From 930e68e348dbd5da12168be052cd44b88099a0f6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jo=C3=A3o=20Felipe=20Santos?= <santosjf@pm.me>
Date: Mon, 15 Jun 2026 11:38:11 -0700
Subject: [PATCH] Fix GCC 9.4 build: gate std::atomic<shared_ptr> on C++20
 feature macro

std::atomic<std::shared_ptr<T>> is a C++20 library feature that libstdc++
only supports from GCC 12. The fallback to the deprecated std::atomic_*
free-function overloads was keyed on _LIBCPP_VERSION, so GCC 9.4 (libstdc++,
not libc++) took the unsupported branch and failed to compile.

Key the selection on the standard __cpp_lib_atomic_shared_ptr feature-test
macro instead. It is undefined on both libc++ and pre-12 libstdc++, routing
both to the free-function fallback, and defined on GCC 12+ where the member
std::atomic<std::shared_ptr> path is used. No behavior change for any
compiler that previously built.
---
 NAM/wavenet/slimmable.cpp | 18 +++++++++---------
 NAM/wavenet/slimmable.h   | 20 +++++++++++++++-----
 2 files changed, 24 insertions(+), 14 deletions(-)

diff --git a/NAM/wavenet/slimmable.cpp b/NAM/wavenet/slimmable.cpp
index 69f077bc..6248618b 100644
--- a/NAM/wavenet/slimmable.cpp
+++ b/NAM/wavenet/slimmable.cpp
@@ -291,45 +291,45 @@ bool is_full_size(const std::vector<wavenet::LayerArrayParams>& params, const st
 
 } // anonymous namespace
 
-#ifdef _LIBCPP_VERSION
+#if NAM_HAS_ATOMIC_SHARED_PTR
 void SlimmableWavenet::_pending_clear_release()
 {
-  std::atomic_store_explicit(&_pending_staged, std::shared_ptr<StagedSlimModel>{}, std::memory_order_release);
+  _pending_staged.store({}, std::memory_order_release);
 }
 
 std::shared_ptr<SlimmableWavenet::StagedSlimModel> SlimmableWavenet::_pending_load_acquire() const
 {
-  return std::atomic_load_explicit(&_pending_staged, std::memory_order_acquire);
+  return _pending_staged.load(std::memory_order_acquire);
 }
 
 void SlimmableWavenet::_pending_store_release(std::shared_ptr<StagedSlimModel> p)
 {
-  std::atomic_store_explicit(&_pending_staged, std::move(p), std::memory_order_release);
+  _pending_staged.store(std::move(p), std::memory_order_release);
 }
 
 std::shared_ptr<SlimmableWavenet::StagedSlimModel> SlimmableWavenet::_pending_exchange_take_acq_rel()
 {
-  return std::atomic_exchange_explicit(&_pending_staged, std::shared_ptr<StagedSlimModel>{}, std::memory_order_acq_rel);
+  return _pending_staged.exchange({}, std::memory_order_acq_rel);
 }
 #else
 void SlimmableWavenet::_pending_clear_release()
 {
-  _pending_staged.store({}, std::memory_order_release);
+  std::atomic_store_explicit(&_pending_staged, std::shared_ptr<StagedSlimModel>{}, std::memory_order_release);
 }
 
 std::shared_ptr<SlimmableWavenet::StagedSlimModel> SlimmableWavenet::_pending_load_acquire() const
 {
-  return _pending_staged.load(std::memory_order_acquire);
+  return std::atomic_load_explicit(&_pending_staged, std::memory_order_acquire);
 }
 
 void SlimmableWavenet::_pending_store_release(std::shared_ptr<StagedSlimModel> p)
 {
-  _pending_staged.store(std::move(p), std::memory_order_release);
+  std::atomic_store_explicit(&_pending_staged, std::move(p), std::memory_order_release);
 }
 
 std::shared_ptr<SlimmableWavenet::StagedSlimModel> SlimmableWavenet::_pending_exchange_take_acq_rel()
 {
-  return _pending_staged.exchange({}, std::memory_order_acq_rel);
+  return std::atomic_exchange_explicit(&_pending_staged, std::shared_ptr<StagedSlimModel>{}, std::memory_order_acq_rel);
 }
 #endif
 
diff --git a/NAM/wavenet/slimmable.h b/NAM/wavenet/slimmable.h
index 28a3ab57..df57b052 100644
--- a/NAM/wavenet/slimmable.h
+++ b/NAM/wavenet/slimmable.h
@@ -3,9 +3,19 @@
 #include <memory>
 #include <vector>
 
-#ifdef _LIBCPP_VERSION
-// libc++: std::atomic<std::shared_ptr<T>> is not viable; staging uses deprecated atomic_* free functions.
+// std::atomic<std::shared_ptr<T>> requires C++20 library support (libstdc++ >= GCC 12).
+// Where it is unavailable -- libc++ (any version so far) and older libstdc++ such as the
+// one shipped with GCC 9 -- fall back to the deprecated std::atomic_* free-function
+// overloads for shared_ptr, which provide the same acquire/release semantics. Keyed on the
+// C++20 feature-test macro rather than on a specific standard library so the right path is
+// chosen for every compiler.
+#if defined(__cpp_lib_atomic_shared_ptr) && __cpp_lib_atomic_shared_ptr >= 201711L
+  #define NAM_HAS_ATOMIC_SHARED_PTR 1
 #else
+  #define NAM_HAS_ATOMIC_SHARED_PTR 0
+#endif
+
+#if NAM_HAS_ATOMIC_SHARED_PTR
   #include <atomic>
 #endif
 
@@ -71,11 +81,11 @@ class SlimmableWavenet : public DSP, public SlimmableModel
     std::shared_ptr<DSP> model;
     std::vector<int> channels;
   };
-#ifdef _LIBCPP_VERSION
+#if NAM_HAS_ATOMIC_SHARED_PTR
+  std::atomic<std::shared_ptr<StagedSlimModel>> _pending_staged;
+#else
   /// Staged model; synchronized via deprecated std::atomic_* overloads for shared_ptr only.
   std::shared_ptr<StagedSlimModel> _pending_staged;
-#else
-  std::atomic<std::shared_ptr<StagedSlimModel>> _pending_staged;
 #endif
 
   std::vector<int> _current_channels;