution; the same issue would also 
apply to ?: of TARGET_EXPR when it's a subexpression rather than the 
full expression, like f(true ? A{} : B{}).

Another simple approach, but more general, would be to routinely strip 
TARGET_EXPR from the operands of ?: like we do in various other places 
in constexpr.c.

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/105550
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (struct constexpr_ctx): Add a tree member.
> 	(init_subob_ctx): Set it.
> 	(cxx_eval_constant_expression): Don't initialize a temporary object
> 	if potential_prvalue_result_of says true.
> 	(cxx_eval_outermost_constant_expr): Adjust the ctx initializer.  Set
> 	ctx.full_expr.
> 	* cp-tree.h (potential_prvalue_result_of): Declare.
> 	* typeck2.cc (potential_prvalue_result_of): No longer static.  Return
> 	if full_expr is null.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-elision1.C: New test.
> ---
>   gcc/cp/constexpr.cc                           | 33 +++++++++---
>   gcc/cp/cp-tree.h                              |  1 +
>   gcc/cp/typeck2.cc                             |  4 +-
>   .../g++.dg/cpp1y/constexpr-elision1.C         | 53 +++++++++++++++++++
>   4 files changed, 82 insertions(+), 9 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 45208478c3f..73880fb089e 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -1129,6 +1129,9 @@ struct constexpr_ctx {
>     tree ctor;
>     /* The object we're building the CONSTRUCTOR for.  */
>     tree object;
> +  /* The whole initializer expression.  Currently only used when the whole
> +     expression is a TARGET_EXPR.  */
> +  tree full_expr;
>     /* If inside SWITCH_EXPR.  */
>     constexpr_switch_state *css_state;
>     /* The aggregate initialization context inside which this one is nested.  This
> @@ -4700,8 +4703,11 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
>     new_ctx.ctor = elt;
>   
>     if (TREE_CODE (value) == TARGET_EXPR)
> -    /* Avoid creating another CONSTRUCTOR when we expand the TARGET_EXPR.  */
> -    value = TARGET_EXPR_INITIAL (value);
> +    {
> +      new_ctx.full_expr = value;
> +      /* Avoid creating another CONSTRUCTOR when we expand the TARGET_EXPR.  */
> +      value = TARGET_EXPR_INITIAL (value);
> +    }
>   }
>   
>   /* We're about to process an initializer for a class or array TYPE.  Make
> @@ -6822,6 +6828,14 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
>   	    break;
>   	  }
>   	gcc_checking_assert (!TARGET_EXPR_DIRECT_INIT_P (t));
> +	/* This TARGET_EXPR may be nested inside another TARGET_EXPR, but
> +	   still serve as the initializer for the same object as the outer
> +	   TARGET_EXPR, as in
> +	     A a = true ? A{} : A{};
> +	   so we can't materialize a temporary.  IOW, don't set ctx->object
> +	   to the TARGET_EXPR's slot.  */
> +	const bool prvalue = potential_prvalue_result_of (t, ctx->full_expr);
> +	gcc_checking_assert (!prvalue || lval == vc_prvalue);
>   	/* Avoid evaluating a TARGET_EXPR more than once.  */
>   	tree slot = TARGET_EXPR_SLOT (t);
>   	if (tree *p = ctx->global->values.get (slot))
> @@ -6831,7 +6845,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
>   	    r = *p;
>   	    break;
>   	  }
> -	if ((AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
> +	if (!prvalue && (AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
>   	  {
>   	    /* We're being expanded without an explicit target, so start
>   	       initializing a new object; expansion with an explicit target
