[ET-VK] Fused RMSNorm operator to fix fp16 overflow

Fused RMSNorm operator that performs squaring, mean, rsqrt, and
weight scaling in a single shader dispatch. All accumulation is done
in fp32 regardless of input dtype, preventing fp16 overflow when
residual stream values exceed sqrt(65504) ≈ 256.

The Python reference impl (`rms_norm_impl`) must preserve the input
dtype — PyTorch type promotion would otherwise produce fp32 output
from fp16 inputs, and the FusePatternsPass re-trace would propagate
that incorrect dtype through the graph.

Authored by Claude.

Differential Revision: [D99841211](https://our.internmc.facebook.com/intern/diff/D99841211/)

ghstack-source-id: 364237333
Pull Request resolved: #18772

Author: ssjia

backends/vulkan/custom_ops_lib.py

@@ -959,3 +959,24 @@ def select_as_symint_impl(x: torch.Tensor, dim: int, index: int):
 lib.define(f"{name}(Tensor x, int dim, int index) -> SymInt")
 lib.impl(name, select_as_symint_impl, "Meta")
 select_as_symint_op = getattr(getattr(torch.ops, namespace), name)
+
+################
+## rms_norm ##
+################
+
+
+def rms_norm_impl(
+    x: torch.Tensor,
+    weight: torch.Tensor,
+    eps: float,
+) -> torch.Tensor:
+    input_dtype = x.dtype
+    variance = x.float().pow(2).mean(-1, keepdim=True)
+    x_normed = x.float() * torch.rsqrt(variance + eps)
+    return (x_normed * weight.float()).to(input_dtype)
+
+
+name = "rms_norm"
+lib.define(f"{name}(Tensor x, Tensor weight, float eps) -> Tensor")
+lib.impl(name, rms_norm_impl, "CompositeExplicitAutograd")
+rms_norm_op = getattr(getattr(torch.ops, namespace), name)

backends/vulkan/op_registry.py

@@ -1606,6 +1606,21 @@ def register_native_layer_norm():
     )
 
 
+# =============================================================================
+# RmsNorm.cpp
+# =============================================================================
+
+
+@update_features(exir_ops.edge.et_vk.rms_norm.default)
+def register_rms_norm():
+    return OpFeatures(
+        inputs_storage=utils.CONTIGUOUS_ANY,
+        inputs_dtypes=utils.FP_T,
+        supports_prepacking=True,
+        supports_resize=True,
+    )
+
+
 #######################
 ## Utility functions ##
 #######################

backends/vulkan/patterns/BUCK

@@ -16,6 +16,7 @@ fbcode_target(_kind = runtime.python_library,
         "quantized_convolution.py",
         "quantized_binary.py",
         "quantized_unary.py",
+        "rms_norm.py",
         "sdpa.py",
         "select_as_symint.py",
     ],

backends/vulkan/patterns/__init__.py

@@ -16,6 +16,8 @@
 
 import executorch.backends.vulkan.patterns.quantized_unary  # noqa
 
+import executorch.backends.vulkan.patterns.rms_norm  # noqa
+
 import executorch.backends.vulkan.patterns.rope  # noqa
 
