BuildSystem is used by PETSc for configuration testing. Written solely in Python, it consists of a number of objects running in a coordinated fashion. Below we describe the main objects involved, and the organization of both the files and in-memory objects during the configure run. However, first we discuss the configure and build process at a higher level.
What is a build?#
The build stage compiles source to object files, stores them somehow (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. However, other parts of Make are not as useful, and we should distinguish the two.
Make uses a single predicate, “older than”, to decide whether to apply a rule. This is a disaster. A useful upgrade to make would expand the list of available predicates, including things like “md5sum has changed” and “flags have changed”. There have been attempts to use Make to determine whether a file has changed, for example by using stamp files. However, it cannot be done without severe contortions which make it much harder to see what Make is doing and maintain the system. Right now, we can combine make with the ccache utility to minimize recompiling and relinking.
Why is configure necessary?#
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?#
PETSc’s fully functional configure model
BuildSystem has also been used as the
configuration tool for other open sources packages. As more than a few 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.
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 SCons, CMake, or Autoconf.
Instead, a flat namespace is used, mirroring the situation in C. 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
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
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.
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.
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
try_compile() test in Autoconf and CMake requires
the entire list of libraries be present in the
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
libstdc++, but they are working with a C project, no
straightforward method exists to add this dependency.
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, builds, and tests the package for inclusion. The native
configure and build system for the package is used, and special support
exists for GNU and CMake packages. No similar system exists in the other
tools, which rely on static declarations, such as
FindPackage.cmake files, that are not tested and often become
obsolete. They also require that any dependent packages use the same
configuration and build system.
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 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.
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 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
#!/usr/bin/env python import os execfile(os.path.join(os.path.dirname(__file__), 'config', 'configure.py'))
configure.py script constructs a tree of configure modules and
executes the configure process over it. A minimal version of this would
package = 'PETSc' def configure(configure_options): # Command line arguments take precedence (but don't destroy argv) 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()
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.
config/package/Configure.py module determines how the tree of
Configure objects is built and how the configure information is output.
configure() method of the module will be run by the
object created at the top level. A minimal
configure() method would look
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
Dump method runs over the tree of configure modules, and outputs
the data necessary for building, usually employing the
addDefine() methods. These
methods funnel output to the include and make files defined by the
framework object, and set at the beginning of this
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
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
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
arch modules define the project root
directory and a build name so that multiple independent builds can be
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.
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,
Output configure header
Log filesystem actions
Children may be added to the Framework using
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
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
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
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
require() calls should be made in that method. The Framework will
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
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
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
addPrototype() also put
information in this header file. Using
addMakeRule() will add make macros and rules to the output makefiles
specified in the framework. In addition we provide
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.