Add epilogue subtiling

stack-info: PR: #948, branch: PaulZhang12/stack/14

Author: PaulZhang12

examples/matmul.py

@@ -28,12 +28,13 @@
 @helion.kernel(
     # static_shapes=True gives a performance boost for matmuls
     static_shapes=True,
-    # Disable autotung over unrolling/range_num_stages
+    # Disable autotuning over range_num_stages
     # tl.dot is pipelined with num_stages
     autotune_config_overrides={
         "range_unroll_factors": [0, 0],
         "range_num_stages": [0, 0],
     },
+    allow_epilogue_subtiling=True,
 )
 def matmul(
     x: Tensor,

helion/_compiler/device_function.py

@@ -462,8 +462,9 @@ def tensor_descriptor_arg(
         self, fake_value: torch.Tensor, block_size: list[int | torch.SymInt]
     ) -> TensorDescriptorArg:
         host_function = HostFunction.current()
-        block_size_expr = ", ".join(map(self.literal_expr, block_size))
+        block_size_expr = ", ".join(self.literal_expr(dim) for dim in block_size)
         key = (fake_value, block_size_expr)
+
         if key not in self._tensor_descriptor_args:
             origin = host_function.tensor_to_origin[fake_value]
             desc_name = self.new_var(origin.suggest_var_name() + "_desc")
@@ -556,22 +557,6 @@ def _format_constexpr_value(self, value: object) -> str:
         if isinstance(value, (torch.SymInt, torch.SymFloat, torch.SymBool)):
             value = value._sympy_()
 
-        # Handle sympy expressions (sanitize by replacing triton_helpers functions)
-        if isinstance(value, sympy.Expr):
-            sanitized = value.replace(  # pyright: ignore[reportAttributeAccessIssue]
-                lambda node: isinstance(node, sympy.Function)
-                and getattr(node.func, "__name__", "")
-                == "triton_helpers.div_floor_integer",
-                lambda node: sympy.floor(node.args[0] / node.args[1]),  # pyright: ignore[reportAttributeAccessIssue]
-            ).replace(  # pyright: ignore[reportAttributeAccessIssue]
-                lambda node: isinstance(node, sympy.Function)
-                and getattr(node.func, "__name__", "")
-                == "triton_helpers.remainder_integer",
-                lambda node: sympy.Mod(node.args[0], node.args[1]),  # pyright: ignore[reportAttributeAccessIssue]
-            )
-            expr = cast("sympy.Expr", sanitized)
-            return HostFunction.current().sympy_expr(expr)
-
         return HostFunction.current().literal_expr(value)
 
     def _tensor_property(
@@ -749,11 +734,19 @@ def current() -> DeviceFunction:
 
 
 class HelionTritonPrinter(TritonPrinter):
-    """Custom Triton printer that avoids wrapping float literals in tl.full().
-
-    Inductor's default TritonPrinter prints SymPy Float as a 0-D Triton value
-    via tl.full([], <val>, tl.float64). We override this to emit the raw numeric
-    literal, letting downstream type promotion and casts handle dtype.
+    """Custom Triton printer that does the following:
+
+    - Avoids wrapping float literals in tl.full().
+     Inductor's default TritonPrinter prints SymPy Float as a 0-D Triton value
+     via tl.full([], <val>, tl.float64). We override this to emit the raw numeric
+     literal, letting downstream type promotion and casts handle dtype.
+
+    - Avoids triton_helpers.div_floor_integer(...) calls when both operands are
+      provably non-negative integers. TritonPrinter by default converts
+      floor(u1/2) to triton_helpers.div_floor_integer(...). We override this to
+      emit u1 // 2 only when the numerator is known to be non-negative and the
+      denominator is a positive integer, so that we keep helper calls for cases
+      that rely on floor semantics with mixed signs.
     """
 
     def _print_Float(self, expr: sympy.Expr) -> str:
@@ -762,6 +755,53 @@ def _print_Float(self, expr: sympy.Expr) -> str:
     def _print_ToFloat(self, expr: sympy.Expr) -> str:
         return f"{expr} + 0.0"
 
