[PATCH] Revert __HAVE_64B_ATOMICS configure check
Adhemerval Zanella
adhemerval.zanella@linaro.org
Thu Nov 13 17:55:24 GMT 2025
The 53807741fb44edb8e7c094cb5e7d4ff4e92a6ec1 added a configure check for
64-bit atomic operations on some 32-bit ABIs where they were not
previously enabled. However, at least for s390 [1] and i686, it is not
fully safe to use 64-bit atomic primitives.
Assume 64-bit atomic support from __WORDSIZE, which maps to how glibc
defines it before the broken change (m68k still explicitly sets
__HAVE_64B_ATOMICS to 1, but since it does not have multicore support,
it should not matter).
Checked on x86_64-linux-gnu and i686-linux-gnu.
[1] https://sourceware.org/pipermail/libc-alpha/2025-November/172458.html
[2] https://sourceware.org/pipermail/libc-alpha/2025-November/172461.html
---
config.h.in | 3 ---
configure | 42 --------------------------------
configure.ac | 27 --------------------
include/atomic.h | 5 ----
sysdeps/alpha/atomic-machine.h | 7 +++++-
sysdeps/generic/atomic-machine.h | 9 +++++++
sysdeps/riscv/atomic-machine.h | 2 ++
sysdeps/sparc/atomic-machine.h | 6 +++--
sysdeps/x86/atomic-machine.h | 2 ++
9 files changed, 23 insertions(+), 80 deletions(-)
diff --git a/config.h.in b/config.h.in
index 4204dbf1236..a7cc17df8ed 100644
--- a/config.h.in
+++ b/config.h.in
@@ -222,9 +222,6 @@
/* An integer used to scale the timeout of test programs. */
#define TIMEOUTFACTOR 1
-/* Set to 1 if 64 bit atomics are supported. */
-#undef __HAVE_64B_ATOMICS 0
-
/*
*/
diff --git a/configure b/configure
index df51b0c1a37..d1e956cc3d3 100755
--- a/configure
+++ b/configure
@@ -7702,48 +7702,6 @@ if test "$libc_cv_gcc_builtin_memset" = yes ; then
fi
-{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for 64-bit atomic support" >&5
-printf %s "checking for 64-bit atomic support... " >&6; }
-if test ${libc_cv_gcc_has_64b_atomics+y}
-then :
- printf %s "(cached) " >&6
-else case e in #(
- e) cat > conftest.c <<\EOF
-typedef struct { long long t; } X;
-extern void has_64b_atomics(void);
-void f(void)
-{
- X x;
- /* Use address of structure with 64-bit type. This avoids incorrect
- implementations which return true even if long long is not 64-bit aligned.
- This works on GCC and LLVM - other cases have bugs and they disagree. */
- _Static_assert (__atomic_always_lock_free (sizeof (x), &x), "no_64b_atomics");
-}
-EOF
-if { ac_try='${CC-cc} -O2 -S conftest.c'
- { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
- (eval $ac_try) 2>&5
- ac_status=$?
- printf "%s\n" "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
- test $ac_status = 0; }; };
-then
- libc_cv_gcc_has_64b_atomics=yes
-else
- libc_cv_gcc_has_64b_atomics=no
-fi
-rm -f conftest* ;;
-esac
-fi
-{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_has_64b_atomics" >&5
-printf "%s\n" "$libc_cv_gcc_has_64b_atomics" >&6; }
-if test "$libc_cv_gcc_has_64b_atomics" = yes; then
- printf "%s\n" "#define __HAVE_64B_ATOMICS 1" >>confdefs.h
-
-else
- printf "%s\n" "#define __HAVE_64B_ATOMICS 0" >>confdefs.h
-
- fi
-
{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for redirection of built-in functions" >&5
printf %s "checking for redirection of built-in functions... " >&6; }
if test ${libc_cv_gcc_builtin_redirection+y}
diff --git a/configure.ac b/configure.ac
index dd0b7a4c7c4..35f69f99c1e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1493,33 +1493,6 @@ if test "$libc_cv_gcc_builtin_memset" = yes ; then
AC_DEFINE(HAVE_BUILTIN_MEMSET)
fi
-AC_CACHE_CHECK(for 64-bit atomic support, libc_cv_gcc_has_64b_atomics, [dnl
-cat > conftest.c <<\EOF
-typedef struct { long long t; } X;
-extern void has_64b_atomics(void);
-void f(void)
-{
- X x;
- /* Use address of structure with 64-bit type. This avoids incorrect
- implementations which return true even if long long is not 64-bit aligned.
