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| SuperPascal | |
|---|---|
| Paradigm | concurrent,imperative,structured |
| Family | WirthPascal |
| Designed by | Per Brinch Hansen |
| First appeared | 1993; 33 years ago (1993) |
| Stable release | 1 / 1993; 33 years ago (1993) |
| Typing discipline | Strong |
| Website | brinch-hansen |
| Influenced by | |
| Communicating sequential processes,Pascal,Concurrent Pascal,Joyce,occam | |
SuperPascal is an imperative,concurrent computingprogramming language developed byPer Brinch Hansen.[1] It was designed as apublication language: a thinking tool to enable the clear and concise expression of concepts in parallel programming. This is in contrast with implementation languages which are often complicated with machine details and historical conventions. It was created to address the need at the time for a parallel publication language. Arguably, few languages today are expressive and concise enough to be used as thinking tools.
SuperPascal is based onNiklaus Wirth's sequential languagePascal, extending it with features for safe and efficient concurrency. Pascal itself was used heavily as a publication language in the 1970s. It was used to teachstructured programming practices and featured in text books, for example, oncompilers[2] and programming languages.[3] Hansen had earlier developed the languageConcurrent Pascal,[4] one of the earliest concurrent languages for the design ofoperating systems andreal-time control systems.
The requirements of SuperPascal were based on the experience gained by Hansen over three years in developing a set of model parallel programs, which implemented methods for common problems incomputer science.[5] This experimentation allowed him to make the following conclusions about the future of scientificparallel computing:
These then led to the following requirements for a parallel publication language:
The key ideas in the design of SuperPascal was to provide asecure programming, with abstract concepts for parallelism.[6][7]
SuperPascal issecure in that it should enable its compiler andruntime system to detect as many cases as possible in which the language concepts break down and produce meaningless results.[8] SuperPascal imposes restrictions on the use of variables that enable a single-pass compiler to check that parallel processes are disjoint, even if the processes use procedures with global variables, eliminating time-dependent errors. Several features in Pascal were ambiguous or insecure and were omitted from SuperPascal, such as labels andgoto statements, pointers and forward declarations.[6]
The parallel features of SuperPascal are a subset ofoccam 2, with the added generality of dynamic process arrays and recursive parallel processes.[7]
Aparallel statement denotes that the fixed number of statements it contains must be executed in parallel. For example:
parallel source() | sink()end
Aforall statement denotes the parallel execution of a statement by a dynamic number of processes, for example:
forall i := 0 to 10 do something()
Parallel processes communicate by sending typed messages through channels created dynamically. Channels are not variables in themselves, but are identified by a unique value known as thechannel reference, which are held bychannel variables. A channel is declared, for example, by the declaration
typechannel=*(boolean,integer);varc:channel;
which defines a new (mixed) type namedchannel and a variable of this type namedc. A mixed type channel is restricted to transmitting only the specified types, in this case boolean and integer values. The channelc is initialised by theopen statement:
open(c)
Message communication is then achieved with thesend(channel, value) andreceive(channel, variable) statements. The expression or variable providing the value forsend, and the variable inreceive, must both be of the same type as the first channel argument. The following example shows the use of these functions in a process that receives a value from theleft channel and outputs it on theright one.
varleft,right:channel;a:number;receive(left,a);send(right,a)
The functionssend andreceive can both take multiple input and output arguments respectively:
send(channel, e1, e2,..., en);receive(channel, v1, v2,..., vn)
The followingruntime communication errors can occur:
Recursive procedures can be combined withparallel andforall statements to create parallel recursive processes. The following example shows how apipeline of processes can be recursively defined using aparallel statement.
procedurepipeline(min,max:integer;left,right:channel);varmiddle:channel;beginifmin<maxthenbeginopen(middle);parallelnode(min,left,middle)|pipeline(min+1,max,middle,right)endendelsenode(min,left,right)end;
Another example is the recursive definition of a processtree:
proceduretree(depth:integer,bottom:channel);varleft,right:channel;beginifdepth>0thenbeginopen(left,right);paralleltree(depth-1,left)|tree(depth-1,right)|root(bottom,left,right)endendelseleaf(bottom)
The most difficult aspect of concurrent programming isunpredictable ornon-reproducible behaviour caused bytime-dependent errors. Time-dependent errors are caused byinterference between parallel processes, due to variable updates or channel conflicts. If processes sharing a variable, update it at unpredictable times, the resulting behaviour of the program is time-dependent. Similarly, if two processes simultaneously try to send or receive on a shared channel, the resulting effect is time-dependent.
SuperPascal enforces certain restrictions on the use of variables and communication to minimise or eliminate time-dependent errors. With variables, a simple rule is required: parallel processes can only update disjoint sets of variables.[1] For example, in aparallel statement atarget variable cannot be updated by more than a single process, but anexpression variable (which can't be updated) may be used by multiple processes. In some circumstances, when a variable such as an array is thetarget of multiple parallel processes, and the programmer knows its element-wise usage isdisjoint, then the disjointness restriction may be overridden with a preceding[sic] statement.
SuperPascal is ablock structured language, with the same basic syntax as Pascal. A program consists of aheader,global variable definitions,function orprocedure definitions and amain procedure. Functions and procedures consists ofblocks, where a block is a set ofstatements. Statements areseparated by semicolons, as opposed to languages likeC orJava, where they areterminated by semicolons.
The following is an example of a complete SuperPascal program, which constructs apipeline communication structure with 100 nodes. A master node sends an integer token to the first node, this is then passed along the pipeline and incremented at each step, and finally received by the master node and printed out.
programpipeline;constlen=100;typechannel=*(integer);varleft,right:channel;value:integer;procedurenode(i:integer;left,right:channel);varvalue:integer;beginreceive(left,value);send(right,value+1)end;procedurecreate(left,right:channel);typerow=array[0..len]ofchannel;varc:row;i:integer;beginc[0]:=left;c[len]:=right;fori:=1tolen-1doopen(c[i]);foralli:=1tolendonode(i,c[i-1],c[i])end;beginopen(left,right);parallelsend(left,0)|create(left,right)|receive(right,value)end;writeln('The resulting value is ',value)end.
The SuperPascal software can be accessed freely from the Brinch Hansen Archive.[9] It consists of a compiler and interpreter, which are both written in normal, sequential Pascal (ISO Level 1 standard Pascal). This is supported by the GNU Pascal compiler and newer versions of theFree Pascal compiler (2.7.1+) with the-Miso switch, with the following respective small modifications to the code.
For GPC, the fileinterpret.p uses the non-standardclock function (line 1786), which is used to obtain the system time. Instead, the Extended PascalgetTimeStamp function can be used (which is supported by the GNU Pascal compiler), by declaring a variable of typeTimeStamp, setting that with the current time usinggetTimeStamp and assigning theSecond field of theTimeStamp to the variablet.
Free Pascal also needs a solution to the above "clock" problem (On windows, just declare gettickcount as external with "clock" as name). Further, the reset/rewrites that are marked as non-standard in the source code must be changed to assign/reset (or rewrite) pairs. (GPC probably only errors on this if you enable strict flags), and the C preprocessor commands #include 'xx' must be changed to {$include 'xx'}.
{ Time code for readtime in Freepascal on unix systems }FunctionFpTime(vartloc:integer):integer;externalname'FPC_SYSC_TIME';procedurereadtime(vart:integer);begin{ A nonstandard function reads the processor time in ms }t:=fptime(t);end;
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