c++: GC collects live data when synthesizing operator== [PR99831]

Here we crash in reshape_init because we're accessing ggc_freed
& poisoned data: since r277865 in defaulted_late_check we call
synthesize_method here:

  if (kind == sfk_comparison)
    {
      /* If the function was declared constexpr, check that the definition
         qualifies.  Otherwise we can define the function lazily.  */
      if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
        synthesize_method (fn);
      return;
    }

which in this test triggers when we're processing the string<"a">{} in
the static_assert.  First, we create a CONSTRUCTOR for the "{}" in
cp_parser_functional_cast, then we call finish_compound_literal which
calls complete_type and that results in garbage collection, which then
frees the CONSTRUCTOR {} we created when parsing the braced-list in
string<"a">{} -- at this point, it's not referenced by anything.
(That's not the case for 'type' in finish_compound_literal: the symbol
table contains a node for operator==, so ggc_mark_roots goes and marks
the fn decl, its type, its arguments etc., as used, so we don't collect
it.)

We could just bump function_depth around the new call to
synthesize_method.  Alternatively, as in this patch, we can create a new
GC root to hold the expression list.  Since I'm adding a new global, I
felt compelled to get rid of one, and I actually did find an unused one!
So, 86 it.  But I'm also happy to just go with bumping function_depth.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/10?

gcc/cp/ChangeLog:

	PR c++/99831
	* parser.c (cp_parser_functional_cast): Make the expression list
	a GC root.
	* pt.c: Remove the saved_trees global.

gcc/testsuite/ChangeLog:

	PR c++/99831
	* g++.dg/other/gc6.C: New test.
---
 gcc/cp/parser.c                  | 20 ++++++++++++--------
 gcc/cp/pt.c                      |  1 -
 gcc/testsuite/g++.dg/other/gc6.C | 16 ++++++++++++++++
 3 files changed, 28 insertions(+), 9 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/other/gc6.C


base-commit: 5f00df5925082c7b66da91270f2ed29bf4818c93

On 4/1/21 2:15 PM, Marek Polacek wrote:
> Here we crash in reshape_init because we're accessing ggc_freed
> & poisoned data: since r277865 in defaulted_late_check we call
> synthesize_method here:
> 
>    if (kind == sfk_comparison)
>      {
>        /* If the function was declared constexpr, check that the definition
>           qualifies.  Otherwise we can define the function lazily.  */
>        if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
>          synthesize_method (fn);
>        return;
>      }
> 
> which in this test triggers when we're processing the string<"a">{} in
> the static_assert.  First, we create a CONSTRUCTOR for the "{}" in
> cp_parser_functional_cast, then we call finish_compound_literal which
> calls complete_type and that results in garbage collection, which then
> frees the CONSTRUCTOR {} we created when parsing the braced-list in
> string<"a">{} -- at this point, it's not referenced by anything.
> (That's not the case for 'type' in finish_compound_literal: the symbol
> table contains a node for operator==, so ggc_mark_roots goes and marks
> the fn decl, its type, its arguments etc., as used, so we don't collect
> it.)
> 
> We could just bump function_depth around the new call to
> synthesize_method.  Alternatively, as in this patch, we can create a new
> GC root to hold the expression list.  Since I'm adding a new global, I
> felt compelled to get rid of one, and I actually did find an unused one!
> So, 86 it.  But I'm also happy to just go with bumping function_depth.

I think bumping function_depth is the right solution; this patch only 
deals with one source of unreachable expressions that could be collected 
prematurely if complete_type results in GC running.

