Fix for threading issues around global session & epoch ids. (#3232)

* Fix for threading issues around global session & epoch ids.

* Make m_epochId atomic for thread visibility.

3 files changed, 9 insertions, 9 deletions

diff --git a/source/slang/slang-ast-base.cpp b/source/slang/slang-ast-base.cpp
index 5b4a4ea0c..7fa0a8886 100644
--- a/source/slang/slang-ast-base.cpp
+++ b/source/slang/slang-ast-base.cpp
@@ -23,7 +23,7 @@ void NodeBase::_initDebug(ASTNodeType inAstNodeType, ASTBuilder* inAstBuilder)
 DeclRefBase* Decl::getDefaultDeclRef()
 {
     auto astBuilder = getCurrentASTBuilder();
-    if (astBuilder->getEpoch() != m_defaultDeclRefEpoch || !m_defaultDeclRef)
+    if (astBuilder && astBuilder->getEpoch() != m_defaultDeclRefEpoch || !m_defaultDeclRef)
     {
         m_defaultDeclRef = astBuilder->getOrCreate<DirectDeclRef>(this);
         m_defaultDeclRefEpoch = astBuilder->getEpoch();
diff --git a/source/slang/slang-ast-builder.cpp b/source/slang/slang-ast-builder.cpp
index 4a3756c1f..b2d1e5c09 100644
--- a/source/slang/slang-ast-builder.cpp
+++ b/source/slang/slang-ast-builder.cpp
@@ -221,12 +221,6 @@ Decl* SharedASTBuilder::tryFindMagicDecl(const String& name)
 
 // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ASTBuilder !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 
-Index& _getGlobalASTEpochId()
-{
-    static thread_local Index epochId = 1;
-    return epochId;
-}
-
 ASTBuilder::ASTBuilder(SharedASTBuilder* sharedASTBuilder, const String& name):
     m_sharedASTBuilder(sharedASTBuilder),
     m_name(name),
@@ -260,12 +254,12 @@ ASTBuilder::~ASTBuilder()
 
 Index ASTBuilder::getEpoch()
 {
-    return _getGlobalASTEpochId();
+    return getSharedASTBuilder()->m_session->m_epochId;
 }
 
 void ASTBuilder::incrementEpoch()
 {
-    _getGlobalASTEpochId()++;
+    getSharedASTBuilder()->m_session->m_epochId++;
 }
 
 NodeBase* ASTBuilder::createByNodeType(ASTNodeType nodeType)
diff --git a/source/slang/slang-compiler.h b/source/slang/slang-compiler.h
index 95995349a..aae18d08f 100755
--- a/source/slang/slang-compiler.h
+++ b/source/slang/slang-compiler.h
@@ -3008,6 +3008,12 @@ namespace Slang
             /// Get the downstream compiler for a transition
         IDownstreamCompiler* getDownstreamCompiler(CodeGenTarget source, CodeGenTarget target);
         
+        // This needs to be atomic not because of contention between threads as `Session` is
+        // *not* multithreaded, but can be used exclusively on one thread at a time.
+        // The need for atomic is purely for visibility. If the session is used on a different 
+        // thread we need to be sure any changes to m_epochId are visible to this thread.
+        std::atomic<Index> m_epochId = 1;
+
         Scope* baseLanguageScope = nullptr;
         Scope* coreLanguageScope = nullptr;
         Scope* hlslLanguageScope = nullptr;