Only enable mimalloc when stdatomic.h is available

Author: tiran

Doc/using/configure.rst

@@ -264,7 +264,8 @@ also be used to improve performance.
 
 .. cmdoption:: --with-mimalloc
 
-   Enable :ref:`mimalloc <mimalloc>` memory allocator (disabled by default).
+   Enable :ref:`mimalloc <mimalloc>` memory allocator. mimalloc is enabled
+   by default when compiler and platform provide C11 ``stdatomic.h``.
 
    See also :envvar:`PYTHONMALLOC` environment variable.
 

configure

@@ -4597,16 +4597,62 @@ then
 fi
 AC_MSG_RESULT($with_doc_strings)
 
+# Check for stdatomic.h, required for mimalloc.
+AC_CACHE_CHECK([for stdatomic.h], [ac_cv_header_stdatomic_h], [
+AC_LINK_IFELSE(
+[
+  AC_LANG_SOURCE([[
+    #include <stdatomic.h>
+    atomic_int int_var;
+    atomic_uintptr_t uintptr_var;
+    int main() {
+      atomic_store_explicit(&int_var, 5, memory_order_relaxed);
+      atomic_store_explicit(&uintptr_var, 0, memory_order_relaxed);
+      int loaded_value = atomic_load_explicit(&int_var, memory_order_seq_cst);
+      return 0;
+    }
+  ]])
+],[ac_cv_header_stdatomic_h=yes],[ac_cv_header_stdatomic_h=no])
+])
+
+AS_VAR_IF([ac_cv_header_stdatomic_h], [yes], [
+    AC_DEFINE(HAVE_STD_ATOMIC, 1,
+              [Has stdatomic.h with atomic_int and atomic_uintptr_t])
+])
+
+# Check for GCC >= 4.7 and clang __atomic builtin functions
+AC_CACHE_CHECK([for builtin __atomic_load_n and __atomic_store_n functions], [ac_cv_builtin_atomic], [
+AC_LINK_IFELSE(
+[
+  AC_LANG_SOURCE([[
+    int val;
+    int main() {
+      __atomic_store_n(&val, 1, __ATOMIC_SEQ_CST);
+      (void)__atomic_load_n(&val, __ATOMIC_SEQ_CST);
+      return 0;
+    }
+  ]])
+],[ac_cv_builtin_atomic=yes],[ac_cv_builtin_atomic=no])
+])
+
+AS_VAR_IF([ac_cv_builtin_atomic], [yes], [
+    AC_DEFINE(HAVE_BUILTIN_ATOMIC, 1, [Has builtin __atomic_load_n() and __atomic_store_n() functions])
+])
+
 # --with-mimalloc
 AC_MSG_CHECKING([for --with-mimalloc])
 AC_ARG_WITH([mimalloc],
   [AS_HELP_STRING([--with-mimalloc],
-                 [build with mimalloc memory allocator (default is yes)])],
+                 [build with mimalloc memory allocator (default is yes if C11 stdatomic.h is available.)])],
   [],
-  [with_mimalloc="yes"]
+  [with_mimalloc="$ac_cv_header_stdatomic_h"]
 )
 
 if test "$with_mimalloc" != no; then
+  if test "$ac_cv_header_stdatomic_h" != yes; then
+    # mimalloc-atomic.h wants C11 stdatomic.h on POSIX
+    AC_MSG_ERROR([mimalloc requires stdatomic.h, use --without-mimalloc to disable mimalloc.])
+  fi
   with_mimalloc=yes
   AC_DEFINE([WITH_MIMALLOC], [1], [Define if you want to compile in mimalloc memory allocator.])
   AC_SUBST([MIMALLOC_HEADERS], ['$(MIMALLOC_HEADERS)'])
@@ -6749,53 +6795,6 @@ if test "$ac_cv_gcc_asm_for_x87" = yes; then
     esac
 fi
 
-# Check for stdatomic.h
-AC_CACHE_CHECK([for stdatomic.h], [ac_cv_header_stdatomic_h], [
-AC_LINK_IFELSE(
-[
-  AC_LANG_SOURCE([[
-    #include <stdatomic.h>
-    atomic_int int_var;
-    atomic_uintptr_t uintptr_var;
-    int main() {
-      atomic_store_explicit(&int_var, 5, memory_order_relaxed);
-      atomic_store_explicit(&uintptr_var, 0, memory_order_relaxed);
-      int loaded_value = atomic_load_explicit(&int_var, memory_order_seq_cst);
-      return 0;
-    }
-  ]])
-],[ac_cv_header_stdatomic_h=yes],[ac_cv_header_stdatomic_h=no])
-])
-
-AS_VAR_IF([ac_cv_header_stdatomic_h], [yes], [
-    AC_DEFINE(HAVE_STD_ATOMIC, 1,
-              [Has stdatomic.h with atomic_int and atomic_uintptr_t])
-])
-
-if test "$ac_cv_header_stdatomic_h" != yes -a "$with_mimalloc" != no; then
-    # mimalloc-atomic.h wants C11 stdatomic.h on POSIX
-    AC_MSG_ERROR([mimalloc requires stdatomic.h, use --without-mimalloc to disable mimalloc. A future version of Python will require stdatomic.h.])
-fi
-
-# Check for GCC >= 4.7 and clang __atomic builtin functions
-AC_CACHE_CHECK([for builtin __atomic_load_n and __atomic_store_n functions], [ac_cv_builtin_atomic], [
-AC_LINK_IFELSE(
-[
-  AC_LANG_SOURCE([[
-    int val;
-    int main() {
-      __atomic_store_n(&val, 1, __ATOMIC_SEQ_CST);
-      (void)__atomic_load_n(&val, __ATOMIC_SEQ_CST);
-      return 0;
-    }
-  ]])
-],[ac_cv_builtin_atomic=yes],[ac_cv_builtin_atomic=no])
-])
-
-AS_VAR_IF([ac_cv_builtin_atomic], [yes], [
-    AC_DEFINE(HAVE_BUILTIN_ATOMIC, 1, [Has builtin __atomic_load_n() and __atomic_store_n() functions])
-])
-
 # ensurepip option
 AC_MSG_CHECKING(for ensurepip)
 AC_ARG_WITH(ensurepip,
@@ -7455,3 +7454,10 @@ AS_VAR_IF([PY_SUPPORT_TIER], [0], [AC_MSG_WARN([
 Platform "$host" with compiler "$ac_cv_cc_name" is not supported by the
 CPython core team, see https://peps.python.org/pep-0011/ for more information.
 ])])
+
+if test "$ac_cv_header_stdatomic_h" != "yes"; then
+  AC_MSG_NOTICE(m4_normalize([
+    Your compiler or platform does have a working C11 stdatomic.h. A future
+    version of Python may require stdatomic.h.
+  ]))
+fi