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/10?
> 
> gcc/cp/ChangeLog:
> 
> 	PR c++/99831
> 	* parser.c (cp_parser_functional_cast): Make the expression list
> 	a GC root.
> 	* pt.c: Remove the saved_trees global.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/99831
> 	* g++.dg/other/gc6.C: New test.
> ---
>   gcc/cp/parser.c                  | 20 ++++++++++++--------
>   gcc/cp/pt.c                      |  1 -
>   gcc/testsuite/g++.dg/other/gc6.C | 16 ++++++++++++++++
>   3 files changed, 28 insertions(+), 9 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/other/gc6.C
> 
> diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> index d0477c42afe..28d7a8be258 100644
> --- a/gcc/cp/parser.c
> +++ b/gcc/cp/parser.c
> @@ -30569,13 +30569,17 @@ cp_parser_simple_cast_expression (cp_parser *parser)
>   }
>   
>   /* Parse a functional cast to TYPE.  Returns an expression
> -   representing the cast.  */
> +   representing the cast.  Preserve the expression list across
> +   a garbage collect (by making it a GC root), which can occur
> +   in finish_compound_literal -> complete_type, when we synthesize
> +   a constexpr comparison function.  */
> +
> +static GTY(()) tree fncast_expression_list;
>   
>   static cp_expr
>   cp_parser_functional_cast (cp_parser* parser, tree type)
>   {
>     vec<tree, va_gc> *vec;
> -  tree expression_list;
>     cp_expr cast;
>     bool nonconst_p;
>   
> @@ -30588,12 +30592,12 @@ cp_parser_functional_cast (cp_parser* parser, tree type)
>       {
>         cp_lexer_set_source_position (parser->lexer);
>         maybe_warn_cpp0x (CPP0X_INITIALIZER_LISTS);
> -      expression_list = cp_parser_braced_list (parser, &nonconst_p);
> -      CONSTRUCTOR_IS_DIRECT_INIT (expression_list) = 1;
> +      fncast_expression_list = cp_parser_braced_list (parser, &nonconst_p);
> +      CONSTRUCTOR_IS_DIRECT_INIT (fncast_expression_list) = 1;
>         if (TREE_CODE (type) == TYPE_DECL)
>   	type = TREE_TYPE (type);
>   
> -      cast = finish_compound_literal (type, expression_list,
> +      cast = finish_compound_literal (type, fncast_expression_list,
>   				      tf_warning_or_error, fcl_functional);
>         /* Create a location of the form:
>   	    type_name{i, f}
> @@ -30612,10 +30616,10 @@ cp_parser_functional_cast (cp_parser* parser, tree type)
>   						 /*allow_expansion_p=*/true,
>   						 /*non_constant_p=*/NULL);
>     if (vec == NULL)
> -    expression_list = error_mark_node;
> +    fncast_expression_list = error_mark_node;
>     else
>       {
> -      expression_list = build_tree_list_vec (vec);
> +      fncast_expression_list = build_tree_list_vec (vec);
>         release_tree_vector (vec);
>       }
>   
> @@ -30626,7 +30630,7 @@ cp_parser_functional_cast (cp_parser* parser, tree type)
>        finishing at the closing paren.  */
>     location_t combined_loc = make_location (start_loc, start_loc,
>   					   parser->lexer);
> -  cast = build_functional_cast (combined_loc, type, expression_list,
> +  cast = build_functional_cast (combined_loc, type, fncast_expression_list,
>                                   tf_warning_or_error);
>     
>     /* [expr.const]/1: In an integral constant expression "only type
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index dc6f2f37f9b..7956e83c1de 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -65,7 +65,6 @@ static GTY(()) struct pending_template *last_pending_template;
>   int processing_template_parmlist;
>   static int template_header_count;
>   
> -static GTY(()) tree saved_trees;
>   static vec<int> inline_parm_levels;
>   
>   static GTY(()) struct tinst_level *current_tinst_level;
> diff --git a/gcc/testsuite/g++.dg/other/gc6.C b/gcc/testsuite/g++.dg/other/gc6.C
> new file mode 100644
> index 00000000000..ff45dd313d6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/other/gc6.C
> @@ -0,0 +1,16 @@
> +// PR c++/99831
> +// { dg-do compile { target c++20 } }
> +// { dg-options "--param ggc-min-heapsize=0 --param ggc-min-expand=0" }
> +
> +template <int N> struct S {
> +  constexpr S(const char (&str)[N]) : value{} { }
> +  char value[N];
> +};
> +template <S> struct string {
> +  constexpr bool operator==(const string &) const = default;
> +};
> +template <S L2> void operator+(string<L2>) {
> +  char value[1];
> +  S{value};
> +}
> +static_assert(string<"a">{} == string<"a">{});
> 
> base-commit: 5f00df5925082c7b66da91270f2ed29bf4818c93
>

On Thu, Apr 01, 2021 at 03:09:44PM -0400, Jason Merrill wrote:
> On 4/1/21 2:15 PM, Marek Polacek wrote:
> > Here we crash in reshape_init because we're accessing ggc_freed
> > & poisoned data: since r277865 in defaulted_late_check we call
> > synthesize_method here:
> > 
> >    if (kind == sfk_comparison)
> >      {
> >        /* If the function was declared constexpr, check that the definition
> >           qualifies.  Otherwise we can define the function lazily.  */
> >        if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
> >          synthesize_method (fn);
> >        return;
> >      }
> > 
> > which in this test triggers when we're processing the string<"a">{} in
> > the static_assert.  First, we create a CONSTRUCTOR for the "{}" in
> > cp_parser_functional_cast, then we call finish_compound_literal which
> > calls complete_type and that results in garbage collection, which then
> > frees the CONSTRUCTOR {} we created when parsing the braced-list in
> > string<"a">{} -- at this point, it's not referenced by anything.
> > (That's not the case for 'type' in finish_compound_literal: the symbol
> > table contains a node for operator==, so ggc_mark_roots goes and marks
> > the fn decl, its type, its arguments etc., as used, so we don't collect
> > it.)
> > 
> > We could just bump function_depth around the new call to
> > synthesize_method.  Alternatively, as in this patch, we can create a new
> > GC root to hold the expression list.  Since I'm adding a new global, I
> > felt compelled to get rid of one, and I actually did find an unused one!
> > So, 86 it.  But I'm also happy to just go with bumping function_depth.
> 
> I think bumping function_depth is the right solution; this patch only deals
> with one source of unreachable expressions that could be collected
> prematurely if complete_type results in GC running.