> @@ -7748,8 +7762,8 @@ 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,
> +  constexpr_ctx ctx = { &global_ctx, nullptr, nullptr, NULL_TREE, NULL_TREE,
> +			NULL_TREE, nullptr, nullptr, allow_non_constant, strict,
>   			manifestly_const_eval || !allow_non_constant };
>   
>     /* Turn off -frounding-math for manifestly constant evaluation.  */
> @@ -7834,9 +7848,12 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
>         if (object && DECL_P (object))
>   	global_ctx.values.put (object, ctx.ctor);
>         if (TREE_CODE (r) == TARGET_EXPR)
> -	/* Avoid creating another CONSTRUCTOR when we expand the
> -	   TARGET_EXPR.  */
> -	r = TARGET_EXPR_INITIAL (r);
> +	{
> +	  ctx.full_expr = r;
> +	  /* Avoid creating another CONSTRUCTOR when we expand the
> +	     TARGET_EXPR.  */
> +	  r = TARGET_EXPR_INITIAL (r);
> +	}
>       }
>   
>     auto_vec<tree, 16> cleanups;
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index d77fd1eb8a9..c3c1f5889a3 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -8160,6 +8160,7 @@ extern tree build_functional_cast		(location_t, tree, tree,
>   						 tsubst_flags_t);
>   extern tree add_exception_specifier		(tree, tree, tsubst_flags_t);
>   extern tree merge_exception_specifiers		(tree, tree);
> +extern bool potential_prvalue_result_of		(tree, tree);
>   
>   /* in mangle.cc */
>   extern void init_mangle				(void);
> diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
> index 1a96be3d412..6578d5ed6d2 100644
> --- a/gcc/cp/typeck2.cc
> +++ b/gcc/cp/typeck2.cc
> @@ -1383,9 +1383,11 @@ digest_init_flags (tree type, tree init, int flags, tsubst_flags_t complain)
>   
>      FULL_EXPR is the whole expression, SUBOB is its TARGET_EXPR subobject.  */
>   
> -static bool
> +bool
>   potential_prvalue_result_of (tree subob, tree full_expr)
>   {
> +  if (!full_expr)
> +    return false;
>     if (subob == full_expr)
>       return true;
>     else if (TREE_CODE (full_expr) == TARGET_EXPR)
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
> new file mode 100644
> index 00000000000..b225ae29cde
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
> @@ -0,0 +1,53 @@
> +// PR c++/105550
> +// { dg-do compile { target c++14 } }
> +
> +struct A {
> +  const A *p = this;
> +};
> +
> +struct B {
> +  const B *p = this;
> +  constexpr operator A() const { return {}; }
> +};
> +
> +constexpr A
> +bar (A)
> +{
> +  return {};
> +}
> +
> +constexpr A baz() { return {}; }
> +
> +struct E {
> +  A a1 = true ? A{} : A{};
> +  A a2 = true ? A{} : B{};
> +  A a3 = false ? A{} : B{};
> +  A a4 = false ? B{} : B{};
> +  A a5 = A{};
> +  A a6 = B{};
> +  A a7 = false ? B{} : (true ? A{} : A{});
> +  A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
> +  A a9 = (A{});
> +  A a10 = (true, A{});
> +  A a11 = bar (A{});
> +  A a12 = baz ();
> +  A a13 = (A{}, A{});
> +};
> +
> +constexpr E e{};
> +
> +constexpr A a1 = true ? A{} : A{};
> +constexpr A a2 = true ? A{} : B{};
> +constexpr A a3 = false ? A{} : B{};
> +constexpr A a4 = false ? B{} : B{};
> +constexpr A a5 = A{};
> +constexpr A a6 = B{};
> +constexpr A a7 = false ? B{} : (true ? A{} : A{});
> +constexpr A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
> +constexpr A a9 = (A{});
> +constexpr A a10 = (true, A{});
> +constexpr A a11 = bar (A{});
> +//static_assert(a10.p == &a10, ""); // bug, 105619
> +constexpr A a12 = baz ();
> +//static_assert(a11.p == &a11, ""); // bug, 105619
> +constexpr A a13 = (A{}, A{});
> 
> base-commit: 97dc78d705a90c1ae83c78a7f2e24942cc3a6257

