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* [gcc r13-7509] libstdc++: Disable cacheline alignment for DJGPP [PR109741]
@ 2023-06-29 23:01 Jonathan Wakely
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From: Jonathan Wakely @ 2023-06-29 23:01 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
https://gcc.gnu.org/g:dbd4acd72274f3b3d542ebf68f9962eda8f8b769
commit r13-7509-gdbd4acd72274f3b3d542ebf68f9962eda8f8b769
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Tue May 16 15:09:20 2023 +0100
libstdc++: Disable cacheline alignment for DJGPP [PR109741]
DJGPP (and maybe other targets) uses MAX_OFILE_ALIGNMENT=16 which means
that globals (and static objects) can't have alignment greater than 16.
This causes an error for the locks defined in src/c++11/shared_ptr.cc
because we try to align them to the cacheline size, to avoid false
sharing.
Add a configure check for the increased alignment, and live with false
sharing where we can't increase the alignment.
libstdc++-v3/ChangeLog:
PR libstdc++/109741
* acinclude.m4 (GLIBCXX_CHECK_ALIGNAS_CACHELINE): Define.
* config.h.in: Regenerate.
* configure: Regenerate.
* configure.ac: Use GLIBCXX_CHECK_ALIGNAS_CACHELINE.
* src/c++11/shared_ptr.cc (__gnu_internal::get_mutex): Do not
align lock table if not supported. use __GCC_DESTRUCTIVE_SIZE
instead of hardcoded 64.
(cherry picked from commit 94a311abf783de754f0f1b2d4c1f00a9788e795b)
Diff:
---
libstdc++-v3/acinclude.m4 | 25 ++++++++++++++++++
libstdc++-v3/config.h.in | 4 +++
libstdc++-v3/configure | 49 ++++++++++++++++++++++++++++++++++++
libstdc++-v3/configure.ac | 3 +++
libstdc++-v3/src/c++11/shared_ptr.cc | 8 ++++--
5 files changed, 87 insertions(+), 2 deletions(-)
diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 42a8e7a775e..af978e3f50c 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -5246,6 +5246,31 @@ AC_DEFUN([GLIBCXX_ZONEINFO_DIR], [
fi
])
+dnl
+dnl Check whether lock tables can be aligned to avoid false sharing.
+dnl
+dnl Defines:
+dnl _GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE if objects with static storage
+dnl duration can be aligned to std::hardware_destructive_interference_size.
+dnl
+AC_DEFUN([GLIBCXX_CHECK_ALIGNAS_CACHELINE], [
+ AC_LANG_SAVE
+ AC_LANG_CPLUSPLUS
+
+ AC_MSG_CHECKING([whether static objects can be aligned to the cacheline size])
+ AC_TRY_COMPILE(, [struct alignas(__GCC_DESTRUCTIVE_SIZE) Aligned { };
+ alignas(Aligned) static char buf[sizeof(Aligned) * 16];
+ ], [ac_alignas_cacheline=yes], [ac_alignas_cacheline=no])
+ if test "$ac_alignas_cacheline" = yes; then
+ AC_DEFINE_UNQUOTED(_GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE, 1,
+ [Define if global objects can be aligned to
+ std::hardware_destructive_interference_size.])
+ fi
+ AC_MSG_RESULT($ac_alignas_cacheline)
+
+ AC_LANG_RESTORE
+])
+
# Macros from the top-level gcc directory.
m4_include([../config/gc++filt.m4])
m4_include([../config/tls.m4])
diff --git a/libstdc++-v3/config.h.in b/libstdc++-v3/config.h.in
index a336daf2303..86c8eeef8ac 100644
--- a/libstdc++-v3/config.h.in
+++ b/libstdc++-v3/config.h.in
@@ -819,6 +819,10 @@
/* Define if the compiler supports C++11 atomics. */
#undef _GLIBCXX_ATOMIC_BUILTINS
+/* Define if global objects can be aligned to
+ std::hardware_destructive_interference_size. */
+#undef _GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE
+
/* Define to use concept checking code from the boost libraries. */
#undef _GLIBCXX_CONCEPT_CHECKS
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index d4286b67a73..776c6b7ba58 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -71611,6 +71611,55 @@ _ACEOF
fi
+# For src/c++11/shared_ptr.cc alignment.
+
+
+ ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+
+ { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether static objects can be aligned to the cacheline size" >&5
+$as_echo_n "checking whether static objects can be aligned to the cacheline size... " >&6; }
+ cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h. */
+
+int
+main ()
+{
+struct alignas(__GCC_DESTRUCTIVE_SIZE) Aligned { };
+ alignas(Aligned) static char buf[sizeof(Aligned) * 16];
+
+ ;
+ return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+ ac_alignas_cacheline=yes
+else
+ ac_alignas_cacheline=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ if test "$ac_alignas_cacheline" = yes; then
+
+cat >>confdefs.h <<_ACEOF
+#define _GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE 1
+_ACEOF
+
+ fi
+ { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_alignas_cacheline" >&5
+$as_echo "$ac_alignas_cacheline" >&6; }
+
+ ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
+
+
# Define documentation rules conditionally.
# See if makeinfo has been installed and is modern enough
diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
index 0dd550a4b4b..df01f58bd83 100644
--- a/libstdc++-v3/configure.ac
+++ b/libstdc++-v3/configure.ac
@@ -538,6 +538,9 @@ GLIBCXX_EMERGENCY_EH_ALLOC
# For src/c++20/tzdb.cc defaults.
GLIBCXX_ZONEINFO_DIR
+# For src/c++11/shared_ptr.cc alignment.
+GLIBCXX_CHECK_ALIGNAS_CACHELINE
+
# Define documentation rules conditionally.
# See if makeinfo has been installed and is modern enough
diff --git a/libstdc++-v3/src/c++11/shared_ptr.cc b/libstdc++-v3/src/c++11/shared_ptr.cc
index 74e879e5828..ae47ca03301 100644
--- a/libstdc++-v3/src/c++11/shared_ptr.cc
+++ b/libstdc++-v3/src/c++11/shared_ptr.cc
@@ -34,8 +34,12 @@ namespace __gnu_internal _GLIBCXX_VISIBILITY(hidden)
__gnu_cxx::__mutex&
get_mutex(unsigned char i)
{
- // increase alignment to put each lock on a separate cache line
- struct alignas(64) M : __gnu_cxx::__mutex { };
+#ifdef _GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE
+ // Increase alignment to put each lock on a separate cache line.
+ struct alignas(__GCC_DESTRUCTIVE_SIZE) M : __gnu_cxx::__mutex { };
+#else
+ using M = __gnu_cxx::__mutex;
+#endif
// Use a static buffer, so that the mutexes are not destructed
// before potential users (or at all)
static __attribute__ ((aligned(__alignof__(M))))
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