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Added a comment about the reason why we promote memory_order_consume to memory_order_acquire.

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Andrey Semashev
2014-06-03 23:30:21 +04:00
parent e55bf17309
commit d4ee3a3047
3 changed files with 24 additions and 0 deletions
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8
include/boost/atomic/detail/ops_gcc_arm.hpp
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@@ -79,6 +79,14 @@ namespace detail {
// 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" "bx " #TMPREG "\n" ".arm\n" ".align 4\n" "8:\n"
#define BOOST_ATOMIC_DETAIL_ARM_ASM_END(TMPREG) "adr " #TMPREG ", 9f + 1\n" "bx " #TMPREG "\n" ".thumb\n" ".align 2\n" "9:\n"

8
include/boost/atomic/detail/ops_gcc_ppc.hpp
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@@ -72,6 +72,14 @@ namespace detail {
to pose a problem.
*/
// 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.
struct gcc_ppc_operations_base
{
static BOOST_FORCEINLINE void fence_before(memory_order order) BOOST_NOEXCEPT

8
include/boost/atomic/detail/ops_msvc_arm.hpp
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@@ -43,6 +43,14 @@ namespace boost {
namespace atomics {
namespace detail {
// 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.
struct msvc_arm_operations_base
{
static BOOST_FORCEINLINE void hardware_full_fence() BOOST_NOEXCEPT