Fair enough, let's go back to my original patch.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/10?

And now I should get back to
<https://gcc.gnu.org/pipermail/gcc-patches/2019-August/527699.html>
where we actually want a new GC root...

-- >8 --
Here we crash in reshape_init because we're accessing ggc_freed
& poisoned data: since r277865 in defaulted_late_check we call
synthesize_method here:

  if (kind == sfk_comparison)
    {
      /* If the function was declared constexpr, check that the definition
         qualifies.  Otherwise we can define the function lazily.  */
      if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
        synthesize_method (fn);
      return;
    }

which in this test triggers when we're processing the string<"a">{} in
the static_assert.  First, we create a CONSTRUCTOR for the "{}" in
cp_parser_functional_cast, then we call finish_compound_literal which
calls complete_type and that results in garbage collection, which then
frees the CONSTRUCTOR {} we created when parsing the braced-list in
string<"a">{} -- at this point, it's not referenced by anything.
(That's not the case for 'type' in finish_compound_literal: the symbol
table contains a node for operator==, so ggc_mark_roots goes and marks
the fn decl, its type, its arguments etc., as used, so we don't collect
it.)

We could just bump function_depth around the new call to synthesize_method
to prevent GC.

gcc/cp/ChangeLog:

	PR c++/99831
	* method.c (defaulted_late_check): ++ and -- function_depth around
	the call to synthesize_method.
	* pt.c: Remove the saved_trees global.

gcc/testsuite/ChangeLog:

	PR c++/99831
	* g++.dg/other/gc6.C: New test.
---
 gcc/cp/method.c                  |  7 ++++++-
 gcc/cp/pt.c                      |  1 -
 gcc/testsuite/g++.dg/other/gc6.C | 16 ++++++++++++++++
 3 files changed, 22 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/other/gc6.C

diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index 8ae7496f023..0f416bec35b 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -3131,7 +3131,12 @@ defaulted_late_check (tree fn)
       /* If the function was declared constexpr, check that the definition
 	 qualifies.  Otherwise we can define the function lazily.  */
       if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
-	synthesize_method (fn);
+	{
+	  /* Prevent GC.  */
+	  function_depth++;
+	  synthesize_method (fn);
+	  function_depth--;
+	}
       return;
     }
 
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index dc6f2f37f9b..7956e83c1de 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -65,7 +65,6 @@ static GTY(()) struct pending_template *last_pending_template;
 int processing_template_parmlist;
 static int template_header_count;
 
-static GTY(()) tree saved_trees;
 static vec<int> inline_parm_levels;
 
 static GTY(()) struct tinst_level *current_tinst_level;
diff --git a/gcc/testsuite/g++.dg/other/gc6.C b/gcc/testsuite/g++.dg/other/gc6.C
new file mode 100644
index 00000000000..ff45dd313d6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/other/gc6.C
@@ -0,0 +1,16 @@
+// PR c++/99831
+// { dg-do compile { target c++20 } }
+// { dg-options "--param ggc-min-heapsize=0 --param ggc-min-expand=0" }
+
+template <int N> struct S {
+  constexpr S(const char (&str)[N]) : value{} { }
+  char value[N];
+};
+template <S> struct string {
+  constexpr bool operator==(const string &) const = default;
+};
+template <S L2> void operator+(string<L2>) {
+  char value[1];
+  S{value};
+}
+static_assert(string<"a">{} == string<"a">{});

