atomic/detail/gcc_arm_asm_common.hpp

0001 /*
0002  * Distributed under the Boost Software License, Version 1.0.
0003  * (See accompanying file LICENSE_1_0.txt or copy at
0004  * http://www.boost.org/LICENSE_1_0.txt)
0005  *
0006  * Copyright (c) 2009 Helge Bahmann
0007  * Copyright (c) 2013 Tim Blechmann
0008  * Copyright (c) 2014, 2020 Andrey Semashev
0009  */
0010 /*!
0011  * \file   atomic/detail/gcc_arm_asm_common.hpp
0012  *
0013  * This header contains basic utilities for gcc asm-based ARM backend.
0014  */
0015
0016 #ifndef BOOST_ATOMIC_DETAIL_GCC_ARM_ASM_COMMON_HPP_INCLUDED_
0017 #define BOOST_ATOMIC_DETAIL_GCC_ARM_ASM_COMMON_HPP_INCLUDED_
0018
0019 #include <boost/atomic/detail/config.hpp>
0020 #include <boost/atomic/detail/capabilities.hpp>
0021
0022 #ifdef BOOST_HAS_PRAGMA_ONCE
0023 #pragma once
0024 #endif
0025
0026 // A memory barrier is effected using a "co-processor 15" instruction,
0027 // though a separate assembler mnemonic is available for it in v7.
0028 //
0029 // "Thumb 1" is a subset of the ARM instruction set that uses a 16-bit encoding.  It
0030 // doesn't include all instructions and in particular it doesn't include the co-processor
0031 // instruction used for the memory barrier or the load-locked/store-conditional
0032 // instructions.  So, if we're compiling in "Thumb 1" mode, we need to wrap all of our
0033 // asm blocks with code to temporarily change to ARM mode.
0034 //
0035 // You can only change between ARM and Thumb modes when branching using the bx instruction.
0036 // bx takes an address specified in a register.  The least significant bit of the address
0037 // indicates the mode, so 1 is added to indicate that the destination code is Thumb.
0038 // A temporary register is needed for the address and is passed as an argument to these
0039 // macros.  It must be one of the "low" registers accessible to Thumb code, specified
0040 // using the "l" attribute in the asm statement.
0041 //
0042 // Architecture v7 introduces "Thumb 2", which does include (almost?) all of the ARM
0043 // instruction set.  (Actually, there was an extension of v6 called v6T2 which supported
0044 // "Thumb 2" mode, but its architecture manual is no longer available, referring to v7.)
0045 // So in v7 we don't need to change to ARM mode; we can write "universal
0046 // assembler" which will assemble to Thumb 2 or ARM code as appropriate.  The only thing
0047 // we need to do to make this "universal" assembler mode work is to insert "IT" instructions
0048 // to annotate the conditional instructions.  These are ignored in other modes (e.g. v6),
0049 // so they can always be present.
0050
0051 // A note about memory_order_consume. Technically, this architecture allows to avoid
0052 // unnecessary memory barrier after consume load since it supports data dependency ordering.
0053 // However, some compiler optimizations may break a seemingly valid code relying on data
0054 // dependency tracking by injecting bogus branches to aid out of order execution.
0055 // This may happen not only in Boost.Atomic code but also in user's code, which we have no
0056 // control of. See this thread: http://lists.boost.org/Archives/boost/2014/06/213890.php.
0057 // For this reason we promote memory_order_consume to memory_order_acquire.
0058
0059 #if defined(__thumb__) && !defined(__thumb2__)
0060 #define BOOST_ATOMIC_DETAIL_ARM_ASM_START(TMPREG) "adr " #TMPREG ", 8f\n\t" "bx " #TMPREG "\n\t" ".arm\n\t" ".align 4\n\t" "8:\n\t"
0061 #define BOOST_ATOMIC_DETAIL_ARM_ASM_END(TMPREG)   "adr " #TMPREG ", 9f + 1\n\t" "bx " #TMPREG "\n\t" ".thumb\n\t" ".align 2\n\t" "9:\n\t"
0062 #define BOOST_ATOMIC_DETAIL_ARM_ASM_TMPREG_CONSTRAINT(var) "=&l" (var)
0063 #else
0064 // Indicate that start/end macros are empty and the tmpreg is not needed
0065 #define BOOST_ATOMIC_DETAIL_ARM_ASM_TMPREG_UNUSED
0066 #define BOOST_ATOMIC_DETAIL_ARM_ASM_START(TMPREG)
0067 #define BOOST_ATOMIC_DETAIL_ARM_ASM_END(TMPREG)
0068 #define BOOST_ATOMIC_DETAIL_ARM_ASM_TMPREG_CONSTRAINT(var) "=&l" (var)
0069 #endif
0070
0071 #if defined(BOOST_ATOMIC_DETAIL_ARM_LITTLE_ENDIAN)
0072 #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_LO(arg) "%" BOOST_STRINGIZE(arg)
0073 #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_HI(arg) "%H" BOOST_STRINGIZE(arg)
0074 #else
0075 #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_LO(arg) "%H" BOOST_STRINGIZE(arg)
0076 #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_HI(arg) "%" BOOST_STRINGIZE(arg)
0077 #endif
0078
0079 #endif // BOOST_ATOMIC_DETAIL_GCC_ARM_ASM_COMMON_HPP_INCLUDED_