[PATCH v2] c++: fix broken copy elision with nested TARGET_EXPRs [PR105550]

[Marek Polacek]

On Thu, Jun 02, 2022 at 05:08:54PM -0400, Jason Merrill wrote:
> On 5/26/22 11:01, Marek Polacek wrote:
> > In this problem, we are failing to properly perform copy elision with
> > a conditional operator, so this:
> > 
> >    constexpr A a = true ? A{} : A{};
> > 
> > fails with:
> > 
> >    error: 'A{((const A*)(&<anonymous>))}' is not a constant expression
> > 
> > The whole initializer is
> > 
> >    TARGET_EXPR <D.2395, 1 ? TARGET_EXPR <D.2393, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}> : TARGET_EXPR <D.2394, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}>>
> > 
> > where the outermost TARGET_EXPR is elided, but not the nested ones.
> > Then we end up replacing the PLACEHOLDER_EXPRs with the temporaries the
> > TARGET_EXPRs represent, which is precisely what should *not* happen with
> > copy elision.
> > 
> > I've tried the approach of tweaking ctx->object, but I ran into gazillion
> > problems with that.  I thought that I would let cxx_eval_constant_expression
> > /TARGET_EXPR create a new object only when ctx->object was null, then
> > adjust setting of ctx->object in places like cxx_bind_parameters_in_call
> > and cxx_eval_component_reference but that failed completely.  Sometimes
> > ctx->object has to be reset, sometimes it cannot be reset, 'this' needed
> > special handling, etc.  I gave up.
> > > But now that we have potential_prvalue_result_of, a simple but less
> > elegant solution is the following.  I thought about setting a flag on
> > a TARGET_EXPR to avoid adding ctx.full_expr, but a new flag would be
> > overkill and using TARGET_EXPR_DIRECT_INIT_P broke things.

Sorry it's taken me so long to get back to this.
 
> This doesn't seem like a general solution; the same issue would also apply
> to ?: of TARGET_EXPR when it's a subexpression rather than the full
> expression, like f(true ? A{} : B{}).

You're right.
 
> Another simple approach, but more general, would be to routinely strip
> TARGET_EXPR from the operands of ?: like we do in various other places in
> constexpr.c.

How about this, then?

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

-- >8 --
In this problem, we are failing to properly perform copy elision with
a conditional operator, so this:

  constexpr A a = true ? A{} : A{};

fails with:

  error: 'A{((const A*)(&<anonymous>))}' is not a constant expression

The whole initializer is

  TARGET_EXPR <D.2395, 1 ? TARGET_EXPR <D.2393, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}> : TARGET_EXPR <D.2394, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}>>

where the outermost TARGET_EXPR is elided, but not the nested ones.
Then we end up replacing the PLACEHOLDER_EXPRs with the temporaries the
TARGET_EXPRs represent, which is precisely what should *not* happen with
copy elision.

I've tried the approach of tweaking ctx->object, but I ran into gazillion
problems with that.  I thought that I would let cxx_eval_constant_expression
/TARGET_EXPR create a new object only when ctx->object was null, then
adjust setting of ctx->object in places like cxx_bind_parameters_in_call
and cxx_eval_component_reference but that failed completely.  Sometimes
ctx->object has to be reset, sometimes it cannot be reset, 'this' needed
special handling, etc.  I gave up.

Instead, this patch strips TARGET_EXPRs from the operands of ?: like
we do in various other places in constexpr.c.

