2008-10-17 Michael Snyder <msnyder@vmware.com>

	* gdb.texinfo: Add documentation for reverse execution.

2 files changed, 151 insertions, 0 deletions

diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index fe1dc33..62eb80d 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,7 @@
+2008-10-17  Michael Snyder  <msnyder@vmware.com>
+
+	* gdb.texinfo: Add documentation for reverse execution.
+
 2008-10-16  Thiago Jung Bauermann  <bauerman@br.ibm.com>
 	    Eli Zaretskii  <eliz@gnu.org>
 
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index d150466..016ac07 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -143,6 +143,7 @@ software in general.  We will miss him.
 * Commands::                    @value{GDBN} commands
 * Running::                     Running programs under @value{GDBN}
 * Stopping::                    Stopping and continuing
+* Reverse Execution::           Running programs backward
 * Stack::                       Examining the stack
 * Source::                      Examining source files
 * Data::                        Examining data
@@ -4850,6 +4851,126 @@ When such an event happens, a system call in another thread may return
 prematurely, even though your program does not appear to stop.
 
 
+@node Reverse Execution
+@chapter Running programs backward
+@cindex reverse execution
+@cindex running programs backward
+
+When you are debugging a program, it is not unusual to realize that
+you have gone too far, and some event of interest has already happened.
+If the target environment supports it, @value{GDBN} can allow you to
+``rewind'' the program by running it backward.
+
+A target environment that supports reverse execution should be able
+to ``undo'' the changes in machine state that have taken place as the
+program was executing normally.  Variables, registers etc.@: should
+revert to their previous values.  Obviously this requires a great
+deal of sophistication on the part of the target environment; not
+all target environments can support reverse execution.
+
+When a program is executed in reverse, the instructions that
+have most recently been executed are ``un-executed'', in reverse
+order.  The program counter runs backward, following the previous
+thread of execution in reverse.  As each instruction is ``un-executed'',
+the values of memory and/or registers that were changed by that
+instruction are reverted to their previous states.  After executing
+a piece of source code in reverse, all side effects of that code
+should be ``undone'', and all variables should be returned to their
+prior values@footnote{
+Note that some side effects are easier to undo than others.  For instance,
+memory and registers are relatively easy, but device I/O is hard.  Some
+targets may be able undo things like device I/O, and some may not.
+
+The contract between @value{GDBN} and the reverse executing target
+requires only that the target do something reasonable when
+@value{GDBN} tells it to execute backwards, and then report the 
+results back to @value{GDBN}.  Whatever the target reports back to
+@value{GDBN}, @value{GDBN} will report back to the user.  @value{GDBN}
+assumes that the memory and registers that the target reports are in a
+consistant state, but @value{GDBN} accepts whatever it is given.
+}.
+
+If you are debugging in a target environment that supports
+reverse execution, @value{GDBN} provides the following commands.
+
+@table @code
+@kindex reverse-continue
+@kindex rc @r{(@code{reverse-continue})}
+@item reverse-continue @r{[}@var{ignore-count}@r{]}
+@itemx rc @r{[}@var{ignore-count}@r{]}
+Beginning at the point where your program last stopped, start executing
+in reverse.  Reverse execution will stop for breakpoints and synchronous
+exceptions (signals), just like normal execution.  Behavior of
+asynchronous signals depends on the target environment.
+
+@kindex reverse-step
+@kindex rs @r{(@code{step})}
+@item reverse-step @r{[}@var{count}@r{]}
+Run the program backward until control reaches the start of a
+different source line; then stop it, and return control to @value{GDBN}.
+
+Like the @code{step} command, @code{reverse-step} will only stop
+at the beginning of a source line.  It ``un-executes'' the previously
+executed source line.  If the previous source line included calls to
+debuggable functions, @code{reverse-step} will step (backward) into
+the called function, stopping at the beginning of the @emph{last}
+statement in the called function (typically a return statement).
+
+Also, as with the @code{step} command, if non-debuggable functions are
+called, @code{reverse-step} will run thru them backward without stopping.
+
+@kindex reverse-stepi
+@kindex rsi @r{(@code{reverse-stepi})}
+@item reverse-stepi @r{[}@var{count}@r{]}
+Reverse-execute one machine instruction.  Note that the instruction
+to be reverse-executed is @emph{not} the one pointed to by the program
+counter, but the instruction executed prior to that one.  For instance,
+if the last instruction was a jump, @code{reverse-stepi} will take you
+back from the destination of the jump to the jump instruction itself.
+
+@kindex reverse-next
+@kindex rn @r{(@code{reverse-next})}
+@item reverse-next @r{[}@var{count}@r{]}
+Run backward to the beginning of the previous line executed in
+the current (innermost) stack frame.  If the line contains function
+calls, they will be ``un-executed'' without stopping.  Starting from
+the first line of a function, @code{reverse-next} will take you back
+to the caller of that function, @emph{before} the function was called,
+just as the normal @code{next} command would take you from the last 
+line of a function back to its return to its caller
+@footnote{Unles the code is too heavily optimized.}.
+
+@kindex reverse-nexti
+@kindex rni @r{(@code{reverse-nexti})}
+@item reverse-nexti @r{[}@var{count}@r{]}
+Like @code{nexti}, @code{reverse-nexti} executes a single instruction
+in reverse, except that called functions are ``un-executed'' atomically.
+That is, if the previously executed instruction was a return from
+another instruction, @code{reverse-nexti} will continue to execute
+in reverse until the call to that function (from the current stack
+frame) is reached.
+
+@kindex reverse-finish
+@item reverse-finish
+Just as the @code{finish} command takes you to the point where the
+current function returns, @code{reverse-finish} takes you to the point
+where it was called.  Instead of ending up at the end of the current
+function invocation, you end up at the beginning.
+
+@kindex set exec-direction
+@item set exec-direction
+Set the direction of target execution.
+@itemx set exec-direction reverse
+@cindex execute forward or backward in time
+@value{GDBN} will perform all execution commands in reverse, until the
+exec-direction mode is changed to ``forward''.  Affected commands include
+@code{step, stepi, next, nexti, continue, and finish}.  The @code{return}
+command cannot be used in reverse mode.
+@item set exec-direction forward
+@value{GDBN} will perform all execution commands in the normal fashion.
+This is the default.
+@end table
+
 
