diff --git a/codec2-1.2.0/src/codec2_alloc.cpp b/codec2-1.2.0/src/codec2_alloc.cpp
index 37b7e67a..ec9e806e 100644
--- a/codec2-1.2.0/src/codec2_alloc.cpp
+++ b/codec2-1.2.0/src/codec2_alloc.cpp
@@ -15,12 +15,14 @@ static thread_local O1HeapInstance* Instance_ = NULL;
 void codec2_initialize_realtime(size_t heapSize)
 {
 #if defined(WIN32)
-    Heap_ = (void*)_aligned_malloc(O1HEAP_ALIGNMENT, heapSize);
+    Heap_ = (void*)_aligned_malloc(heapSize, O1HEAP_ALIGNMENT);
 #else
     Heap_ = (void*)aligned_alloc(O1HEAP_ALIGNMENT, heapSize);
 #endif // defined(WIN32)
     assert(Heap_ != NULL);
 
+    memset(Heap_, 0, heapSize);
+
     Instance_ = o1heapInit(Heap_, heapSize);
     assert(Instance_ != NULL);
 }
diff --git a/codec2-1.2.0/src/o1heap.c b/codec2-1.2.0/src/o1heap.c
index 6af213bb..d95707e7 100644
--- a/codec2-1.2.0/src/o1heap.c
+++ b/codec2-1.2.0/src/o1heap.c
@@ -61,8 +61,10 @@
 /// computation, which is available via compiler intrinsics. The default implementation will automatically use
 /// the intrinsics for some of the compilers; for others it will default to the slow software emulation,
 /// which can be overridden by the user via O1HEAP_CONFIG_HEADER. The library guarantees that the argument is positive.
+///
+/// NOTE (MS, 2025-06-04): __builtin_clzl seems broken on LLVM MinGW, so reverting to slower emulation on Windows.
 #if O1HEAP_USE_INTRINSICS && !defined(O1HEAP_CLZ)
-#    if defined(__GNUC__) || defined(__clang__) || defined(__CC_ARM)
+#    if !defined(WIN32) && (defined(__GNUC__) || defined(__clang__) || defined(__CC_ARM))
 #        define O1HEAP_CLZ __builtin_clzl
 #    endif
 #endif