base-commit: 0cf4813202f19768e31666f6aa82bde4dce4065a

On 4/1/21 5:12 PM, Marek Polacek wrote:
> On Thu, Apr 01, 2021 at 03:09:44PM -0400, Jason Merrill wrote:
>> On 4/1/21 2:15 PM, Marek Polacek wrote:
>>> Here we crash in reshape_init because we're accessing ggc_freed
>>> & poisoned data: since r277865 in defaulted_late_check we call
>>> synthesize_method here:
>>>
>>>     if (kind == sfk_comparison)
>>>       {
>>>         /* If the function was declared constexpr, check that the definition
>>>            qualifies.  Otherwise we can define the function lazily.  */
>>>         if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
>>>           synthesize_method (fn);
>>>         return;
>>>       }
>>>
>>> which in this test triggers when we're processing the string<"a">{} in
>>> the static_assert.  First, we create a CONSTRUCTOR for the "{}" in
>>> cp_parser_functional_cast, then we call finish_compound_literal which
>>> calls complete_type and that results in garbage collection, which then
>>> frees the CONSTRUCTOR {} we created when parsing the braced-list in
>>> string<"a">{} -- at this point, it's not referenced by anything.
>>> (That's not the case for 'type' in finish_compound_literal: the symbol
>>> table contains a node for operator==, so ggc_mark_roots goes and marks
>>> the fn decl, its type, its arguments etc., as used, so we don't collect
>>> it.)
>>>
>>> We could just bump function_depth around the new call to
>>> synthesize_method.  Alternatively, as in this patch, we can create a new
>>> GC root to hold the expression list.  Since I'm adding a new global, I
>>> felt compelled to get rid of one, and I actually did find an unused one!
>>> So, 86 it.  But I'm also happy to just go with bumping function_depth.
>>
>> I think bumping function_depth is the right solution; this patch only deals
>> with one source of unreachable expressions that could be collected
>> prematurely if complete_type results in GC running.
> 
> Fair enough, let's go back to my original patch.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/10?

OK.

> And now I should get back to
> <https://gcc.gnu.org/pipermail/gcc-patches/2019-August/527699.html>
> where we actually want a new GC root...
> 
> -- >8 --
> Here we crash in reshape_init because we're accessing ggc_freed
> & poisoned data: since r277865 in defaulted_late_check we call
> synthesize_method here:
> 
>    if (kind == sfk_comparison)
>      {
>        /* If the function was declared constexpr, check that the definition
>           qualifies.  Otherwise we can define the function lazily.  */
>        if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
>          synthesize_method (fn);
>        return;
>      }
> 
> which in this test triggers when we're processing the string<"a">{} in
> the static_assert.  First, we create a CONSTRUCTOR for the "{}" in
> cp_parser_functional_cast, then we call finish_compound_literal which
> calls complete_type and that results in garbage collection, which then
> frees the CONSTRUCTOR {} we created when parsing the braced-list in
> string<"a">{} -- at this point, it's not referenced by anything.
> (That's not the case for 'type' in finish_compound_literal: the symbol
> table contains a node for operator==, so ggc_mark_roots goes and marks
> the fn decl, its type, its arguments etc., as used, so we don't collect
> it.)
> 
> We could just bump function_depth around the new call to synthesize_method
> to prevent GC.
> 
> gcc/cp/ChangeLog:
> 
> 	PR c++/99831
> 	* method.c (defaulted_late_check): ++ and -- function_depth around
> 	the call to synthesize_method.
> 	* pt.c: Remove the saved_trees global.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/99831
> 	* g++.dg/other/gc6.C: New test.
> ---
>   gcc/cp/method.c                  |  7 ++++++-
>   gcc/cp/pt.c                      |  1 -
>   gcc/testsuite/g++.dg/other/gc6.C | 16 ++++++++++++++++
>   3 files changed, 22 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/other/gc6.C
> 
> diff --git a/gcc/cp/method.c b/gcc/cp/method.c
> index 8ae7496f023..0f416bec35b 100644
> --- a/gcc/cp/method.c
> +++ b/gcc/cp/method.c
> @@ -3131,7 +3131,12 @@ defaulted_late_check (tree fn)
>         /* If the function was declared constexpr, check that the definition
>   	 qualifies.  Otherwise we can define the function lazily.  */
>         if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
> -	synthesize_method (fn);
> +	{
> +	  /* Prevent GC.  */
> +	  function_depth++;
> +	  synthesize_method (fn);
> +	  function_depth--;
> +	}
>         return;
>       }
>   
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index dc6f2f37f9b..7956e83c1de 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -65,7 +65,6 @@ static GTY(()) struct pending_template *last_pending_template;
>   int processing_template_parmlist;
>   static int template_header_count;
>   
> -static GTY(()) tree saved_trees;
>   static vec<int> inline_parm_levels;
>   
>   static GTY(()) struct tinst_level *current_tinst_level;
> diff --git a/gcc/testsuite/g++.dg/other/gc6.C b/gcc/testsuite/g++.dg/other/gc6.C
> new file mode 100644
> index 00000000000..ff45dd313d6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/other/gc6.C
> @@ -0,0 +1,16 @@
> +// PR c++/99831
> +// { dg-do compile { target c++20 } }
> +// { dg-options "--param ggc-min-heapsize=0 --param ggc-min-expand=0" }
> +
> +template <int N> struct S {
> +  constexpr S(const char (&str)[N]) : value{} { }
> +  char value[N];
> +};
> +template <S> struct string {
> +  constexpr bool operator==(const string &) const = default;
> +};
> +template <S L2> void operator+(string<L2>) {
> +  char value[1];
> +  S{value};
> +}
> +static_assert(string<"a">{} == string<"a">{});
> 
> base-commit: 0cf4813202f19768e31666f6aa82bde4dce4065a
>