	PR c++/105550

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_conditional_expression): Strip TARGET_EXPRs.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/nsdmi-aggr16.C: Remove FIXME.
	* g++.dg/cpp1y/nsdmi-aggr17.C: Remove FIXME.
	* g++.dg/cpp0x/constexpr-elision1.C: New test.
	* g++.dg/cpp1y/constexpr-elision1.C: New test.
---
 gcc/cp/constexpr.cc                           |  7 +++
 .../g++.dg/cpp0x/constexpr-elision1.C         | 16 ++++++
 .../g++.dg/cpp1y/constexpr-elision1.C         | 53 +++++++++++++++++++
 gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C     |  5 +-
 gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C     |  5 +-
 5 files changed, 80 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-elision1.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 0dc94d9445d..5f7fc6f8f0c 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -3507,6 +3507,13 @@ cxx_eval_conditional_expression (const constexpr_ctx *ctx, tree t,
     val = TREE_OPERAND (t, 1);
   if (TREE_CODE (t) == IF_STMT && !val)
     val = void_node;
+  /* A TARGET_EXPR may be nested inside another TARGET_EXPR, but still
+     serve as the initializer for the same object as the outer TARGET_EXPR,
+     as in
+       A a = true ? A{} : A{};
+     so strip the inner TARGET_EXPR so we don't materialize a temporary.  */
+  if (TREE_CODE (val) == TARGET_EXPR)
+    val = TARGET_EXPR_INITIAL (val);
   return cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
 				       overflow_p, jump_target);
 }
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-elision1.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-elision1.C
new file mode 100644
index 00000000000..9e7b9ec3405
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-elision1.C
@@ -0,0 +1,16 @@
+// PR c++/105550
+// { dg-do compile { target c++11 } }
+
+template <typename, typename> struct pair {
+  constexpr pair(int, int) {}
+};
+template <typename _Tp, typename _Compare>
+pair<const _Tp &, const _Tp &> minmax(const _Tp &__a, const _Tp &__b,
+                                      _Compare) {
+  return 0 ? pair<const _Tp &, const _Tp &>(__b, __a)
+           : pair<const _Tp &, const _Tp &>(__a, __b);
+}
+typedef int value_type;
+typedef int compare_type;
+template pair<const value_type &, const value_type &>
+minmax(const value_type &, const value_type &, compare_type);
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
new file mode 100644
index 00000000000..b225ae29cde
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
@@ -0,0 +1,53 @@
+// PR c++/105550
+// { dg-do compile { target c++14 } }
+
+struct A {
+  const A *p = this;
+};
+
+struct B {
+  const B *p = this;
+  constexpr operator A() const { return {}; }
+};
+
+constexpr A
+bar (A)
+{
+  return {};
+}
+
+constexpr A baz() { return {}; }
+
+struct E {
+  A a1 = true ? A{} : A{};
+  A a2 = true ? A{} : B{};
+  A a3 = false ? A{} : B{};
+  A a4 = false ? B{} : B{};
+  A a5 = A{};
+  A a6 = B{};
+  A a7 = false ? B{} : (true ? A{} : A{});
+  A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
+  A a9 = (A{});
+  A a10 = (true, A{});
+  A a11 = bar (A{});
+  A a12 = baz ();
+  A a13 = (A{}, A{});
+};
+
+constexpr E e{};
+
+constexpr A a1 = true ? A{} : A{};
+constexpr A a2 = true ? A{} : B{};
+constexpr A a3 = false ? A{} : B{};
+constexpr A a4 = false ? B{} : B{};
+constexpr A a5 = A{};
+constexpr A a6 = B{};
+constexpr A a7 = false ? B{} : (true ? A{} : A{});
+constexpr A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
+constexpr A a9 = (A{});
+constexpr A a10 = (true, A{});
+constexpr A a11 = bar (A{});
+//static_assert(a10.p == &a10, ""); // bug, 105619
+constexpr A a12 = baz ();
+//static_assert(a11.p == &a11, ""); // bug, 105619
+constexpr A a13 = (A{}, A{});
diff --git a/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C
index dc6492c1b0b..5e66bdd2370 100644
--- a/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C
+++ b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C
@@ -40,9 +40,8 @@ struct E {
   A d = true ? (false ? A{} : A{}) : (false ? A{} : A{});
 };
 
-// FIXME: When fixing this, also fix nsdmi-aggr17.C.
-constexpr E e;	    // { dg-bogus "" "PR105550" { xfail *-*-* } }
-SA (e.a.p == &e.a); // { dg-bogus "" "PR105550" { xfail *-*-* } }
+constexpr E e;
+SA (e.a.p == &e.a);
 
 E e1 = { };
 
diff --git a/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C
index fc27a2cdac7..ca9637b37eb 100644
--- a/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C
+++ b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C
@@ -56,9 +56,8 @@ struct F {
   A a = true ? A{x} : A{x};
 };
 
-// FIXME: Doesn't work due to PR105550.
-//constexpr F f;
-//SA (f.a.p == &f.a);
+constexpr F f;
+SA (f.a.p == &f.a);
 SA (e.x == 42);
 F f2 = { };
 F f3 = { 42 };

base-commit: 113844d68e94f4e9c0e946db351ba7d3d4a1335a
-- 
2.36.1

Re: [PATCH v2] c++: fix broken copy elision with nested TARGET_EXPRs [PR105550]

[Jason Merrill]

