THE DESIGN OF MPS CONFIGURATION
                           design.mps.config
                           incomplete design
                           richard 1997-02-19

INTRODUCTION

.intro: This document describes how the MPS configuration is parameterized so 
that it can target different architectures, operating systems, build 
environments, varieties, and products.

.bg: For background see [build system mail, configuration mail, 
meeting.general.something]


Document History

.hist.0: Initial draft created by Richard Brooksby <richard> on 1997-02-19 
based on discussions of configuration at meeting.general.1997-02-05.

.hist.1: Various improvements and clarifications to the draft discussed between 
Richard and Nick Barnes <nickb> at meeting.general.1997-02-19.


REQUIREMENTS

.req.arch: Allow architecture specific configurations of the MPS.

.req.os: Allow operating system specific configurations of the MPS.

.req.builder: Allow build environment (compiler, etc.) specific configurations 
of the MPS.

.req.prod: Allow product specific
configurations of the MPS.  [This requirement has been retired on
2012-09-03.  Client-specific customisation of the MPS will be handled by
configuration management, while the MPS source remains generic, to
reduce costs and increase reliability.  RB 2012-09-03]

.req.var: Allow configurations with different amounts of instrumentation 
(assertions, metering, etc.).

.req.impact: The configuration system should have a minimal effect on 
maintainability of the implementation.

.req.port: The system should be easy to port across operating systems.

.req.maint: Maintenance of the configuration and build system should not 
consume much developer time.



DEFINITIONS

.def.platform: A platform is a combination of an architecture (.def.arch), an 
operating system (.def.os), and a builder (.def.builder).  The set of supported 
platforms is platform.*.

.def.arch: An architecture is processor type with associated calling 
conventions and other binary interface stuff.

.def.os: An operating system is the interface to external resources.

.def.builder: A builder is the tools (C compiler, etc.) used to make the target 
(.def.target).

.def.var: A variety is a combination of annotations such as assertions, 
metering, etc.

.def.prod: A product is the intended product into which the MPS will fit, e.g. 
ScriptWorks, Dylan, etc.

.def.target: The target is the result of the build.


OVERVIEW

- No automatically generated code.  Use only C compiler and linker.
- Simple build function (design.mps.buildsys.????)
- Avoid conditional code spaghetti in implementations.
- Dependency on a particular configuration should be minimized and localized 
when developing code.


THE BUILD SYSTEM


Abstract Build Function

.build.fun: The MPS implementation assumes only a simple "build function" which 
takes a set of sources, possibly in several languages, compiles them with a set 
of predefined preprocessor symbols, and links the result with a set of 
libraries to form the target:

  target := build(<defs>, <srcs>, <libs>)

.build.sep: Separate compilation and linkage can be seen as a memoization of 
this function, and is not strictly necessary for the build.

.build.cc: A consequence of this approach is that it should always be possible 
to build a complete target with a single UNIX command line calling the compiler 
driver (usually "cc" or "gcc"), for example:

  cc -o main -DCONFIG_VAR_DF foo.c bar.c baz.s -lz

.build.defs: The "defs" are the set of preprocessor macros which are to be 
predefined when compiling the module sources.

  CONFIG_VAR_<variety-code>

The variety-codes are the letter code that appears after "variety." in the 
tag of the relevant variety document (see variety.*) converted to upper case.  
Currently (2012-09-03): RASH, HOT, COOL, DIAG, TI

If no CONFIG_VAR is present, HOT is assumed in .

.build.srcs: The "srcs" are the set of sources that must be compiled in order 
to build the target.  The set of sources may vary depending on the 
configuration.  For example, different sets of sources may be required to build 
on different architectures.  [This is a dependency between the makefile (or whatever) 
and the module configuration in config.h.]

.build.libs: The "libs" are the set of libraries to which the compiled sources 
must be linked in order to build the target.  For example, when building a test 
program, it might include the ANSI C library and an operating system interface 
library.


File Structure

.file.dir: Each product consists of a single directory (corresponding to a HOPE 
compound) containing all the sources for the whole family of targets.  
.file.base: The names of sources must be unique in the first eight characters 
in order to conform to FAT filesystem naming restrictions.  .file.ext: The 
extension may be up to three characters and directly indicates the source 
language.

[Where is the set of valid extensions and languages defined?]


Modules and Naming

.mod.unique: Each module has an identifier which is unique within the MPS.  
.mod.impls: Each module has one or more implementations which may be in any 
language supported by the relevant build environment.  .mod.primary: The 
primary implementation of a module is written in target-independent ANSI C in a 
source file with the same name as the module.  [This seems to be with an "an" 
suffix now.  GavinM 1997-08-07]  .mod.secondary: The names of other 
implementations should begin with the same prefix (the module id or a shortened 
version of it) and be suffixed with on or more target parameter codes (defined 
below).  In particular, the names of assembly language sources must include the 
target parameter code for the relevant architecture.


