c++: Drop TREE_READONLY on vars (possibly) initialized by tls wrapper [PR109164]

Hi!

The following two testcases are miscompiled, because we keep TREE_READONLY
on the vars even when they are (possibly) dynamically initialized by a TLS
wrapper function.  Normally cp_finish_decl drops TREE_READONLY from vars
which need dynamic initialization, but for TLS we do this kind of
initialization upon every access to those variables.  Keeping them
TREE_READONLY means e.g. PRE can hoist loads from those before loops
which contain the TLS wrapper calls, so we can access the TLS variables
before they are initialized.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2023-03-17  Jakub Jelinek  <jakub@redhat.com>

	PR c++/109164
	* decl2.cc (get_tls_wrapper_fn): Clear TREE_READONLY on variables for
	which a TLS wrapper is added.

	* g++.dg/tls/thread_local13.C: New test.
	* g++.dg/tls/thread_local13-aux.cc: New file.
	* g++.dg/tls/thread_local14.C: New test.
	* g++.dg/tls/thread_local14-aux.cc: New file.


	Jakub

On 3/17/23 13:51, Jakub Jelinek wrote:
> Hi!
> 
> The following two testcases are miscompiled, because we keep TREE_READONLY
> on the vars even when they are (possibly) dynamically initialized by a TLS
> wrapper function.  Normally cp_finish_decl drops TREE_READONLY from vars
> which need dynamic initialization, but for TLS we do this kind of
> initialization upon every access to those variables.

Why not handle this case in cp_finish_decl, too?  That is, add 
DECL_THREAD_LOCAL_P to the TREE_STATIC check in

>       if (var_definition_p
>           /* With -fmerge-all-constants, gimplify_init_constructor                                                                  
>              might add TREE_STATIC to aggregate variables.  */
>           && (TREE_STATIC (decl)
>               || (flag_merge_constants >= 2
>                   && AGGREGATE_TYPE_P (type))))
>         {
>           /* If a TREE_READONLY variable needs initialization                                                                       
>              at runtime, it is no longer readonly and we need to                                                                    
>              avoid MEM_READONLY_P being set on RTL created for it.  */

?