On 6/24/22 20:30, Marek Polacek wrote:
> On Thu, Jun 02, 2022 at 05:08:54PM -0400, Jason Merrill wrote:
>> On 5/26/22 11:01, Marek Polacek wrote:
>>> In this problem, we are failing to properly perform copy elision with
>>> a conditional operator, so this:
>>>
>>>     constexpr A a = true ? A{} : A{};
>>>
>>> fails with:
>>>
>>>     error: 'A{((const A*)(&<anonymous>))}' is not a constant expression
>>>
>>> The whole initializer is
>>>
>>>     TARGET_EXPR <D.2395, 1 ? TARGET_EXPR <D.2393, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}> : TARGET_EXPR <D.2394, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}>>
>>>
>>> where the outermost TARGET_EXPR is elided, but not the nested ones.
>>> Then we end up replacing the PLACEHOLDER_EXPRs with the temporaries the
>>> TARGET_EXPRs represent, which is precisely what should *not* happen with
>>> copy elision.
>>>
>>> I've tried the approach of tweaking ctx->object, but I ran into gazillion
>>> problems with that.  I thought that I would let cxx_eval_constant_expression
>>> /TARGET_EXPR create a new object only when ctx->object was null, then
>>> adjust setting of ctx->object in places like cxx_bind_parameters_in_call
>>> and cxx_eval_component_reference but that failed completely.  Sometimes
>>> ctx->object has to be reset, sometimes it cannot be reset, 'this' needed
>>> special handling, etc.  I gave up.
>>>> But now that we have potential_prvalue_result_of, a simple but less
>>> elegant solution is the following.  I thought about setting a flag on
>>> a TARGET_EXPR to avoid adding ctx.full_expr, but a new flag would be
>>> overkill and using TARGET_EXPR_DIRECT_INIT_P broke things.
> 
> Sorry it's taken me so long to get back to this.
>   
>> This doesn't seem like a general solution; the same issue would also apply
>> to ?: of TARGET_EXPR when it's a subexpression rather than the full
>> expression, like f(true ? A{} : B{}).
> 
> You're right.
>   
>> Another simple approach, but more general, would be to routinely strip
>> TARGET_EXPR from the operands of ?: like we do in various other places in
>> constexpr.c.
> 
> How about this, then?
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >8 --
> In this problem, we are failing to properly perform copy elision with
> a conditional operator, so this:
> 
>    constexpr A a = true ? A{} : A{};
> 
> fails with:
> 
>    error: 'A{((const A*)(&<anonymous>))}' is not a constant expression
> 
> The whole initializer is
> 
>    TARGET_EXPR <D.2395, 1 ? TARGET_EXPR <D.2393, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}> : TARGET_EXPR <D.2394, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}>>
> 
> where the outermost TARGET_EXPR is elided, but not the nested ones.
> Then we end up replacing the PLACEHOLDER_EXPRs with the temporaries the
> TARGET_EXPRs represent, which is precisely what should *not* happen with
> copy elision.
> 
> I've tried the approach of tweaking ctx->object, but I ran into gazillion
> problems with that.  I thought that I would let cxx_eval_constant_expression
> /TARGET_EXPR create a new object only when ctx->object was null, then
> adjust setting of ctx->object in places like cxx_bind_parameters_in_call
> and cxx_eval_component_reference but that failed completely.  Sometimes
> ctx->object has to be reset, sometimes it cannot be reset, 'this' needed
> special handling, etc.  I gave up.
> 
> Instead, this patch strips TARGET_EXPRs from the operands of ?: like
> we do in various other places in constexpr.c.
> 
> 	PR c++/105550
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (cxx_eval_conditional_expression): Strip TARGET_EXPRs.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/nsdmi-aggr16.C: Remove FIXME.
> 	* g++.dg/cpp1y/nsdmi-aggr17.C: Remove FIXME.
> 	* g++.dg/cpp0x/constexpr-elision1.C: New test.
> 	* g++.dg/cpp1y/constexpr-elision1.C: New test.
> ---
>   gcc/cp/constexpr.cc                           |  7 +++
>   .../g++.dg/cpp0x/constexpr-elision1.C         | 16 ++++++
>   .../g++.dg/cpp1y/constexpr-elision1.C         | 53 +++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C     |  5 +-
>   gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C     |  5 +-