Build System Rationale

.build.rat: This simple design makes it possible to build the MPS using many 
different tools.  Microsoft Visual C++, Metrowerks Codewarrior, and other 
graphical development tools do not support much in the way of generated 
sources, staged building, or other such stuff.  The Visual C and Metrowerks 
"project" files correspond closely to a closure of the build function 
(.build.fun).  The simplicity of the build function has also made it easy to 
set up builds using NMAKE (DOS), MPW (Macintosh), and to get the MPS up and 
running on other platforms such as FreeBSD and Linux in very little time.  The 
cost of maintaining the build systems on these various platforms is also 
reduced to a minimum, allowing the MM Group to concentrate on primary 
development.  The source code is kept simple and straightforward.  When looking 
at MPS sources you can tell exactly what is going to be generated with very 
little context.  The sources are not munged beyond the standard ANSI C 
preprocessor.

.build.port: The portability requirement (.req.port) implies that the build 
system must use only standard tools that will be available on all conceivable 
target platforms.  Experience of development environments on the Macintosh 
(Metrowerks Codewarrior) and Windows NT (Visual C++) indicates that we cannot 
assume much sophistication in the use of file structure by development 
environments.  The best that we can hope for is the ability to combine a fixed 
list of source files, libraries, and predefined preprocessor symbols into a 
single target.

.build.maint: The maintainability requirement (.req.maint) implies that we 
don't spend time trying to develop a set of tools to support anything more 
complicated than the simple build function described above.  The effort in 
constructing and maintaining a portable system of this kind is considerable.  
Such efforts have failed in EP.


IMPLEMENTATION

[ Now in impl.h.config, may symbols out of date.  GavinM 1997-08-07 ]

.impl: The two implementation files impl.h.config and impl.h.mpstd can be seen 
as preprocessor programs which "accept" build parameters and "emit" 
configuration parameters (.fig.impl).  The build parameters are defined either 
by the builder (in the case of target detection) or by the build function (in 
the case of selecting the variety).

.fig.impl:

   build parameters                       configuration parameters

   CONFIG_VAR_COOL     -->  config.h  --> ASSERT_AND_CHECK, TELEMETRY, etc.

   _WIN32              -->  mpstd.h   --> MPS_OS_W3, etc.

.impl.dep: No source code, other than the directives in impl.h.config and 
impl.h.mpstd, should depend on any build parameters.  That is, identifiers 
beginning "CONFIG_" should only appear in impl.h.config.  Code may depend on 
configuration parameters in certain, limited ways, as defined below (.conf).


Target Platform Detection

.pf: The target platform is "detected" by the preprocessor directives in 
impl.h.mpstd.
.pf.form: This file consists of sets of directives of the form:

  #elif <conjunction of builder predefinitions>
  #define MPS_PF_<platform code>
  #define MPS_OS_<operating system code>
  #define MPS_ARCH_<architecture code>
  #define MPS_BUILD_<builder code>
  #define MPS_T_WORD           <word type>
  #define MPS_WORD_SHIFT       <word shift>
  #define MPS_PF_ALIGN         <minimum alignment>

.pf.detect: The conjunction of builder predefinitions is a constant expression 
which detects the target platform.  It is a logical AND of expressions which 
look for preprocessor symbols defined by the build environment to indicate the 
target.  These must be accompanied by a reference to the build tool 
documentation from which the symbols came.  For example:

  /* Visual C++ 2.0, Books Online, C/C++ Book, Preprocessor Reference, */
  /* Chapter 1: The Preprocessor, Macros, Predefined 

  #elif defined(_MSC_VER) && defined(_WIN32) && defined(_M_IX86)

.pf.codes: The declarations of the platform, operating system, architecture, 
and builder codes define preprocessor macros corresponding the the target 
detected (.pfm.detect).  For example:

  #define MPS_PF_W3I3MV
  #define MPS_OS_W3
  #define MPS_ARCH_I3
  #define MPS_BUILD_MV

.pf.word: The declaration of MPS_T_WORD defines the unsigned integral type 
which corresponds, on the detected target, to the machine word.  It is used to 
defined the MPS Word type (design.mps.type.word).  [Insert backwards ref 
there.]  For example:

  #define MPS_T_WORD      unsigned long

.pf.word-width: The declaration of MPS_WORD_WIDTH defines the number of bits in 
the type defined by MPS_T_WORD (.pf.word) on the target.  For example:

