Disable 2CTA fwd non-causal on CUDA 12 to work around codegen regression

flash_attn/cute/utils.py

@@ -60,12 +60,33 @@
 _fa_disable_2cta_enabled: bool = os.environ.get("FA_DISABLE_2CTA", "0") == "1"
 
 
+def _is_cuda_12() -> bool:
+    """Check if the CUDA toolkit version is 12.x.
+
+    2CTA forward non-causal has a codegen regression on CUDA 12 that causes
+    ~18% slowdown compared to 1CTA. This is fixed in CUDA 13.x.
+    """
+    try:
+        import torch
+
+        cuda_version = torch.version.cuda
+        if cuda_version is not None:
+            major = cuda_version.split(".")[0]
+            return int(major) == 12
+    except Exception:
+        pass
+    return False
+
+
+_fa_disable_2cta_cuda12: bool = _is_cuda_12()
+
+
 def _get_use_clc_scheduler_default() -> bool:
     return _fa_clc_enabled
 
 
 def _get_disable_2cta_default() -> bool:
-    return _fa_disable_2cta_enabled
+    return _fa_disable_2cta_enabled or _fa_disable_2cta_cuda12
 
 
 def _compute_base_hash(func: Callable) -> str: