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<== Date ==> <== Thread ==>

Subject: Experimental Sequencer Release
From: Benjamin Franksen <benjamin.franksen@bessy.de>
To: tech-talk@aps.anl.gov, core-talk@aps.anl.gov
Date: Wed, 5 May 2010 12:50:57 +0200
Hello Everybody

I have been working on a new, improved version of the sequencer. What 
follows is copied verbatim from the README-2.0.98 file in the release. One 
item I have not yet thought about is how to organize collaboration. The 
current version (2.0.12) is hosted at 
http://epics.web.psi.ch/software/sequencer/, however there seems to be no 
public repository there, only releases, so I started my work based on the 
latest released tar ball, and recorded my changes in darcs repository (see 
below under Download). Anyway, I am open to suggestions.

EPICS sequencer 2.0.98 (darcs tag R2-0-98)
------------------------------------------

This is an alpha release for an upcoming seq-2.1.0.

Most of the changes relative to seq-2.0.12 are to the SNL compiler, but a 
small number of changes have been made to the runtime library as well.

The extensions are mostly conservative: existing SNL programs should compile 
and work just as with seq-2.0.12 (with one exception, see next paragraph). 
This is not easy to guarantee, however, as there are many corner cases where 
the manual is imprecise and the code was convoluted (and possibly 
erroneous), especially with regard to the "syncQ" feature. See also my 
request below.

There is one (mis-)feature I have removed: 2.0.12 (and earlier versions) 
allow more than one entry or exit block inside the same state. This has no 
semantic value at all, the action statements are simply concatenated as if 
they had been written in one block. So if you (for whatever reason, which? 
Tell me!) relied on this, then for each state you'll have to merge all its 
entry blocks into one (and similar for its exit blocks).

What's in it for you?
=====================

The most important extensions are local definitions and the new state change 
command. Suggestions, criticism, or encouragements regarding these 
extensions are welcome, of course. (Send them to tech-talk@aps.anl.gov, 
core-talk@aps.anl.gov, or benjamin.franksen@bessy.de).

Local Definitions
-----------------

Here, 'definitions' is to be understood as in the SNL manual, i.e. options, 
variable declarations, assign, monitor, sync, and syncQ constructs. These 
definitions have to appear (in any order) right after the opening brace and 
before any other content (code, states,transitions), similar as in C. 
However, not every definition is allowed everywhere:

 * option definitions are restricted just as before, i.e. at the top level
   (for program options) and inside a state (for state options)
 * assign, monitor, sync, and syncQ can appear inside a state set (ss
   <state_set_name> {...}) and inside a state (state <state_name> {...}), in
   addition to the top level
 * foreign declarations (see below) and event flag declarations are
   restricted to the top-level
 * variable declarations can appear at the start of any block (state set,
   state, transition, entry, exit, and compound statement blocks); their
   scope is always limited (statically) to the smallest enclosing block

Local variable declarations come in two flavours, depending on where they 
appear:

(1) Variables of /unlimited life time/ are global variables and those which 
are local to a state set or a state clause. Only variables of this sort can 
be assigned to a process variable, monitored, synced etc.

(2) Variables declared in any other block have lifetime limited to the 
enclosing block, they disappear when the block exits, just as block local 
variables in C.

Variable declarations are restricted to the small set of types offered by 
SNL just as before. Scalar variable declarations may be initialized with an 
arbitrary expression (for top-level variables the C compiler will only allow 
constant expressions, but this isn't checked by the SNL compiler).

State Change Command
--------------------

This is an experimental feature. It adds a new primitive action statement

    state <state-name>;

Its operational meaning is to immediately return from the enclosing 
transition action block and to enter the named state, instead of the default 
state that is given after the block as before. Entry and exit blocks are 
respected exactly as with all other state changes.

I have termed this an experimental feature because I am not sure it is good 
to offer something like that. It is certainly similar to a "goto", in that 
it enables unstructured control flow. I am interested in your opinion!

