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# BuildSystem

`BuildSystem` (located in `config/BuildSystem`) configures PETSc before PETSc is compiled with make. It is much like GNU Autoconf (configure) but written in Python especially for PETSc.

## What is a build?

The build stage compiles source to object files, stores them (usually in archives), and links shared libraries and executables. These are mechanical operations that reduce to applying a construction rule to sets of files. The Make tool is great at this job.

## Why is configure necessary?

The `configure` program is designed to assemble all information and preconditions necessary for the build stage. This is a far more complicated task, heavily dependent on the local hardware and software environment. It is also the source of nearly every build problem. The most crucial aspect of a configure system is not performance, scalability, or even functionality, but debuggability. Configuration failure is at least as common as success, due to broken tools, operating system upgrades, hardware incompatibilities, user error, and a host of other reasons. Problem diagnosis is the single biggest bottleneck for development and maintenance time. Unfortunately, current systems are built to optimize the successful case rather than the unsuccessful. In PETSc, we have developed the `BuildSystem` package to remedy the shortcomings of configuration systems such as Autoconf, CMake, and SCons.

## Why use PETSc BuildSystem?

As several configuration tools currently exist, it is instructive to consider why PETSc would choose to create another from scratch. Below we list features and design considerations which lead us to prefer `BuildSystem` to the alternatives.

### Namespacing

`BuildSystem` wraps collections of related tests in Python modules, which also hold the test results. Thus results are accessed using normal Python namespacing. As rudimentary as this sounds, no namespacing beyond the use of variable name prefixes is present in Autoconf, CMake, and SCons. Instead, a flat namespace is used. This tendency appears again when composing command lines for external tools, such as the compiler and linker. In the traditional configure tools, options are aggregated in a single bucket variable, such as `INCLUDE` or `LIBS`, whereas in `BuildSystem` one can trace the provenance of a flag before it is added to the command line. CMake also makes the unfortunate decision to force all link options to resolve to full paths, which causes havoc with compiler-private libraries.

### Explicit control flow

The `BuildSystem` configure modules mentioned above, containing one `Configure` object per module, are organized explicitly into a directed acyclic graph (DAG). The user indicates dependence, an edge in the dependence graph, with a single call, `requires('path.to.other.test', self)`, which not only structures the DAG, but returns the `Configure` object. The caller can then use this object to access the results of the tests run by the dependency, achieving test and result encapsulation simply.

### Multi-language tests

`BuildSystem` maintains an explicit language stack, so that the current language can be manipulated by the test environment. A compile or link can be run using any language, complete with the proper compilers, flags, libraries, etc., with a single call. This automation is crucial for cross-language tests, which are thinly supported in current tools. In fact, the design of these tools inhibits this kind of check. The `check_function_exists()` call in Autoconf and CMake looks only for the presence of a particular symbol in a library, and fails in C++ and on Microsoft Windows, whereas the equivalent `BuildSystem` test can also take a declaration. The `try_compile()` test in Autoconf and CMake requires the entire list of libraries be present in the `LIBS` variable, providing no good way to obtain libraries from other tests in a modular fashion. As another example, if the user has a dependent library that requires `libstdc++`, but they are working with a C project, no straightforward method exists to add this dependency.

### Subpackages

The most complicated, yet perhaps most useful, part of `BuildSystem` is support for dependent packages. It provides an object scaffolding for including a 3rd party package (more than 100 are now available) so that PETSc downloads and builds the package for use by PETSc. The native configure and build system for the package is used, and special support exists for Autoconf and CMake packages. No similar system exists in the other tools, which rely on static declarations, such as `pkg-config` or `FindPackage.cmake` files, that are not tested and often become obsolete.

### Batch environments

Most systems, such as Autoconf and CMake, do not actually run tests in a batch environment, but rather require a direct specification, in CMake a “platform file”. This requires a human expert to write and maintain the platform file. Alternatively, `BuildSystem` submits a dynamically generated set of tests to the batch system, enabling automatic cross-configuration and cross-compilation.

### Caching

Caching often seems like an attractive option since configuration can be quite time-consuming, and both Autoconf and CMake enable caching by default. However, no system has the ability to reliably invalidate the cache when the environment for the configuration changes. For example, a compiler or library dependency may be upgraded on the system. Moreover, dependencies between cached variables are not tracked, so that even if some variables are correctly updated after an upgrade, others which depend on them may not be. Moreover, CMake mixes together information which is discovered automatically with that explicitly provided by the user, which is often not tested.

### Concision

The cognitive load is usually larger for larger code bases, and our observation is that the addition of logic to Autoconf and CMake is often quite cumbersome and verbose as they do not employ a modern, higher level language. Although `BuildSystem` itself is not widely used, it has the advantage of being written in Python, a widely-understood, high-level language.

## High level organization

A minimal `BuildSystem` setup consists of a `config` directory off the package root, which contains all the Python necessary to run (in addition to the `BuildSystem` source). At minimum, the `config` directory contains `configure.py`, which is executed to run the configure process, and a module for the package itself. For example, PETSc contains `config/PETSc/petsc.py`. It is also common to include a top level `configure` file to execute the configure, as this looks like Autotools,

    #!/usr/bin/env python3
    import os
    execfile(os.path.join(os.path.dirname(__file__), 'config', 'configure.py'))

The `configure.py` script constructs a tree of configure modules and executes the configure process over it. A minimal version of this would be

    package = 'PETSc'

    def configure(configure_options):
      # Command line arguments take precedence (but don't destroy argv[0])
      sys.argv = sys.argv[:1] + configure_options + sys.argv[1:]
      framework = config.framework.Framework(['--configModules='+package+'.Configure', '--optionsModule='+package+'.compilerOptions']+sys.argv[1:], loadArgDB = 0)
      framework.setup()
      framework.configure(out = sys.stdout)
      framework.storeSubstitutions(framework.argDB)
      framework.printSummary()
      framework.argDB.save(force = True)
      framework.logClear()
      framework.closeLog()

    if __name__ == '__main__':
      configure([])

The PETSc `configure.py` is quite a bit longer than this, as it performs specialized command line processing, error handling, and integrating logging with the rest of PETSc.

The `config/package/Configure.py` module determines how the tree of `Configure` objects is built and how the configure information is output. The `configure()` method of the module will be run by the `Framework` object created at the top level. A minimal `configure()` method would look like

    def configure(self):
      self.framework.header          = self.arch.arch+'/include/'+self.project+'conf.h'
      self.framework.makeMacroHeader = self.arch.arch+'/conf/'+self.project+'variables'
      self.framework.makeRuleHeader  = self.arch.arch+'/conf/'+self.project+'rules'

      self.Dump()
      self.logClear()
      return

The `Dump` method runs over the tree of configure modules, and outputs the data necessary for building, usually employing the `addMakeMacro()`, `addMakeRule()` and `addDefine()` methods. These methods funnel output to the include and make files defined by the framework object, and set at the beginning of this `configure()` method. There is also some simple information that is often used, which we define in the initializer,

    def __init__(self, framework):
      config.base.Configure.__init__(self, framework)
      self.Project      = 'PETSc'
      self.project      = self.Project.lower()
      self.PROJECT      = self.Project.upper()
      self.headerPrefix = self.PROJECT
      self.substPrefix  = self.PROJECT
      self.framework.Project = self.Project
      return

More sophisticated configure assemblies, like PETSc, output some other custom information, such as information about the machine, configure process, and a script to recreate the configure run.

The `Package` configure module has two other main functions. First, top level options can be defined in the `setupHelp()` method,

    def setupHelp(self, help):
      import nargs
      help.addArgument(self.Project, '-prefix=<path>', nargs.Arg(None, '', 'Specify location to install '+self.Project+' (eg. /usr/local)'))
      help.addArgument(self.Project, '-load-path=<path>', nargs.Arg(None, os.path.join(os.getcwd(), 'modules'), 'Specify location of auxiliary modules'))
      help.addArgument(self.Project, '-with-shared-libraries', nargs.ArgBool(None, 0, 'Make libraries shared'))
      help.addArgument(self.Project, '-with-dynamic-loading', nargs.ArgBool(None, 0, 'Make libraries dynamic'))
      return

This uses the `BuildSystem` help facility that is used to define options for all configure modules. The first argument groups these options into a section named for the package. The second task is to build the tree of modules for the configure run, using the `setupDependencies()` method. A simple way to do this is by explicitly declaring dependencies,

    def setupDependencies(self, framework):
        config.base.Configure.setupDependencies(self, framework)
        self.setCompilers  = framework.require('config.setCompilers',                self)
        self.arch          = framework.require(self.Project+'.utilities.arch',       self.setCompilers)
        self.projectdir    = framework.require(self.Project+'.utilities.projectdir', self.arch)
        self.compilers     = framework.require('config.compilers',                   self)
        self.types         = framework.require('config.types',                       self)
        self.headers       = framework.require('config.headers',                     self)
        self.functions     = framework.require('config.functions',                   self)
        self.libraries     = framework.require('config.libraries',                   self)

        self.compilers.headerPrefix  = self.headerPrefix
        self.types.headerPrefix      = self.headerPrefix
        self.headers.headerPrefix    = self.headerPrefix
        self.functions.headerPrefix  = self.headerPrefix
        self.libraries.headerPrefix  = self.headerPrefix

