1 /* 2 * Distributed under the Boost Software License, Version 1.0. 3 * (See accompanying file LICENSE_1_0.txt or copy at 4 * http://www.boost.org/LICENSE_1_0.txt) 5 * 6 * Copyright (c) 2009 Helge Bahmann 7 * Copyright (c) 2013 Tim Blechmann 8 * Copyright (c) 2014, 2020 Andrey Semashev 9 */ 10 /*! 11 * \file atomic/detail/gcc_arm_asm_common.hpp 12 * 13 * This header contains basic utilities for gcc asm-based ARM backend. 14 */ 15 16 #ifndef BOOST_ATOMIC_DETAIL_GCC_ARM_ASM_COMMON_HPP_INCLUDED_ 17 #define BOOST_ATOMIC_DETAIL_GCC_ARM_ASM_COMMON_HPP_INCLUDED_ 18 19 #include <boost/atomic/detail/config.hpp> 20 #include <boost/atomic/detail/capabilities.hpp> 21 22 // A memory barrier is effected using a "co-processor 15" instruction, 23 // though a separate assembler mnemonic is available for it in v7. 24 // 25 // "Thumb 1" is a subset of the ARM instruction set that uses a 16-bit encoding. It 26 // doesn't include all instructions and in particular it doesn't include the co-processor 27 // instruction used for the memory barrier or the load-locked/store-conditional 28 // instructions. So, if we're compiling in "Thumb 1" mode, we need to wrap all of our 29 // asm blocks with code to temporarily change to ARM mode. 30 // 31 // You can only change between ARM and Thumb modes when branching using the bx instruction. 32 // bx takes an address specified in a register. The least significant bit of the address 33 // indicates the mode, so 1 is added to indicate that the destination code is Thumb. 34 // A temporary register is needed for the address and is passed as an argument to these 35 // macros. It must be one of the "low" registers accessible to Thumb code, specified 36 // using the "l" attribute in the asm statement. 37 // 38 // Architecture v7 introduces "Thumb 2", which does include (almost?) all of the ARM 39 // instruction set. (Actually, there was an extension of v6 called v6T2 which supported 40 // "Thumb 2" mode, but its architecture manual is no longer available, referring to v7.) 41 // So in v7 we don't need to change to ARM mode; we can write "universal 42 // assembler" which will assemble to Thumb 2 or ARM code as appropriate. The only thing 43 // we need to do to make this "universal" assembler mode work is to insert "IT" instructions 44 // to annotate the conditional instructions. These are ignored in other modes (e.g. v6), 45 // so they can always be present. 46 47 // A note about memory_order_consume. Technically, this architecture allows to avoid 48 // unnecessary memory barrier after consume load since it supports data dependency ordering. 49 // However, some compiler optimizations may break a seemingly valid code relying on data 50 // dependency tracking by injecting bogus branches to aid out of order execution. 51 // This may happen not only in Boost.Atomic code but also in user's code, which we have no 52 // control of. See this thread: http://lists.boost.org/Archives/boost/2014/06/213890.php. 53 // For this reason we promote memory_order_consume to memory_order_acquire. 54 55 #if defined(__thumb__) && !defined(__thumb2__) 56 #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" 57 #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" 58 #define BOOST_ATOMIC_DETAIL_ARM_ASM_TMPREG_CONSTRAINT(var) "=&l" (var) 59 #else 60 // Indicate that start/end macros are empty and the tmpreg is not needed 61 #define BOOST_ATOMIC_DETAIL_ARM_ASM_TMPREG_UNUSED 62 #define BOOST_ATOMIC_DETAIL_ARM_ASM_START(TMPREG) 63 #define BOOST_ATOMIC_DETAIL_ARM_ASM_END(TMPREG) 64 #define BOOST_ATOMIC_DETAIL_ARM_ASM_TMPREG_CONSTRAINT(var) "=&l" (var) 65 #endif 66 67 #if defined(BOOST_ATOMIC_DETAIL_ARM_LITTLE_ENDIAN) 68 #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_LO(arg) "%" BOOST_STRINGIZE(arg) 69 #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_HI(arg) "%H" BOOST_STRINGIZE(arg) 70 #else 71 #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_LO(arg) "%H" BOOST_STRINGIZE(arg) 72 #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_HI(arg) "%" BOOST_STRINGIZE(arg) 73 #endif 74 75 #endif // BOOST_ATOMIC_DETAIL_GCC_ARM_ASM_COMMON_HPP_INCLUDED_ 76