| /* |
| * Distributed under the Boost Software License, Version 1.0. |
| * (See accompanying file LICENSE_1_0.txt or copy at |
| * http://www.boost.org/LICENSE_1_0.txt) |
| * |
| * Copyright (c) 2009 Helge Bahmann |
| * Copyright (c) 2013 Tim Blechmann |
| * Copyright (c) 2014, 2020 Andrey Semashev |
| */ |
| /*! |
| * \file atomic/detail/gcc_arm_asm_common.hpp |
| * |
| * This header contains basic utilities for gcc asm-based ARM backend. |
| */ |
| |
| #ifndef BOOST_ATOMIC_DETAIL_GCC_ARM_ASM_COMMON_HPP_INCLUDED_ |
| #define BOOST_ATOMIC_DETAIL_GCC_ARM_ASM_COMMON_HPP_INCLUDED_ |
| |
| #include <boost/atomic/detail/config.hpp> |
| #include <boost/atomic/detail/capabilities.hpp> |
| |
| // A memory barrier is effected using a "co-processor 15" instruction, |
| // though a separate assembler mnemonic is available for it in v7. |
| // |
| // "Thumb 1" is a subset of the ARM instruction set that uses a 16-bit encoding. It |
| // doesn't include all instructions and in particular it doesn't include the co-processor |
| // instruction used for the memory barrier or the load-locked/store-conditional |
| // instructions. So, if we're compiling in "Thumb 1" mode, we need to wrap all of our |
| // asm blocks with code to temporarily change to ARM mode. |
| // |
| // You can only change between ARM and Thumb modes when branching using the bx instruction. |
| // bx takes an address specified in a register. The least significant bit of the address |
| // indicates the mode, so 1 is added to indicate that the destination code is Thumb. |
| // A temporary register is needed for the address and is passed as an argument to these |
| // macros. It must be one of the "low" registers accessible to Thumb code, specified |
| // using the "l" attribute in the asm statement. |
| // |
| // Architecture v7 introduces "Thumb 2", which does include (almost?) all of the ARM |
| // instruction set. (Actually, there was an extension of v6 called v6T2 which supported |
| // "Thumb 2" mode, but its architecture manual is no longer available, referring to v7.) |
| // So in v7 we don't need to change to ARM mode; we can write "universal |
| // assembler" which will assemble to Thumb 2 or ARM code as appropriate. The only thing |
| // we need to do to make this "universal" assembler mode work is to insert "IT" instructions |
| // to annotate the conditional instructions. These are ignored in other modes (e.g. v6), |
| // so they can always be present. |
| |
| // A note about memory_order_consume. Technically, this architecture allows to avoid |
| // unnecessary memory barrier after consume load since it supports data dependency ordering. |
| // However, some compiler optimizations may break a seemingly valid code relying on data |
| // dependency tracking by injecting bogus branches to aid out of order execution. |
| // This may happen not only in Boost.Atomic code but also in user's code, which we have no |
| // control of. See this thread: http://lists.boost.org/Archives/boost/2014/06/213890.php. |
| // For this reason we promote memory_order_consume to memory_order_acquire. |
| |
| #if defined(__thumb__) && !defined(__thumb2__) |
| #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" |
| #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" |
| #define BOOST_ATOMIC_DETAIL_ARM_ASM_TMPREG_CONSTRAINT(var) "=&l" (var) |
| #else |
| // Indicate that start/end macros are empty and the tmpreg is not needed |
| #define BOOST_ATOMIC_DETAIL_ARM_ASM_TMPREG_UNUSED |
| #define BOOST_ATOMIC_DETAIL_ARM_ASM_START(TMPREG) |
| #define BOOST_ATOMIC_DETAIL_ARM_ASM_END(TMPREG) |
| #define BOOST_ATOMIC_DETAIL_ARM_ASM_TMPREG_CONSTRAINT(var) "=&l" (var) |
| #endif |
| |
| #if defined(BOOST_ATOMIC_DETAIL_ARM_LITTLE_ENDIAN) |
| #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_LO(arg) "%" BOOST_STRINGIZE(arg) |
| #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_HI(arg) "%H" BOOST_STRINGIZE(arg) |
| #else |
| #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_LO(arg) "%H" BOOST_STRINGIZE(arg) |
| #define BOOST_ATOMIC_DETAIL_ARM_ASM_ARG_HI(arg) "%" BOOST_STRINGIZE(arg) |
| #endif |
| |
| #endif // BOOST_ATOMIC_DETAIL_GCC_ARM_ASM_COMMON_HPP_INCLUDED_ |
