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# 7. Configuration

## 7.1. 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]

## 7.2. History

.hist.0: Initial draft created by Richard Brooksby 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 at meeting.general.1997-02-19.

.hist.2: Converted from MMInfo database design document. Richard Brooksby, 2002-06-07.

.hist.3: Updated for variety-reform branch, to remove untrue things, though the document could do with a rewrite. Richard Brooksby, 2012-09-03.

.hist.4: Converted to reStructuredText. Gareth Rees, 2013-03-19.

## 7.3. 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 was 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.]

.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.

## 7.4. 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.

## 7.5. 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.

## 7.6. The build system

### 7.6.1. 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, of the form:

    CONFIG_VAR_<variety-code>

A variety-code is a code that appears after `variety.` in the tag of the relevant variety document (see design.mps.variety), converted to upper case. Currently (2012-09-03) the valid variety codes are `RASH`, `HOT`, `COOL`, `DIAG`, and `TI`.

If no `CONFIG_VAR_` is present, `HOT` is assumed in `config.h`.

.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.

### 7.6.2. 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?]

### 7.6.3. 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 (plus the `an` suffix if there are secondary implementations: see .mod.secondary).

..mod.secondary: The names of other implementations should begin with the same prefix (the module identifier, 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.

### 7.6.4. 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.

## 7.7. Implementation

.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).

Build parameter → | Header → | Configuration parameters
--- | --- | ---
`CONFIG_VAR_COOL` | `config.h` | `AVER_AND_CHECK`, `EVENT_ALL`, etc.
`_WIN32` | `mpstd.h` | `MPS_OS_W3`, `MPS_BUILD_MV`, 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).

### 7.7.1. 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 conjunction 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 Macros.
     */

    #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). 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 base-2 logarithm 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

### 7.7.2. 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), for example, `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 an `#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.]

## 7.8. 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).

### 7.8.1. 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>

### 7.8.2. 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.

### 7.8.3. 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 is allowed:

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

And so is this:

    #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 is not allowed:

    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.

## 7.9. Procedures

[Adding an architecture, etc.]

## 7.10. Notes

What about constants?

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