Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!hc!lll-winken!uunet!mcvax!ukc!icdoc!syma!aarons From: aarons@syma.sussex.ac.uk (Aaron Sloman) Newsgroups: comp.arch Subject: Re: Independent Architecture Complilers Summary: a two level VM aids portability of multiple languages Keywords: Poplog, portability Message-ID: <933@syma.sussex.ac.uk> Date: 28 Apr 89 06:44:17 GMT References: <2889@rti.UUCP> Organization: School of Cognitive & Computing Sciences, Sussex Univ. UK Lines: 218 tassos@rti.UUCP (Tassos Markas) writes: > Date: 14 Apr 89 01:20:12 GMT > Organization: Research Triangle Institute, RTP, NC > > I'm looking for independent architecture compilers. > This compiler should accept a high level langauage (I would prefer C) and > produce microcode that can be easily retargeted for any system architecture. > The Poplog two-level virtual machine with the two levels linked by a machine-independend and language independent compiler provides relatively easy portability for incremental compilers for a range of high level languages, though not C at present. It also makes it relatively easy to add a new new high level language that immediately runs (with a rich environment) on all the target architectures. I append a more detailed description. I hope it is of some interest, and I apologise for its length. Although Sussex University has a commercial interest in Poplog I have tried to avoid raising any commercial issues. --------------------- Poplog provides development tools for a range of languages: Common Lisp, Prolog, ML and POP-11 (a Lisp-like language with a more readable Pascal-like syntax). It also provides tools for adding new incremental compilers. It might be possible to add an incremental compiler for C, though it would not run very fast. However, from late 1989 we expect to give users access to a C-like extension to Pop-11 that is used for developing and porting Poplog. It is not quite as fast as C but provides far more facilities. Before I describe porting I need to explain how the running system works. The mechanisms described below were designed and implemented by John Gibson, at Sussex University. All the languages in Poplog compile to a common virtual machine, the Poplog VM which is then compiled to native machine code. First an over-simplified description: The Poplog system allows different languages to share a common store manager, and common data-types, so that a program in one language can call another and share data-structures. There is also a common interface to the host operating system and an "external" interface, to non-Poplog languages (C, Fortran, etc). The Poplog languages are incrementally compiled for rapid development and testing: individual procedures can rapidly be compiled, tested, modified and re-compiled and are immediately automatically linked in to the rest of the system, old versions being garbage collected if no longer pointed to. The languages are all implemented using a set of tools for adding new incremental compilers. These tools include procedures for breaking up a text stream into items, and tools for planting VM instructions when procedures are compiled. These tools are used by the Poplog developers to implement the four Poplog languages but are also available for users to implement new languages suited to particular applications. All this makes it possible to build a range of portable incremental compilers for different sorts of programming languages. POP-11, PROLOG, COMMON LISP and ML all compile to a common internal representation, and share machine-specific run-time code generators. Thus several different machine-independent "front ends" for different languages can share a machine-specific "back end" which compiles to native machine code, which runs far more quickly than if the new language had been interpreted. The actual story is more complicated: there are two Poplog virtual machines, a high level and a low level one, both of which are language independent and machine independent. The high level VM has powerful instructions, which makes it convenient as a target language for compilers for high level languages. This includes special facilities to support Prolog operations, dynamic and lexical scoping of variables, procedure definitions, procedure calls, suspending and resuming processes, and so on. Because these are quite sophisticated operations, the mapping from the Poplog VM to native machine code is still fairly complex. So there is a machine independent and language independent intermediate compiler which compiles from the high level VM to to a low level VM, doing a considerable amount of optimisation on the way. A machine-specific back-end then translates the low-level VM to native machine code, except when porting or re-building the system. In the latter case the final stage is translation to assembly language. (See diagram below.) The bulk of the core Poplog system is written in an extended dialect of POP-11, with provision for C-like addressing modes, for efficiency. We call it SYSPOP. The system sources, mostly written in SYSPOP, are also compiled to the high-level VM, and then to the low level VM. But instead of then being translated to machine code, the low level instructions are automatically translated to assembly language files for the target machine. This is much easier than producing object files, because there is a fairly straight-forward mapping from the low level VM to assembly language, and the programs that do the translation don't have to worry about formats for object files: we leave that