bkietz commented on a change in pull request #7493:
URL: https://github.com/apache/arrow/pull/7493#discussion_r442924149
##########
File path: cpp/src/arrow/util/atomic_shared_ptr.h
##########
@@ -24,7 +24,9 @@
namespace arrow {
namespace internal {
-#if !defined(__clang__) && defined(__GNUC__) && __GNUC__ < 5
+// https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html#abi.versioning
+// https://gcc.gnu.org/develop.html#timeline
+#if defined(__GLIBCXX__) && __GLIBCXX__ < 20150422
// atomic shared_ptr operations only appeared in gcc 5,
// emulate them with unsafe ops on gcc 4.x.
Review comment:
```suggestion
#if defined(__GLIBCXX__) && __GLIBCXX__ < 20150422
// Atomic shared_ptr operations only appeared in gcc 5's libstdc++,
// emulate them with unsafe ops if unavailable.
//
// The libstdc++ version is a the encoded release date of gcc 5, see
//
https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html#abi.versioning.__GLIBCXX__
```
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