77 lines
2.6 KiB
C++
77 lines
2.6 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <folly/CPortability.h>
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// FOLLY_KEEP
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//
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// When applied to a function, prevents removal of the function.
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//
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// Functions may be removed when building with both function-sections and
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// gc-sections. It is tricky to keep them; this utility captures the steps
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// required.
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//
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// In order for this mechanism to work, this header must be included. This
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// mechanism relies on the hidden global variable below.
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//
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// The linker, when asked to with --gc-sections, may throw out unreferenced
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// sections. When the compiler emits each function into its own section, as it
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// does when asked to with -ffunction-sections, the linker will throw out
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// unreferenced functions. The key is to move all kept functions into a single
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// section, avoiding the behavior of -ffunction-sections, and then force the
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// compiler to emit some reference to at least one function in that section.
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// This way, the linker will see at least one reference to the kept section,
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// and so will not throw it out.
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#if __GNUC__ && __linux__
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#define FOLLY_KEEP [[gnu::section(".text.folly.keep")]]
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#else
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#define FOLLY_KEEP
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#endif
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#if __GNUC__ && __linux__
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#define FOLLY_KEEP_DETAIL_ATTR_NAKED [[gnu::naked]]
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#define FOLLY_KEEP_DETAIL_ATTR_NOINLINE [[gnu::noinline]]
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#else
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#define FOLLY_KEEP_DETAIL_ATTR_NAKED
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#define FOLLY_KEEP_DETAIL_ATTR_NOINLINE
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#endif
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namespace folly {
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namespace detail {
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// marking this as [[gnu::naked]] gets clang not to emit any text for this
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FOLLY_KEEP FOLLY_KEEP_DETAIL_ATTR_NAKED static void keep_anchor() {}
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class keep {
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public:
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// must accept the anchor function as an argument
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FOLLY_KEEP_DETAIL_ATTR_NOINLINE explicit keep(void (*)()) noexcept {}
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};
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// noinline ctor and trivial dtor minimize the text size of this
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static keep keep_instance{keep_anchor};
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// weak and noinline to prevent the compiler from eliding calls
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template <typename... T>
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FOLLY_ATTR_WEAK FOLLY_NOINLINE void keep_sink(T&&...) {}
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template <typename... T>
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FOLLY_ATTR_WEAK FOLLY_NOINLINE void keep_sink_nx(T&&...) noexcept {}
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} // namespace detail
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} // namespace folly
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