to the assembler and linker supplied by the manufacturer. In fact, the system sources need facilities not available to users, so the two intermediate virtual machines are slightly enhanced for SYSPOP. The following diagram summarises the situation. {POP-11, COMMON LISP, PROLOG, ML, SYSPOP, etc} | Compile to [language specific] | V [High level VM] (extended for SYSPOP) | Optimise & compile to | V [Low level VM] (modified for SYSPOP) | Compile (translate) to [machine specific] | V [Native machine instructions] [or assembler - for SYSPOP] So for ordinary users compiling or re-compiling procedures during software development the built in machine code generator is used and compilation is very fast, with no linking required. For rebuilding the whole system the back end is changed to generate assembler, and rebuilding is much slower. But it does not need to be done very often. All the compilers and translators are implemented in Poplog (mostly in POP-11). Only the last stage is machine specific. The low level VM is at a level that makes it possible on the VAX, for example, to generate approximately one machine instruction per low level VM instruction. So writing the code generator for something like a VAX or M68020 was relatively easy. For a RISC machine the task is a little more complicated. Porting to a new computer requires the run-time "back end", i.e. the low level VM compiler, to be changed and also the system-building tools which output assembly language programs for the target machine. There are also a few hand-coded assembly files which have to be re-written for each machine. Thereafter all the high level languages have incremental compilers for the new machine. (The machine-independent system building tools perform rather complex tasks, such as creating a dictionary of procedure names and system variables that have to be accessible to users at run time. So besides translating system source files, the tools create additional assembler files and also check for consistency between the different system source files.) The Poplog VM provides a varied, extendable set of data-types and operations thereon, including facilities for logic programming, list, record and array processing, 'number crunching', sophisticated control structures (e.g. co-routines), 'active variables' and 'exit actions', that is instructions executed whenever a procedure exits, whether normally or abnormally. Indefinite precision arithmetic, ratios and complex numbers are accessible to all the languages that need them. Both dynamic and lexical scoping of variables are provided. A tree-structured "section" mechanism (partly like packages) gives further support for modular design. External modules (e.g. programs in C or Fortran) can be dynamically linked in and unlinked. A set of facilities for accessing the operating system is also provided. The VM facilities are relatively easy to port to a range of computers and operating systems because the core system is mostly implemented in SYSPOP, and is largely machine independent. Only the machine-dependent portions mentioned above (e.g. run-time code generator, and translator from low level VM to assembler), plus a small number of assembler files need be changed for a new machine (unless the operating system is also new). Since the translators are all written in a high level AI language, altering them is relatively easy. Porting requires compiling all the SYSPOP system sources, to generate the corresponding new assmbler files, then moving them and the hand-made assembler files to the new machine, where they are assembled then linked. The same process is used to rebuild the system on an existing machine when new features are added deep in the system. Much of the system is in source libraries compiled as needed by users, and modifying those components does not require re-building. Using this mechanism an experienced programmer with no prior knowledge of Poplog or the target processor was able to port Poplog to a RISC machine in about 7 months. But for the usual crop of bugs in the operating system, assembler, and other software of the new machine the actual porting time would have been shorter. In general, extra time is required for user testing, producing system specific documentation, tidying up loose ends etc. Thus 7 to 12 months work ports incremental compilers for four sophisticated languages, a screen editor, and a host of utilities. Any other languages implemented by users using the compiler-building tools should also run immediately. So in principle this mechanism allows a fixed amount of work to port an indefinitely large number of incremental compilers. Additional work will be required if the operating system is different from Unix or VMS, or if a machine specific window manager has to be provided. This should not be necessary for workstations supporting X-windows. POPLOG is too big for 80286-based PCs. Currently it runs on VAX (VMS/Unix), Sun2, Sun3, Sun4(SPARC), Sun386i (Road-runner), HP 9000 300 series workstations with HPUX Apollo 680?0 worstations with Bsd Unix Sequent Symmetry with Dynix Orion 1/05 (with Clipper). This version is not supported at present Aaron Sloman, School of Cognitive and Computing Sciences, Univ of Sussex, Brighton, BN1 9QN, England INTERNET: aarons%uk.ac.sussex.cogs@nsfnet-relay.ac.uk aarons%uk.ac.sussex.cogs%nsfnet-relay.ac.uk@relay.cs.net JANET aarons@cogs.sussex.ac.uk BITNET: aarons%uk.ac.sussex.cogs@uk.ac or aarons%uk.ac.sussex.cogs%ukacrl.bitnet@cunyvm.cuny.edu UUCP: ...mcvax!ukc!cogs!aarons or aarons@cogs.uucp