[gcc r9-9432] c++: Ensure correct destruction order of local statics [PR99613]

[gcc r9-9432] c++: Ensure correct destruction order of local statics [PR99613]

[Jakub Jelinek]

https://gcc.gnu.org/g:cb6efb9d909323e41fa0a8abbe805eb32d1659ea

commit r9-9432-gcb6efb9d909323e41fa0a8abbe805eb32d1659ea
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Mar 16 21:17:44 2021 +0100

    c++: Ensure correct destruction order of local statics [PR99613]
    
    As mentioned in the PR, if end of two constructions of local statics
    is strongly ordered, their destructors should be run in the reverse order.
    As we run __cxa_guard_release before calling __cxa_atexit, it is possible
    that we have two threads that access two local statics in the same order
    for the first time, one thread wins the __cxa_guard_acquire on the first
    one but is rescheduled in between the __cxa_guard_release and __cxa_atexit
    calls, then the other thread is scheduled and wins __cxa_guard_acquire
    on the second one and calls __cxa_quard_release and __cxa_atexit and only
    afterwards the first thread calls its __cxa_atexit.  This means a variable
    whose completion of the constructor strongly happened after the completion
    of the other one will be destructed after the other variable is destructed.
    
    The following patch fixes that by swapping the __cxa_guard_release and
    __cxa_atexit calls.
    
    2021-03-16  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/99613
            * decl.c (expand_static_init): For thread guards, call __cxa_atexit
            before calling __cxa_guard_release rather than after it.  Formatting
            fixes.
    
    (cherry picked from commit 1703937a05b8b95bc29d2de292387dfd9eb7c9a3)

Diff:
---
 gcc/cp/decl.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 7350212b8e7..1b20d13e4b3 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -8448,17 +8448,25 @@ expand_static_init (tree decl, tree init)
 
 	  /* Do the initialization itself.  */
 	  init = add_stmt_to_compound (begin, init);
-	  init = add_stmt_to_compound
-	    (init, build2 (MODIFY_EXPR, void_type_node, flag, boolean_true_node));
-	  init = add_stmt_to_compound
-	    (init, build_call_n (release_fn, 1, guard_addr));
+	  init = add_stmt_to_compound (init,
+				       build2 (MODIFY_EXPR, void_type_node,
+					       flag, boolean_true_node));
+
+	  /* Use atexit to register a function for destroying this static
+	     variable.  Do this before calling __cxa_guard_release.  */
+	  init = add_stmt_to_compound (init, register_dtor_fn (decl));
+
+	  init = add_stmt_to_compound (init, build_call_n (release_fn, 1,
+							   guard_addr));
 	}
       else
-	init = add_stmt_to_compound (init, set_guard (guard));
+	{
+	  init = add_stmt_to_compound (init, set_guard (guard));
 
-      /* Use atexit to register a function for destroying this static
-	 variable.  */
-      init = add_stmt_to_compound (init, register_dtor_fn (decl));
+	  /* Use atexit to register a function for destroying this static
+	     variable.  */
+	  init = add_stmt_to_compound (init, register_dtor_fn (decl));
+	}
 
       finish_expr_stmt (init);