The `projectdir` and `arch` modules define the project root directory and a build name so that multiple independent builds can be managed. The `Framework.require()` method creates an edge in the dependency graph for configure modules, and returns the module object so that it can be queried after the configure information is determined. Setting the header prefix routes all the defines made inside those modules to our package configure header. We can also automatically create configure modules based upon what we see on the filesystem,

    for utility in os.listdir(os.path.join('config', self.Project, 'utilities')):
      (utilityName, ext) = os.path.splitext(utility)
      if not utilityName.startswith('.') and not utilityName.startswith('#') and ext == '.py' and not utilityName == '__init__':
        utilityObj                    = self.framework.require(self.Project+'.utilities.'+utilityName, self)
        utilityObj.headerPrefix       = self.headerPrefix
        utilityObj.archProvider       = self.arch
        utilityObj.languageProvider   = self.languages
        utilityObj.precisionProvider  = self.scalartypes
        utilityObj.installDirProvider = self.installdir
        utilityObj.externalPackagesDirProvider = self.externalpackagesdir
        setattr(self, utilityName.lower(), utilityObj)

The provider modules customize the information given to the module based upon settings for our package. For example, PETSc can be compiled with a scalar type that is single, double, or quad precision, and thus has a `precisionProvider`. If a package does not have this capability, the provider setting can be omitted.

## Main objects

### Framework

The `config.framework.Framework` object serves as the central control for a configure run. It maintains a graph of all the configure modules involved, which is also used to track dependencies between them. It initiates the run, compiles the results, and handles the final output. It maintains the help list for all options available in the run. The `setup()` method performs generic `Script` setup and then is called recursively on all the child modules. The `cleanup()` method performs the final output and logging actions,

- Substitute files

- Output configure header

- Log filesystem actions

Children may be added to the Framework using `addChild()` or `getChild()`, but the far more frequent method is to use `require()`. Here a module is requested, as in `getChild()`, but it is also required to run before another module, usually the one executing the `require()`. This provides a simple local interface to establish dependencies between the child modules, and provides a partial order on the children to the Framework.

A backwards compatibility mode is provided for which the user specifies a configure header and set of files to experience substitution, mirroring the common usage of Autoconf. Slight improvements have been made in that all defines are now guarded, various prefixes are allowed for defines and substitutions, and C specific constructs such as function prototypes and typedefs are removed to a separate header. However, this is not the intended future usage. The use of configure modules by other modules in the same run provides a model for the suggested interaction of a new build system with the Framework. If a module requires another, it merely executes a `require()`. For instance, the PETSc configure module for hypre requires information about MPI, and thus contains

    self.mpi = self.framework.require("config.packages.MPI", self)

Notice that passing self for the last arguments means that the MPI module will run before the hypre module. Furthermore, we save the resulting object as `self.mpi` so that we may interrogate it later. hypre can initially test whether MPI was indeed found using `self.mpi.found`. When hypre requires the list of MPI libraries in order to link a test object, the module can use `self.mpi.lib`.

### Base

The `config.base.Configure` is the base class for all configure objects. It handles several types of interaction. First, it has hooks that allow the Framework to initialize it correctly. The Framework will first instantiate the object and call `setupDependencies()`. All `require()` calls should be made in that method. The Framework will then call `configure()`. If it succeeds, the object will be marked as configured. Second, all configure tests should be run using `executeTest()` which formats the output and adds metadata for the log.

Third, all tests that involve preprocessing, compiling, linking, and running operator through `base`. Two forms of this check are provided for each operation. The first is an “output” form which is intended to provide the status and complete output of the command. The second, or “check” form will return a success or failure indication based upon the status and output. The routines are

    outputPreprocess(), checkPreprocess(), preprocess()
    outputCompile(),    checkCompile()
    outputLink(),       checkLink()
    outputRun(),        checkRun()

The language used for these operation is managed with a stack, similar to Autoconf, using `pushLanguage()` and `popLanguage()`. We also provide special forms used to check for valid compiler and linker flags, optionally adding them to the defaults.

    checkCompilerFlag(), addCompilerFlag()
    checkLinkerFlag(),   addLinkerFlag()

You can also use `getExecutable()` to search for executables.

After configure tests have been run, various kinds of output can be generated.A #define statement can be added to the configure header using `addDefine()`, and `addTypedef()` and `addPrototype()` also put information in this header file. Using `addMakeMacro()` and `addMakeRule()` will add make macros and rules to the output makefiles specified in the framework. In addition we provide `addSubstitution()` and `addArgumentSubstitution()` to mimic the behavior of Autoconf if necessary. The object may define a `headerPrefix` member, which will be appended, followed by an underscore, to every define which is output from it. Similarly, a `substPrefix` can be defined which applies to every substitution from the object. Typedefs and function prototypes are placed in a separate header in order to accommodate languages such as Fortran whose preprocessor can sometimes fail at these statements.