+    def _is_nonnegative(self, expr: sympy.Expr) -> bool:
+        if expr.is_nonnegative is True or expr.is_zero is True:
+            return True
+        if expr.is_positive is True:
+            return True
+        try:
+            host_fn = HostFunction.current()
+        except NoCurrentFunction:
+            host_fn = None
+        if host_fn is not None:
+            origin_info = host_fn.expr_to_origin.get(expr)
+            if origin_info and isinstance(
+                origin_info.origin, (BlockSizeOrigin, TensorSizeOrigin)
+            ):
+                return True
+        if isinstance(expr, sympy.Symbol) and expr.name.startswith("_BLOCK_SIZE_"):
+            return True
+        if isinstance(expr, sympy.Number):
+            return bool(expr >= 0)
+        return False
+
+    def _format_trunc_div(self, lhs: sympy.Expr, rhs: sympy.Expr) -> str:
+        lhs_str = self._print(lhs)
+        rhs_str = self._print(rhs)
+        if not (lhs.is_Integer or lhs.is_Symbol):
+            lhs_str = f"({lhs_str})"
+        if not (rhs.is_Integer or rhs.is_Symbol):
+            rhs_str = f"({rhs_str})"
+        return f"{lhs_str} // {rhs_str}"
+
+    def _print_floor(self, expr: sympy.Expr) -> str:
+        inner = expr.args[0]
+        numer, denom = inner.as_numer_denom()
+        if (
+            isinstance(denom, sympy.Integer)
+            and denom > 1
+            and self._is_nonnegative(numer)
+        ):
+            return self._format_trunc_div(numer, denom)
+        return super()._print_floor(expr)
+
+    def _print_FloorDiv(self, expr: sympy.Expr) -> str:
+        lhs, rhs = expr.args
+        if isinstance(rhs, sympy.Integer) and rhs > 0 and self._is_nonnegative(lhs):
+            return self._format_trunc_div(lhs, rhs)
+        return super()._print_FloorDiv(expr)
+
 
 def texpr(expr: sympy.Expr) -> str:
     return HelionTritonPrinter().doprint(expr)

helion/_compiler/device_ir.py

@@ -63,6 +63,7 @@
 from .type_propagation import _eval_binary
 from .type_propagation import _eval_compare
 from .type_propagation import _eval_unary
+from .utils import _use_epilogue_subtile
 
 if TYPE_CHECKING:
     from collections.abc import Callable
@@ -1191,6 +1192,11 @@ def lower_to_device_ir(func: HostFunction) -> DeviceIR:
             total_load_count, loads_without_eviction_policy, store_count
         )
 
+        if _use_epilogue_subtile():
+            for graph in device_ir.graphs:
+                # Epilogue subtiling only for Blackwell
+                epilogue_subtiling_pass(graph.graph, store_count)
+
         return device_ir
 
 
@@ -1348,3 +1354,69 @@ def remove_unnecessary_tile_index(graph: torch.fx.Graph) -> None:
                 user.args = tuple(new_args)
         if len(node.users) == 0:
             graph.erase_node(node)
+
+
+def epilogue_subtiling_pass(graph: torch.fx.Graph, store_count: int) -> None:
+    """
+    Replace epilogue subtile with a tunable value.
+    """
+    if store_count == 0:
+        return
+
+    from ..autotuner.config_fragment import EnumFragment
+    from ..autotuner.config_fragment import ListOf
+    from ..autotuner.config_spec import VALID_EPILOGUE_SUBTILE_SIZES
+    from .inductor_lowering import PointwiseLowering
+
+    env = CompileEnvironment.current()
+    # Register a tunable for epilogue subtile for all device stores
+    fragment = ListOf(
+        EnumFragment(choices=VALID_EPILOGUE_SUBTILE_SIZES), length=store_count
+    )
+    env.config_spec.epilogue_subtiling = fragment
+
+    def collect_pointwise_epilogue_nodes(
+        store_node: torch.fx.Node,
+    ) -> dict[torch.fx.Node, None]:
+        """Recursively collect all pointwise nodes that can be subtiled in the epilogue.
+
+        Starting from a store node, traverse backwards through all input nodes,
+        collecting pointwise operations until we hit non-pointwise nodes.
+        Only include pointwise nodes that have a single user to ensure they can be fused.
+        """
+        # dict to preserve order
+        pointwise_nodes = {}
+        visited = set()
+        stack = [store_node.args[2]]  # Start with the value being stored
+
+        while stack:
+            current = stack.pop()
+            if current in visited or not isinstance(current, torch.fx.Node):
+                continue
+
+            visited.add(current)
+
+            lowering = current.meta.get("lowering")
+            # Check if this is a pointwise operation with only one user
+            if isinstance(lowering, PointwiseLowering) and len(current.users) == 1:
+                if current not in pointwise_nodes:
+                    pointwise_nodes[current] = None
+                stack.extend(current.all_input_nodes)
+
+        return pointwise_nodes
+
+    from ..language import store as store_api
+
+    stores = set()
+
+    for node in graph.nodes:
+        if node.op == "call_function" and node.target == store_api:
+            stores.add(node)
+            # Collect all pointwise nodes that can be subtiled in the epilogue
+            pointwise_nodes = collect_pointwise_epilogue_nodes(node)
+            if pointwise_nodes:
+                # Mark all collected pointwise nodes for epilogue subtiling
+                for pw_node in pointwise_nodes:
+                    pw_node.meta["epilogue_subtile"] = True
+                # Store the set of pointwise nodes in the store node's metadata
+                node.meta["pointwise_epilogue_nodes"] = pointwise_nodes