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diff --git a/third_party/highway/hwy/per_target.h b/third_party/highway/hwy/per_target.h
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+++ b/third_party/highway/hwy/per_target.h
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+// Copyright 2022 Google LLC
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef HIGHWAY_HWY_PER_TARGET_H_
+#define HIGHWAY_HWY_PER_TARGET_H_
+
+#include <stddef.h>
+
+// Per-target functions.
+
+namespace hwy {
+
+// Returns size in bytes of a vector, i.e. `Lanes(ScalableTag<uint8_t>())`.
+//
+// Do not cache the result, which may change after calling DisableTargets, or
+// if software requests a different vector size (e.g. when entering/exiting SME
+// streaming mode). Instead call this right before the code that depends on the
+// result, without any DisableTargets or SME transition in-between. Note that
+// this involves an indirect call, so prefer not to call this frequently nor
+// unnecessarily.
+size_t VectorBytes();
+
+}  // namespace hwy
+
+#endif  // HIGHWAY_HWY_PER_TARGET_H_