>   5 files changed, 80 insertions(+), 6 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-elision1.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 0dc94d9445d..5f7fc6f8f0c 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -3507,6 +3507,13 @@ cxx_eval_conditional_expression (const constexpr_ctx *ctx, tree t,
>       val = TREE_OPERAND (t, 1);
>     if (TREE_CODE (t) == IF_STMT && !val)
>       val = void_node;
> +  /* A TARGET_EXPR may be nested inside another TARGET_EXPR, but still
> +     serve as the initializer for the same object as the outer TARGET_EXPR,
> +     as in
> +       A a = true ? A{} : A{};
> +     so strip the inner TARGET_EXPR so we don't materialize a temporary.  */
> +  if (TREE_CODE (val) == TARGET_EXPR)
> +    val = TARGET_EXPR_INITIAL (val);
>     return cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
>   				       overflow_p, jump_target);
>   }
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-elision1.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-elision1.C
> new file mode 100644
> index 00000000000..9e7b9ec3405
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-elision1.C
> @@ -0,0 +1,16 @@
> +// PR c++/105550
> +// { dg-do compile { target c++11 } }
> +
> +template <typename, typename> struct pair {
> +  constexpr pair(int, int) {}
> +};
> +template <typename _Tp, typename _Compare>
> +pair<const _Tp &, const _Tp &> minmax(const _Tp &__a, const _Tp &__b,
> +                                      _Compare) {
> +  return 0 ? pair<const _Tp &, const _Tp &>(__b, __a)
> +           : pair<const _Tp &, const _Tp &>(__a, __b);
> +}
> +typedef int value_type;
> +typedef int compare_type;
> +template pair<const value_type &, const value_type &>
> +minmax(const value_type &, const value_type &, compare_type);
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
> new file mode 100644
> index 00000000000..b225ae29cde
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
> @@ -0,0 +1,53 @@
> +// PR c++/105550
> +// { dg-do compile { target c++14 } }
> +
> +struct A {
> +  const A *p = this;
> +};
> +
> +struct B {
> +  const B *p = this;
> +  constexpr operator A() const { return {}; }
> +};
> +
> +constexpr A
> +bar (A)
> +{
> +  return {};
> +}
> +
> +constexpr A baz() { return {}; }
> +
> +struct E {
> +  A a1 = true ? A{} : A{};
> +  A a2 = true ? A{} : B{};
> +  A a3 = false ? A{} : B{};
> +  A a4 = false ? B{} : B{};
> +  A a5 = A{};
> +  A a6 = B{};
> +  A a7 = false ? B{} : (true ? A{} : A{});
> +  A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
> +  A a9 = (A{});
> +  A a10 = (true, A{});
> +  A a11 = bar (A{});
> +  A a12 = baz ();
> +  A a13 = (A{}, A{});
> +};
> +
> +constexpr E e{};
> +
> +constexpr A a1 = true ? A{} : A{};
> +constexpr A a2 = true ? A{} : B{};
> +constexpr A a3 = false ? A{} : B{};
> +constexpr A a4 = false ? B{} : B{};
> +constexpr A a5 = A{};
> +constexpr A a6 = B{};
> +constexpr A a7 = false ? B{} : (true ? A{} : A{});
> +constexpr A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
> +constexpr A a9 = (A{});
> +constexpr A a10 = (true, A{});
> +constexpr A a11 = bar (A{});
> +//static_assert(a10.p == &a10, ""); // bug, 105619
> +constexpr A a12 = baz ();
> +//static_assert(a11.p == &a11, ""); // bug, 105619
> +constexpr A a13 = (A{}, A{});
> diff --git a/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C
> index dc6492c1b0b..5e66bdd2370 100644
> --- a/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C
> +++ b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr16.C
> @@ -40,9 +40,8 @@ struct E {
>     A d = true ? (false ? A{} : A{}) : (false ? A{} : A{});
>   };
>   
> -// FIXME: When fixing this, also fix nsdmi-aggr17.C.
> -constexpr E e;	    // { dg-bogus "" "PR105550" { xfail *-*-* } }
> -SA (e.a.p == &e.a); // { dg-bogus "" "PR105550" { xfail *-*-* } }
> +constexpr E e;
> +SA (e.a.p == &e.a);
>   
>   E e1 = { };
>   
> diff --git a/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C
> index fc27a2cdac7..ca9637b37eb 100644
> --- a/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C
> +++ b/gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr17.C
> @@ -56,9 +56,8 @@ struct F {
>     A a = true ? A{x} : A{x};
>   };
>   
> -// FIXME: Doesn't work due to PR105550.
> -//constexpr F f;
> -//SA (f.a.p == &f.a);
> +constexpr F f;
> +SA (f.a.p == &f.a);
>   SA (e.x == 42);
>   F f2 = { };
>   F f3 = { 42 };
> 
> base-commit: 113844d68e94f4e9c0e946db351ba7d3d4a1335a