 @node Stack
 @chapter Examining the Stack
@@ -24565,6 +24686,22 @@ breakpoint at @var{addr}.
 Don't use this packet.  Use the @samp{Z} and @samp{z} packets instead
 (@pxref{insert breakpoint or watchpoint packet}).
 
+@item bc
+@cindex @samp{bc} packet
+Backward continue.  Execute the target system in reverse.  No parameter.
+@xref{Reverse Execution}, for more information.
+
+Reply:
+@xref{Stop Reply Packets}, for the reply specifications.
+
+@item bs
+@cindex @samp{bs} packet
+Backward single step.  Execute one instruction in reverse.  No parameter.
+@xref{Reverse Execution}, for more information.
+
+Reply:
+@xref{Stop Reply Packets}, for the reply specifications.
+
 @item c @r{[}@var{addr}@r{]}
 @cindex @samp{c} packet
 Continue.  @var{addr} is address to resume.  If @var{addr} is omitted,
@@ -25182,6 +25319,16 @@ hex.
 The packet indicates that the loaded libraries have changed.
 @value{GDBN} should use @samp{qXfer:libraries:read} to fetch a new
 list of loaded libraries.  @var{r} is ignored.
+
+@cindex replay log events, remote reply
+@item replaylog
+The packet indicates that the target cannot continue replaying 
+logged execution events, because it has reached the end (or the
+beginning when executing backward) of the log.  The value of @var{r}
+will be either @samp{begin} or @samp{end}.  @xref{Reverse Execution}, 
+for more information.
+
+
 @end table
 
 @item W @var{AA}