Minor Extensions/Improvements
-----------------------------

 * You can avoid the usual 'warning: variable xxx used but not defined' by
   declaring foreign (i.e. C) variables, using a /foreign declaration
   statement/. The syntax is simple:

      declare xxx;

   declares that  xxx  is defined somewhere outside the control of the SNL
   compiler. Foreign declarations may appear only at the top-level scope.
 * Fixed the generated line markers, so that error and warning messages now
   correctly point to the source location (this was seriously broken in the
   old version).
 * The syntax now accepts a larger subset of C. For instance, 'character'
   literals are now recognized, as well as the 'continue' statement.

Download
========

Releases can be downloaded from

   http://www-csr.bessy.de/control/SoftDist/sequencer/releases/

The darcs repository is located at

   http://www-csr.bessy.de/control/SoftDist/sequencer/repo/

Build and Install
=================

As usual, you need to adapt configure/RELEASE so that EPICS_BASE refers to 
the install location of the EPICS base you want to use. This version has 
been tested with base-3.14.8.2 and base-3.14.11.

Apart from EPICS base (and its dependencies, e.g. Perl), building this 
version of seq requires an additional tool named re2c to be installed. This 
can be downloaded from http://sourceforge.net/projects/re2c/files/ (sources 
and Windows binaries), the home page is http://re2c.org/. If you are on a 
linux system, you will probably want to use the re2c package your 
distribution provides.

Internals
=========

The compiler is not a re-write from scratch, but changes are numerous and 
pervasive.

The ancient versions of yacc and lexx that are bundled with EPICS base (in a 
modified version and thus never upgraded) are no longer used. Since lex/yacc 
suffers from severe backward compatibility disease (witness all the 
traditional-C stuff they still carry around, global vars and everything), I 
decided to look for something better. Shying away from more radical steps 
(for instance, it would have been much, much easier to re-implement the 
whole compiler in Haskell) because of all the usual issues involved 
(portability, nobody else would understand the code, etc, etc), I chose a 
more conservative approch: the new snc version uses re2c as the lexer 
generator, and lemon as the parser generator. Re2c is available for many 
platforms (including Windows), whereas lemon consists of just one source 
file (plus one template file) and so can be easily bundled with the 
sequencer. Both tools generate very fast code (reportedly better than 
lex/yacc).

Other internal changes include:

 * use standard ANSI C
 * clean separation between compiler stages: lexing, parsing, analysis,
   code generation
 * no global variables, very few static ones
 * unified error, warning, and debug reporting
 * improved type safety by using unions instead of casts
   (plus a number of supporting macros) e.g. for the various syntactic
   constructs; added many new struct types
 * use a hash table (the gpHash from libCom) for name lookup instead of
   doing linear search all over the place
 * complete re-implementation of lexing and parsing (using re2c and
   lemon); the new parser spec has only three parsing conflicts and
   these are unavoidable: one is the well-known if-else ambiguity, the
   remaining two are due to escaped C code, where the parser cannot
   decide whether it is a declaration or a statement (the old version
   had a total of 744 conflicts)
 * generated code contains fewer '#define's making accidental name
   clashes less probable
 * the interface between the sequencer library and the generated code
   is now more type safe (no more XXX_FUNC casts, SS_ID and USER_VAR
   became anonymous struct types)
 * in order to implement the state change command, an additional argument
   is needed for the action callback

Request for Help
================

For better test coverage, I would like you all to SEND ME SNL PROGRAMS. 
Especially if you are using the 'syncQ' construct. These must be self-
contained, i.e. they should not depend on external header files, or if they 
do, please send them along. I do not necessarily plan to run these programs, 
just compile them and see that the result is identical (up to improvements) 
to what the previous version generates.

Reporting Bugs
==============

I am glad to receive bug reports, questions, comments, whatever.  Send them 
to <benjamin.franksen@bessy.de>

Cheers
Ben

Replies:
Re: Experimental Sequencer Release Andrew Johnson

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