[gcc r11-3572] libstdc++: Fix test_and_acquire / set_and_release for EABI guard variables

[gcc r11-3572] libstdc++: Fix test_and_acquire / set_and_release for EABI guard variables

[Jonathan Wakely]

https://gcc.gnu.org/g:d1ac0f0dfba9d04bcf38132b3ae45db572e7f1ea

commit r11-3572-gd1ac0f0dfba9d04bcf38132b3ae45db572e7f1ea
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 30 21:14:43 2020 +0100

    libstdc++: Fix test_and_acquire / set_and_release for EABI guard variables
    
    The default definitions of _GLIBCXX_GUARD_TEST_AND_ACQUIRE and
    _GLIBCXX_GUARD_SET_AND_RELEASE in libsupc++/guard.cc only work for the
    generic (IA64) ABI, because they test/set the first byte of the guard
    variable. For EABI we need to use the least significant bit, which means
    using the first byte is wrong for big endian targets.
    
    This has been wrong since r224411, but previously it only caused poor
    performance. The _GLIBCXX_GUARD_TEST_AND_ACQUIRE at the very start of
    __cxa_guard_acquire would always return false even if the initialization
    was actually complete. Before my r11-3484 change the atomic compare
    exchange would have loaded the correct value, and then returned 0 as
    expected when the initialization is complete. After my change, in the
    single-threaded case there is no redundant check for init being
    complete, because I foolishly assumed that the check at the start of the
    function actually worked.
    
    The default definition of _GLIBCXX_GUARD_SET_AND_RELEASE is also wrong
    for big endian EABI, but appears to work because it sets the wrong bit
    but then the buggy TEST_AND_ACQUIRE tests that wrong bit as well. Also,
    the buggy SET_AND_RELEASE macro is only used for targets with threads
    enabled but no futex syscalls.
    
    This should fix the regressions introduced by my patch, by defining
    custom versions of the TEST_AND_ACQUIRE and SET_AND_RELEASE macros that
    are correct for EABI.
    
    libstdc++-v3/ChangeLog:
    
            * config/cpu/arm/cxxabi_tweaks.h (_GLIBCXX_GUARD_TEST_AND_ACQUIRE):
            (_GLIBCXX_GUARD_SET_AND_RELEASE): Define for EABI.

Diff:
---
 libstdc++-v3/config/cpu/arm/cxxabi_tweaks.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/config/cpu/arm/cxxabi_tweaks.h b/libstdc++-v3/config/cpu/arm/cxxabi_tweaks.h
index 4eb43c8373c..4fb34869f8a 100644
--- a/libstdc++-v3/config/cpu/arm/cxxabi_tweaks.h
+++ b/libstdc++-v3/config/cpu/arm/cxxabi_tweaks.h
@@ -39,7 +39,7 @@ namespace __cxxabiv1
 
 #ifdef __ARM_EABI__
   // The ARM EABI uses the least significant bit of a 32-bit
-  // guard variable.  */
+  // guard variable.
 #define _GLIBCXX_GUARD_TEST(x) ((*(x) & 1) != 0)
 #define _GLIBCXX_GUARD_SET(x) *(x) = 1
 #define _GLIBCXX_GUARD_BIT 1
@@ -47,6 +47,11 @@ namespace __cxxabiv1
 #define _GLIBCXX_GUARD_WAITING_BIT __guard_test_bit (2, 1)
   typedef int __guard;
 
+#define _GLIBCXX_GUARD_TEST_AND_ACQUIRE(x) \
+  _GLIBCXX_GUARD_TEST(__atomic_load_n(x, __ATOMIC_ACQUIRE))
+#define _GLIBCXX_GUARD_SET_AND_RELEASE(x) \
+  __atomic_store_n(x, 1, __ATOMIC_RELEASE)
+
   // We also want the element size in array cookies.
 #define _GLIBCXX_ELTSIZE_IN_COOKIE 1