> Keeping them
> TREE_READONLY means e.g. PRE can hoist loads from those before loops
> which contain the TLS wrapper calls, so we can access the TLS variables
> before they are initialized.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> 2023-03-17  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/109164
> 	* decl2.cc (get_tls_wrapper_fn): Clear TREE_READONLY on variables for
> 	which a TLS wrapper is added.
> 
> 	* g++.dg/tls/thread_local13.C: New test.
> 	* g++.dg/tls/thread_local13-aux.cc: New file.
> 	* g++.dg/tls/thread_local14.C: New test.
> 	* g++.dg/tls/thread_local14-aux.cc: New file.
> 
> --- gcc/cp/decl2.cc.jj	2023-03-07 21:20:31.800491531 +0100
> +++ gcc/cp/decl2.cc	2023-03-17 12:20:11.960678291 +0100
> @@ -3773,6 +3773,12 @@ get_tls_wrapper_fn (tree var)
>         DECL_BEFRIENDING_CLASSES (fn) = var;
>   
>         set_global_binding (fn);
> +
> +      /* The variable now needs dynamic initialization by the wrapper
> +	 function, we don't want to hoist accesses to it before the
> +	 wrapper.  */
> +      if (TREE_READONLY (var))
> +	TREE_READONLY (var) = 0;
>       }
>     return fn;
>   }
> --- gcc/testsuite/g++.dg/tls/thread_local13.C.jj	2023-03-17 12:28:24.692427351 +0100
> +++ gcc/testsuite/g++.dg/tls/thread_local13.C	2023-03-17 12:30:34.505519746 +0100
> @@ -0,0 +1,21 @@
> +// PR c++/109164
> +// { dg-do run { target c++11 } }
> +// { dg-options "-O2" }
> +// { dg-add-options tls }
> +// { dg-require-effective-target tls_runtime }
> +// { dg-additional-sources "thread_local13-aux.cc" }
> +
> +struct S { virtual void foo (); int s; };
> +extern thread_local S &t;
> +bool bar ();
> +
> +bool
> +baz ()
> +{
> +  while (1)
> +    {
> +      t.foo ();
> +      if (!bar ())
> +        return false;
> +    }
> +}
> --- gcc/testsuite/g++.dg/tls/thread_local13-aux.cc.jj	2023-03-17 12:28:28.721368058 +0100
> +++ gcc/testsuite/g++.dg/tls/thread_local13-aux.cc	2023-03-17 12:37:53.952070861 +0100
> @@ -0,0 +1,35 @@
> +// PR c++/109164
> +
> +struct S { virtual void foo (); int s; };
> +extern bool baz ();
> +
> +void
> +S::foo ()
> +{
> +  if (s != 42)
> +    __builtin_abort ();
> +}
> +
> +S s;
> +
> +S &
> +qux ()
> +{
> +  s.s = 42;
> +  return s;
> +}
> +
> +thread_local S &t = qux ();
> +
> +bool
> +bar ()
> +{
> +  return false;
> +}
> +
> +int
> +main ()
> +{
> +  if (baz ())
> +    __builtin_abort ();
> +}
> --- gcc/testsuite/g++.dg/tls/thread_local14.C.jj	2023-03-17 12:35:48.951905245 +0100
> +++ gcc/testsuite/g++.dg/tls/thread_local14.C	2023-03-17 12:49:03.456249628 +0100
> @@ -0,0 +1,19 @@
> +// PR c++/109164
> +// { dg-do run { target c++11 } }
> +// { dg-options "-O2" }
> +// { dg-add-options tls }
> +// { dg-require-effective-target tls_runtime }
> +// { dg-additional-sources "thread_local14-aux.cc" }
> +
> +extern thread_local const int t;
> +bool bar (int);
> +
> +bool
> +baz ()
> +{
> +  while (1)
> +    {
> +      if (!bar (t))
> +        return false;
> +    }
> +}
> --- gcc/testsuite/g++.dg/tls/thread_local14-aux.cc.jj	2023-03-17 12:36:58.724881322 +0100
> +++ gcc/testsuite/g++.dg/tls/thread_local14-aux.cc	2023-03-17 12:48:53.914389421 +0100
> @@ -0,0 +1,26 @@
> +// PR c++/109164
> +
> +extern bool baz ();
> +
> +int
> +qux ()
> +{
> +  return 42;
> +}
> +
> +extern thread_local const int t = qux ();
> +
> +bool
> +bar (int x)
> +{
> +  if (x != 42)
> +    __builtin_abort ();
> +  return false;
> +}
> +
> +int
> +main ()
> +{
> +  if (baz ())
> +    __builtin_abort ();
> +}
> 
> 	Jakub
>

On Sat, Mar 18, 2023 at 08:39:45AM -0400, Jason Merrill wrote:
> On 3/17/23 13:51, Jakub Jelinek wrote:
> > The following two testcases are miscompiled, because we keep TREE_READONLY
> > on the vars even when they are (possibly) dynamically initialized by a TLS
> > wrapper function.  Normally cp_finish_decl drops TREE_READONLY from vars
> > which need dynamic initialization, but for TLS we do this kind of
> > initialization upon every access to those variables.
> 
> Why not handle this case in cp_finish_decl, too?  That is, add
> DECL_THREAD_LOCAL_P to the TREE_STATIC check in

The patch is mostly about DECL_EXTERNAL cases, the others are supposedly
handled by the var_definition_p code there (or at least I assumed;
testcases certainly test only DECL_EXTERNAL).
I guess it could be done in cp_finish_decl, maybe better next to the
  /* A reference will be modified here, as it is initialized.  */
  if (! DECL_EXTERNAL (decl)
      && TREE_READONLY (decl)
      && TYPE_REF_P (type))
    {
      was_readonly = 1;
      TREE_READONLY (decl) = 0;
    }
spot, but we'd need to export the decl2.cc helpers for it,
because not all DECL_THREAD_LOCAL_P vars need to be treated that way.
  if (VAR_P (decl)
      && CP_DECL_THREAD_LOCAL_P (decl)
      && var_needs_tls_wrapper (decl)
      && (!DECL_EXTERNAL (decl) || flag_extern_tls_init))
    TREE_READONLY (decl) = 0;
where var_needs_tls_wrapper would need to be exported from decl2.cc.
Though, var_needs_tls_wrapper -> var_defined_without_dynamic_init
needs
DECL_NONTRIVIALLY_INITIALIZED_P/DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P,
so perhaps that is accurate only closer to the end of cp_finish_decl?

	Jakub