diff options
| -rw-r--r-- | libitm/ChangeLog | 4 | ||||
| -rw-r--r-- | libitm/libitm.texi | 34 |
2 files changed, 35 insertions, 3 deletions
diff --git a/libitm/ChangeLog b/libitm/ChangeLog index 3e9e159f978..1be02b525b4 100644 --- a/libitm/ChangeLog +++ b/libitm/ChangeLog @@ -1,3 +1,7 @@ +2012-03-02 Torvald Riegel <triegel@redhat.com> + + * libitm.texi: Link to specification and add a usage example. + 2012-02-24 Torvald Riegel <triegel@redhat.com> * retry.cc (GTM::gtm_thread::number_of_threads_changed): Change diff --git a/libitm/libitm.texi b/libitm/libitm.texi index b31657f7f97..6cfcaf9277d 100644 --- a/libitm/libitm.texi +++ b/libitm/libitm.texi @@ -82,8 +82,8 @@ several threads. To activate support for TM in C/C++, the compile-time flag @option{-fgnu-tm} must be specified. This enables TM language-level constructs such as -transaction statements (@code{__transaction}, @pxref{C/C++ Language -Constructs for TM} for details). +transaction statements (e.g., @code{__transaction_atomic}, @pxref{C/C++ +Language Constructs for TM} for details). @c --------------------------------------------------------------------- @c C/C++ Language Constructs for TM @@ -92,7 +92,35 @@ Constructs for TM} for details). @node C/C++ Language Constructs for TM @chapter C/C++ Language Constructs for TM -TODO: link to the C++ TM spec. a few examples. how gcc's support differs. +Transactions are supported in C++ and C in the form of transaction statements, +transaction expressions, and function transactions. In the following example, +both @code{a} and @code{b} will be read and the difference will be written to +@code{c}, all atomically and isolated from other transactions: + +@example +__transaction_atomic @{ c = a - b; @} +@end example + +Therefore, another thread can use the following code to concurrently update +@code{b} without ever causing @code{c} to hold a negative value (and without +having to use other synchronization constructs such as locks or C++11 +atomics): + +@example +__transaction_atomic @{ if (a > b) b++; @} +@end example + +GCC follows the @uref{https://sites.google.com/site/tmforcplusplus/, Draft +Specification of Transactional Language Constructs for C++ (v1.1)} in its +implementation of transactions. + +The precise semantics of transactions are defined in terms of the C++11/C11 +memory model (see the specification). Roughly, transactions provide +synchronization guarantees that are similar to what would be guaranteed when +using a single global lock as a guard for all transactions. Note that like +other synchronization constructs in C/C++, transactions rely on a +data-race-free program (e.g., a nontransactional write that is concurrent +with a transactional read to the same memory location is a data race). @c --------------------------------------------------------------------- @c The libitm ABI |