 import executorch.backends.vulkan.patterns.rope_hf  # noqa

backends/vulkan/patterns/rms_norm.py

@@ -0,0 +1,279 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+from typing import Optional
+
+import torch
+
+from executorch.backends.vulkan.patterns.pattern_registry import (
+    PatternMatch,
+    register_pattern_detector,
+    register_pattern_replacement,
+)
+
+from executorch.exir import ExportedProgram
+from executorch.exir.dialects._ops import ops as exir_ops
+
+
+_CAST_OPS = {
+    exir_ops.edge.aten._to_copy.default,
+    exir_ops.edge.aten.to.dtype,
+}
+
+
+def _skip_casts(node: torch.fx.Node) -> torch.fx.Node:
+    """Unwrap chains of dtype-cast nodes to find the underlying value."""
+    while isinstance(node, torch.fx.Node) and node.target in _CAST_OPS:
+        if node.args and isinstance(node.args[0], torch.fx.Node):
+            node = node.args[0]
+        else:
+            break
+    return node
+
+
+class RmsNormMatch(PatternMatch):
+    """
+    Detects the decomposed RMSNorm pattern, including variants where dtype
+    casts (to_copy) are inserted around the computation.
+
+    The canonical pattern emitted by the Llama RMSNorm implementation is:
+
+      x_orig (any dtype)
+        -> to_copy(fp32) -> x_f32
+           -> mul(x_f32, x_f32) -> mean(dim=-1, keepdim=True)
+           -> add(eps) -> rsqrt -> rstd_f32
+        -> mul(x_f32, rstd_f32) -> norm_f32
+        -> to_copy(orig dtype) -> norm_cast
+      weight -> to_copy(orig dtype) -> weight_cast
+      -> mul(norm_cast, weight_cast)   ← anchor node
+
+    We look through to_copy nodes when comparing tensor identities so that
+    the match also handles fp32-only models where no casts are present.
+
+    The anchor node is the final mul (scale by weight).
+    """
+
+    def __init__(self, final_mul_node: torch.fx.Node) -> None:  # noqa: C901
+        self.anchor_node = final_mul_node
+        self.match_found = False
+        self.all_nodes = [self.anchor_node]
+
+        # final_mul: mul(normalized_cast, weight_cast)
+        # Unwrap casts to reach the underlying norm_mul and weight.
+        norm_mul_node, self.weight_node = self._identify_norm_mul_and_weight(
+            final_mul_node
+        )
+        if norm_mul_node is None:
+            return
+
+        self.all_nodes.append(norm_mul_node)
+
+        # norm_mul: mul(x_f32, rstd_f32)
+        rsqrt_node, x_for_norm = self._identify_rsqrt_and_input(norm_mul_node)
+        if rsqrt_node is None:
+            return
+
+        self.all_nodes.append(rsqrt_node)
+
+        # rsqrt -> add(mean_sq, eps) -> mean(x_sq, dim=-1, keepdim=True)
+        add_node = self._get_single_arg_node(
+            rsqrt_node, exir_ops.edge.aten.rsqrt.default
+        )
+        if add_node is None or add_node.target != exir_ops.edge.aten.add.Tensor:
+            return
+
+        self.all_nodes.append(add_node)
+
+        self.eps_node = None
+        mean_node = None
+        for arg in add_node.args[:2]:
+            if (
+                isinstance(arg, torch.fx.Node)
+                and arg.target == exir_ops.edge.aten.mean.dim
+            ):
+                mean_node = arg
+            else:
+                self.eps_node = arg
+
+        if mean_node is None or self.eps_node is None:
+            return
+
+        self.all_nodes.append(mean_node)
+
+        # Verify mean has keepdim=True and dim=[-1]
+        if len(mean_node.args) < 3:
+            return
+        mean_dims = mean_node.args[1]
+        if mean_dims != [-1]:
+            return
+        if not mean_node.args[2]:
+            return
+
+        # mean's input should be x_sq = mul(x, x) or pow(x, 2)
+        sq_node = mean_node.args[0]
+        if not isinstance(sq_node, torch.fx.Node):
+            return
+
+        self.all_nodes.append(sq_node)
+
+        # Use the fp32 x (x_for_norm) as the canonical fp32 input.
+        # Both mul(x,x) and the norm mul should share the same fp32 source.
+        x_f32 = (
+            _skip_casts(x_for_norm)
+            if isinstance(x_for_norm, torch.fx.Node)
+            else x_for_norm
+        )
+
+        if sq_node.target == exir_ops.edge.aten.mul.Tensor:
+            if sq_node.args[0] != sq_node.args[1]:
+                return
+            sq_input = sq_node.args[0]
+            if not isinstance(sq_input, torch.fx.Node):
+                return
+            if _skip_casts(sq_input) != x_f32 and sq_input != x_for_norm:
+                return
+        elif sq_node.target == exir_ops.edge.aten.pow.Tensor_Scalar:
+            sq_input = sq_node.args[0]
+            if not isinstance(sq_input, torch.fx.Node):
+                return
+            if _skip_casts(sq_input) != x_f32 and sq_input != x_for_norm:
+                return
+            if sq_node.args[1] != 2 and sq_node.args[1] != 2.0:
+                return
+        else:
+            return
+
+        # The canonical input node to expose to the fused op is the original
+        # tensor before any fp32 upcast (i.e. the input to the first to_copy).
+        # If there's no cast, x_for_norm is already the original input.
+        self.input_node = (
+            _skip_casts(x_for_norm)
+            if isinstance(x_for_norm, torch.fx.Node)
+            else x_for_norm
+        )
+        # Also collect the intermediate cast nodes so they can be cleaned up
+        cast_node = x_for_norm
+        while (
+            isinstance(cast_node, torch.fx.Node)
+            and cast_node.target in _CAST_OPS
+            and cast_node not in self.all_nodes
+        ):
+            self.all_nodes.append(cast_node)
+            cast_node = cast_node.args[0] if cast_node.args else cast_node
+
+        self.match_found = True
+
+    def _identify_norm_mul_and_weight(self, final_mul_node):
+        """From mul(norm_cast, weight_cast), unwrap casts and find the
+        underlying norm-mul node and the weight source node."""
+        if len(final_mul_node.args) < 2:
+            return None, None
+
+        a, b = final_mul_node.args[0], final_mul_node.args[1]
+
+        for norm_candidate_raw, weight_candidate_raw in [(a, b), (b, a)]:
+            if not isinstance(norm_candidate_raw, torch.fx.Node):
+                continue
+            norm_candidate = _skip_casts(norm_candidate_raw)
+            if (
+                isinstance(norm_candidate, torch.fx.Node)
+                and norm_candidate.target == exir_ops.edge.aten.mul.Tensor
+                and self._has_rsqrt_ancestor(norm_candidate)
+            ):
+                return norm_candidate, weight_candidate_raw
+
+        return None, None
+
+    def _has_rsqrt_ancestor(self, mul_node):
+        """Check if one of mul_node's args is an rsqrt node (possibly through casts)."""
+        for arg in mul_node.args[:2]:
+            if not isinstance(arg, torch.fx.Node):
+                continue
+            if _skip_casts(arg).target == exir_ops.edge.aten.rsqrt.default:
+                return True
+        return False
+
+    def _identify_rsqrt_and_input(self, norm_mul_node):
+        """From mul(x, rstd), find the rsqrt node and the input x.
+        The rsqrt may be wrapped in a cast node."""
+        if len(norm_mul_node.args) < 2:
+            return None, None
+
+        a, b = norm_mul_node.args[0], norm_mul_node.args[1]
+
+        for rsqrt_candidate_raw, input_candidate in [(a, b), (b, a)]:
+            if not isinstance(rsqrt_candidate_raw, torch.fx.Node):
+                continue
+            rsqrt_candidate = _skip_casts(rsqrt_candidate_raw)
+            if (
+                isinstance(rsqrt_candidate, torch.fx.Node)
+                and rsqrt_candidate.target == exir_ops.edge.aten.rsqrt.default
+            ):
+                return rsqrt_candidate, input_candidate
+
+        return None, None
+
+    def _get_single_arg_node(self, node, expected_target):
+        """Get the single input arg of a unary op node."""
+        if node.target != expected_target:
+            return None
+        if len(node.args) < 1 or not isinstance(node.args[0], torch.fx.Node):
+            return None
+        return node.args[0]
+
+
+@register_pattern_detector("rms_norm")
+def find_rms_norm_patterns(
+    node: torch.fx.Node,
+) -> Optional[RmsNormMatch]:
+    if node.target != exir_ops.edge.aten.mul.Tensor:
+        return None
+
+    matched_pattern = RmsNormMatch(node)
+    if matched_pattern.match_found:
+        return matched_pattern
+
+    return None
+
+
+##
+## Pattern Replacement
+##
+
+
+def _extract_eps_value(eps_node) -> float:
+    if isinstance(eps_node, (int, float)):
+        return float(eps_node)
+    if isinstance(eps_node, torch.fx.Node) and "val" in eps_node.meta:
+        val = eps_node.meta["val"]
+        if isinstance(val, torch.Tensor):
+            return float(val.item())
+        if isinstance(val, (int, float)):
+            return float(val)
+    raise ValueError(f"Cannot extract epsilon value from {eps_node}")
+
+
+@register_pattern_replacement("rms_norm")
+def replace_rms_norm_with_fused_op(
+    ep: ExportedProgram,
+    graph_module: torch.fx.GraphModule,
+    match: RmsNormMatch,
+):
+    eps_val = _extract_eps_value(match.eps_node)
+
+    with graph_module.graph.inserting_before(match.anchor_node):
+        rms_norm_node = graph_module.graph.create_node(
+            "call_function",
+            exir_ops.edge.et_vk.rms_norm.default,
+            args=(
+                match.input_node,
+                match.weight_node,
+                eps_val,
+            ),
+        )
+
+    rms_norm_node.meta["val"] = match.anchor_node.meta["val"]
+    match.anchor_node.replace_all_uses_with(rms_norm_node)