- This works on GCC and LLVM - other cases have bugs and they disagree. */
- _Static_assert (__atomic_always_lock_free (sizeof (x), &x), "no_64b_atomics");
-}
-EOF
-dnl
-if AC_TRY_COMMAND([${CC-cc} -O2 -S conftest.c]);
-then
- libc_cv_gcc_has_64b_atomics=yes
-else
- libc_cv_gcc_has_64b_atomics=no
-fi
-rm -f conftest* ])
-if test "$libc_cv_gcc_has_64b_atomics" = yes; then
- AC_DEFINE(__HAVE_64B_ATOMICS, 1)
-else
- AC_DEFINE(__HAVE_64B_ATOMICS, 0)
- fi
-
AC_CACHE_CHECK(for redirection of built-in functions, libc_cv_gcc_builtin_redirection, [dnl
cat > conftest.c <<\EOF
extern char *strstr (const char *, const char *) __asm ("my_strstr");
diff --git a/include/atomic.h b/include/atomic.h
index bac7423ec54..7c0eac365ed 100644
--- a/include/atomic.h
+++ b/include/atomic.h
@@ -117,11 +117,6 @@
#endif
-/* This is equal to 1 iff the architecture supports 64b atomic operations. */
-#ifndef __HAVE_64B_ATOMICS
-#error Unable to determine if 64-bit atomics are present.
-#endif
-
/* The following functions are a subset of the atomic operations provided by
C11. Usually, a function named atomic_OP_MO(args) is equivalent to C11's
atomic_OP_explicit(args, memory_order_MO); exceptions noted below. */
diff --git a/sysdeps/alpha/atomic-machine.h b/sysdeps/alpha/atomic-machine.h
index 9aec2317488..b7ce8c4726d 100644
--- a/sysdeps/alpha/atomic-machine.h
+++ b/sysdeps/alpha/atomic-machine.h
@@ -15,6 +15,11 @@
License along with the GNU C Library. If not, see
<https://www.gnu.org/licenses/>. */
-#include <stdint.h>
+#ifndef _ALPHA_ATOMIC_MACHINE_H
+#define _ALPHA_ATOMIC_MACHINE_H
+
+#include_next <atomic-machine.h>
#define atomic_write_barrier() __asm ("wmb" : : : "memory");
+
+#endif
diff --git a/sysdeps/generic/atomic-machine.h b/sysdeps/generic/atomic-machine.h
index 9f8528c1f2a..d9960486031 100644
--- a/sysdeps/generic/atomic-machine.h
+++ b/sysdeps/generic/atomic-machine.h
@@ -34,4 +34,13 @@
and adaptive mutexes to optimize spin-wait loops.
*/
+#include <bits/wordsize.h>
+
+/* This is equal to 1 iff the architecture supports 64b atomic operations. */
+#if __WORDSIZE == 64
+# define __HAVE_64B_ATOMICS 1
+#else
+# define __HAVE_64B_ATOMICS 0
+#endif
+
#endif /* atomic-machine.h */
diff --git a/sysdeps/riscv/atomic-machine.h b/sysdeps/riscv/atomic-machine.h
index b6494b3c837..e375c6665e9 100644
--- a/sysdeps/riscv/atomic-machine.h
+++ b/sysdeps/riscv/atomic-machine.h
@@ -21,6 +21,8 @@
#ifdef __riscv_atomic
+#include_next <atomic-machine.h>
+
/* Miscellaneous. */
# define asm_amo(which, ordering, mem, value) ({ \
diff --git a/sysdeps/sparc/atomic-machine.h b/sysdeps/sparc/atomic-machine.h
index b8c1e96eb46..2ffcbb2fcf3 100644
--- a/sysdeps/sparc/atomic-machine.h
+++ b/sysdeps/sparc/atomic-machine.h
@@ -16,8 +16,10 @@
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
-#ifndef _ATOMIC_MACHINE_H
-#define _ATOMIC_MACHINE_H 1
+#ifndef _SPARC_ATOMIC_MACHINE_H
+#define _SPARC_ATOMIC_MACHINE_H 1
+
+#include_next <atomic-machine.h>
#ifdef __sparc_v9__
# define atomic_full_barrier() \
diff --git a/sysdeps/x86/atomic-machine.h b/sysdeps/x86/atomic-machine.h
index 0681f579879..c854f40dea2 100644
--- a/sysdeps/x86/atomic-machine.h
+++ b/sysdeps/x86/atomic-machine.h
@@ -19,6 +19,8 @@
#ifndef _X86_ATOMIC_MACHINE_H
#define _X86_ATOMIC_MACHINE_H 1
+#include_next <atomic-machine.h>
+
#define atomic_spin_nop() __asm ("pause")
#endif /* atomic-machine.h */
--
2.43.0
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