Enrico Fraccaroli
49 lines
2.2 KiB
C
49 lines
2.2 KiB
C
/// @file compiler.h
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/// @brief Definition of memory barriers.
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/// @details
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/// Generally speaking, memory barriers prevent the compiler from merging or
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/// refetching reads or writes. Ensuring that the compiler does not fold, spindle,
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/// or otherwise mutilate accesses that either do not require ordering or that
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/// interact with an explicit memory barrier or atomic instruction that provides
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/// the required ordering.
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///
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/// Follows an extract from `LINUX KERNEL MEMORY BARRIERS` by: David Howells,
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/// Paul E. McKenney, Will Deacon, Peter Zijlstra, available at:
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/// https://www.kernel.org/doc/Documentation/memory-barriers.txt
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///
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/// Consider the following abstract model of the system:
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/// @code
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/// : :
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/// +-------+ : +--------+ : +-------+
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/// | CPU 1 |<----->| Memory |<----->| CPU 2 |
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/// +-------+ : +--------+ : +-------+
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/// ^ : ^ : ^
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/// | : | : |
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/// | : v : |
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/// | : +--------+ : |
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/// +---------->| Device |<----------+
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/// : +--------+ :
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/// : :
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/// @endcode
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/// Each CPU executes a program that generates memory access operations. In the
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/// abstract CPU, memory operation ordering is very relaxed, and a CPU may actually
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/// perform the memory operations in any order it likes, provided program causality
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/// appears to be maintained. Similarly, the compiler may also arrange the
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/// instructions it emits in any order it likes, provided it doesn't affect the
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/// apparent operation of the program.
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///
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/// So in the above diagram, the effects of the memory operations performed by a
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/// CPU are perceived by the rest of the system as the operations cross the
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/// interface between the CPU and rest of the system (the dotted lines).
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///
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/// @copyright (c) 2014-2022 This file is distributed under the MIT License.
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/// See LICENSE.md for details.
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#pragma once
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/// @brief Assign the value to the given variable.
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#define WRITE_ONCE(var, val) (*((__volatile__ __typeof__(val) *)(&(var))) = (val))
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/// @brief Read the value from the given variable.
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#define READ_ONCE(var) (*((__volatile__ __typeof__(var) *)(&(var))))
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