From 66198f447f45a87718b6457756976baa8defb2fd Mon Sep 17 00:00:00 2001
From: Jay Berkenbilt <ejb@ql.org>
Date: Fri, 10 Apr 2020 13:30:00 -0400
Subject: [PATCH] Use -latomic if needed with gcc

---
 autofiles.sums |  2 +-
 configure      | 43 +++++++++++++++++++++++++++++++++++++++++++
 configure.ac   | 26 ++++++++++++++++++++++++++
 3 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/autofiles.sums b/autofiles.sums
index e995876b..731de13e 100644
--- a/autofiles.sums
+++ b/autofiles.sums
@@ -1,4 +1,4 @@
-e764290be37a921ecba94755b6da22dd9e9b98644b8fdfdd0fa7f1331ce56a05  configure.ac
+c9b8555013e70c93572e0be5f5c7c03c6b7e0680de4870010945357a02909eaa  configure.ac
 d3f9ee6f6f0846888d9a10fd3dad2e4b1258be84205426cf04d7cef02d61dad7  aclocal.m4
 b0ce6d1dba8effa47d25154b2bb56eddafc997254a0f3f903cf9b6abffc03616  libqpdf/qpdf/qpdf-config.h.in
 5297971a0ef90bcd5563eb3f7127a032bb76d3ae2af7258bf13479caf8983a60  m4/ax_cxx_compile_stdcxx.m4
diff --git a/configure b/configure
index 5a9c614f..9703878f 100755
--- a/configure
+++ b/configure
@@ -16975,6 +16975,49 @@ ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
+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 to use -latomic" >&5
+$as_echo_n "checking whether to use -latomic... " >&6; }
+oLDFLAGS="$LDFLAGS"
+LDFLAGS="$LDFLAGS -Wl,--as-needed -latomic"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <atomic>
+    int main()
+    {
+        static std::atomic<int> a{0};
+        a = a.fetch_add(1);
+        return 0;
+    }
+
+_ACEOF
+if ac_fn_cxx_try_link "$LINENO"; then :
+  qpdf_USE_ATOMIC=1
+else
+  qpdf_USE_ATOMIC=0
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+if test "$qpdf_USE_ATOMIC" = "1"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+  LDFLAGS="$oLDFLAGS"
+fi
+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
+
+
 if test "x$qpdf_OS_SECURE_RANDOM" = "x1"; then
   OLIBS=$LIBS
   LIBS="$LIBS Advapi32.lib"
diff --git a/configure.ac b/configure.ac
index 9a887b90..a3c3d064 100644
--- a/configure.ac
+++ b/configure.ac
@@ -240,6 +240,32 @@ for i in 0 1; do
 done
 AC_LANG_POP
 
+AC_LANG_PUSH([C++])
+AC_MSG_CHECKING(whether to use -latomic)
+dnl On some platforms with some versions of gcc, you have to link with
+dnl -latomic in order for std::atomic to work. Passing --as-needed
+dnl prevents unnecessary linking with -latomic.
+oLDFLAGS="$LDFLAGS"
+LDFLAGS="$LDFLAGS -Wl,--as-needed -latomic"
+AC_LINK_IFELSE([AC_LANG_SOURCE(
+  [[#include <atomic>
+    int main()
+    {
+        static std::atomic<int> a{0};
+        a = a.fetch_add(1);
+        return 0;
+    }
+   ]])],
+   [qpdf_USE_ATOMIC=1],
+   [qpdf_USE_ATOMIC=0])
+if test "$qpdf_USE_ATOMIC" = "1"; then
+  AC_MSG_RESULT(yes)
+else
+  AC_MSG_RESULT(no)
+  LDFLAGS="$oLDFLAGS"
+fi
+AC_LANG_POP
+
 if test "x$qpdf_OS_SECURE_RANDOM" = "x1"; then
   OLIBS=$LIBS
   LIBS="$LIBS Advapi32.lib"