summaryrefslogtreecommitdiff
diff options
context:
space:
mode:
-rw-r--r--libitm/ChangeLog4
-rw-r--r--libitm/libitm.texi34
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