  #define MPS_WORD_WIDTH  32

.pf.word-shift: The declaration of MPS_WORD_SHIFT defines the log to the base 2 
of MPS_WORD_WIDTH.  For example:

  #define MPS_WORD_SHIFT  5

.pf.pf-align: The declaration of MPS_PF_ALIGN defines the minimum alignment 
which must be used for a memory block to permit any normal processor memory 
access.  In other words, it is the maximum alignment required by the processor 
for normal memory access.  For example:

  #define MPS_PF_ALIGN    4


Target Varieties

.var: The target variety is handled by preprocessor directives in 
impl.h.config.  .var.form: The file contains sets of directives of the form:

  #elif defined(CONFIG_VAR_DF)
  #define MPS_VAR_DF
  #define ASSERT_MPSI
  #define ASSERT_MPM
  etc.

.var.detect: The configured variety is one of the variety preprocessor 
definitions passed to the build function (.build.defs), e.g. CONFIG_VAR_DF.  
[These are decoupled so that it's possible to tell the difference between 
overridden settings etc.  Explain.]

.var.symbols: The directives should define whatever symbols are necessary to 
control annotations.  These symbols parameterize other parts of the code, such 
as the declaration of assertions, etc.  The symbols should all begin with the 
prefix "MPS_VAR_".


[Tidy this up:]
Note, anything which can be configured, is configured, even if it's just 
configured to "NONE" meaning nothing.  This makes sure that you can't choose 
something by omission.  Where these symbols are used there will be a #error to 
catch the unused case.
Exception: To allow simple building of the MPS with "cc -c mps.c" we choose
CONFIG_VAR_HOT by default.
[This is a general principle which applies to other configuration stuff too.]


SOURCE CODE CONFIGURATION

.conf: This section describes how the configuration may affect the source code 
of the MPS.

.conf.limit: The form of dependency allowed is carefully limited to ensure that 
code remains maintainable and portable (.req.impact).

.conf.min: The dependency of code on configuration parameters should be kept to 
a minimum in order to keep the system maintainable (.req.impact).


Configuration Parameters

.conf.params: The compilation of a module is parameterized by:

  MPS_ARCH_<arch-code>
  MPS_OS_<os-code>
  MPS_BUILDER_<builder-code>
  MPS_PF_<platform-code>


Abstract and Concrete Module Interfaces

Basic principle: the caller must not be affected by configuration of a module.  
This reduces complexity and dependency of configuration.

All callers use the same abstract interface.  Caller code does not change.

Abstract interface includes:
  - method definitions (logical function prototypes which may be macro methods)
  - names of types
  - names of constants
  - names of structures and fields which form part of the interface, and 
possibly their types, depending on the protocol defined
  - the protocols

The abstract interface to a module may not be altered by a configuration 
parameter.  However, the concrete interface may vary.


Configuring Module Implementations

For example, this isn't allowed, because there is a change in the interface.

  #if defined(PROT_FOO)
  void ProtSpong(Foo foo, Bar bar);
  #else
  int ProtSpong(Bar bar, Foo foo);
  #endif

This example shows how:

  #ifdef PROTECTION
  void ProtSync(Space space);
  /* more decls. */
  #else /* PROTECTION not */
  #define ProtSync(space) NOOP
  /* more decls. */
  #endif /* PROTECTION */

or

  #if defined(PROT_FOO)
  typedef struct ProtStruct {
    int foo;
  } ProtStruct;
  #define ProtSpong(prot)  X((prot)->foo)
  #elif defined(PROT_BAR)
  typedef struct ProtStruct {
    float bar;
  } ProtStruct;
  #define ProtSpong(prot)  Y((prot)->bar)
  #else
  #error "No PROT_* configured."
  #endif

Configuration parameters may not be used to vary implementations in .c files.  
For example, this sort of thing:

  int map(void *base, size_t size)
  {
  #if defined(MPS_OS_W3)
    VirtualAlloc(foo, bar, base, size);
  #elif defined(MPS_OS_SU)
    mmap(base, size, frob);
  #else
  #error "No implementation of map."
  #endif
  }

This leads to extreme code spaghetti.  In effect, it's a "candy machine 
interface" on source code.  This kind of thing should be done by having several 
implementations of the same interface in separate source files.  If this leads 
to duplication of code then that code should be placed in a separate, common 
module.


PROCEDURES

[Adding an architecture, etc.]


NOTES

What about constants?

To do:
- Renaming of some stuff.
- Introduce product selection.
- Change makefiles.
- Eliminate mpmconf.h by moving stuff to config.h.
- Update files to refer to this design document.