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b04a70177b
The old inline assembly provided two output operands that were afterwards combined for a return value. However, the second output operand isn't necessary when using the "Acquiring a Lock" example-code of the ARM manual "Barrier Litmus Tests and Cookbook". This saves two logical operations per acquisition try. Additionally better documentation is now provided. Fixes #1292
69 lines
1.6 KiB
C++
69 lines
1.6 KiB
C++
/*
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* \brief Atomic operations for ARM
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* \author Norman Feske
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* \author Stefan Kalkowski
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* \author Martin Stein
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* \date 2007-04-28
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*/
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/*
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* Copyright (C) 2007-2017 Genode Labs GmbH
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*
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* This file is part of the Genode OS framework, which is distributed
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* under the terms of the GNU Affero General Public License version 3.
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*/
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#ifndef _INCLUDE__SPEC__ARM__CPU__ATOMIC_H_
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#define _INCLUDE__SPEC__ARM__CPU__ATOMIC_H_
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#include <cpu/memory_barrier.h>
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namespace Genode {
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/**
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* Atomic compare and exchange
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*
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* This function compares the value at dest with cmp_val.
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* If both values are equal, dest is set to new_val. If
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* both values are different, the value at dest remains
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* unchanged.
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*
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* Note, that cmpxchg() represents a memory barrier.
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*
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* \return 1 if the value was successfully changed to new_val,
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* 0 if cmp_val and the value at dest differ.
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*/
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inline int cmpxchg(volatile int *dest, int cmp_val, int new_val)
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{
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int result;
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asm volatile (
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/* compare values */
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"1:\n"
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"ldrex %0, [%1]\n"
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"cmp %0, %2\n"
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/* if not equal, return with result 0 */
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"movne %0, #0\n"
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"bne 2f\n"
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/* if equal, try to override memory value exclusively */
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"strex %0, %3, [%1]\n"
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"cmp %0, #0\n"
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/* if access wasn't exclusive, go back to comparison */
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"bne 1b\n"
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/* if access was exclusive, return with result 1 */
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"mov %0, #1\n"
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"2:\n"
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: "=&r" (result) : "r" (dest), "r" (cmp_val), "r" (new_val)
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: "cc");
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Genode::memory_barrier();
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return result;
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}
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}
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#endif /* _INCLUDE__SPEC__ARM__CPU__ATOMIC_H_ */
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