Build System

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Build System

gem5's build system is based on SCons, an open source build system implemented in Python. You can find more information about scons at The main scons file is called SConstruct and is found in the root of the source tree. Additional scons files are named SConscript and are found throughout the tree, usually near the files they're associated with.

Build targets

In gem5, scons build targets are of the form <build dir>/<configuration>/<target>. For example:

scons build/ARM/gem5.opt

The <build dir> part of the target is a directory path that ends in "build". Typically this is simply "build" by itself (as in the example), but you can specify a directory called "build" located somewhere else instead. All targets under the same build dir are assumed to be compiled for the same host platform, and share the same global variable settings. These settings are stored in the build/ file.

The <configuration> part ("ARM" in the example above) selects a set of compile-time build options that control simulator functionality, such as the ISA used, which CPU models are included, which Ruby coherence protocol to use, etc. These per configuration variables are stored in separate files in the build/variables directory. This location makes it possible to clean out a build by deleting the configuration directory completely (e.g., using 'rm -rf') without losing the variable settings.

The first time a particular configuration is built, the configuration name is also used to determine the default per-configuration variable settings by looking for a matching name in the build_opts directory. If you would like to create a new configuration with a name not found there, use the --default flag to tell scons which file in build_opts or build/variables to use for the default.

The build target ("gem5.opt" in the example above) is typically the name of the gem5 binary to build, which specifies the set of compiler flags used. Currently supported versions are gem5.debug, gem5.opt,, and gem5.perf.

  • gem5.debug has optimizations turned off. This ensures that variables won't be optimized out, functions won't be unexpectedly inlined, and control flow will not behave in surprising ways. That makes this version easier to work with in tools like gdb, but without optimizations this version is significantly slower than the others. You should choose it when using tools like gdb and valgrind and don't want any details obscured, but other wise more optimized versions are recommended.
  • gem5.opt has optimizations turned on and debugging functionality like asserts and DPRINTFs left in. This gives a good balance between the speed of the simulation and insight into what's happening in case something goes wrong. This version is best in most circumstances.
  • has optimizations turned on and debugging functionality compiled out. This pulls out all the stops performance wise, but does so at the expense of run time error checking and the ability to turn on debug output. This version is recommended if you're very confident everything is working correctly and want to get peak performance from the simulator.
  • is similar to but also includes instrumentation that allows it to be used with the gprof profiling tool. This version is not needed very often, but can be used to identify the areas of gem5 that should be focused on to improve performance.
  • gem5.perf also includes instrumentation, but does so using google perftools, allowing it to be profiled with google-pprof. This profiling version is complementary to, and can probably replace it for all Linux-based systems.

These versions are summarized in the following table.

Binary name Optimizations Run time debugging support Profiling support
gem5.debug X
gem5.opt X X X X X
gem5.perf X X

Command line options

Scons will recognize the following command line options specific to gem5.

Option Effect
--colors Turn on colorized output
--no-colors Turn off colorized output
--default Override which existing build configuration or build_opts file to use for defaults
--ignore-style Disable style checking hooks
--update-ref Update test reference outputs
--verbose Print full tool command lines

Environment variables

The following environment variables are imported from the host environment for use in scons:

Variable Use
AS Assembler command
AR Archive tool command
CC C compiler command
CXX C++ compiler command
HOME User's home directory
LD_LIBRARY_PATH Path to search for library files at loading time
LIBRARY_PATH Path to search for library files at linking time
PATH Path to search for programs
PROTOC protobuf compiler command
PYTHONPATH Path to search for python files
RANLIB Ranlib command
M5_CONFIG Where to look for the special ".m5" directory
M5_DEFAULT_BINARY The default build target which overrides the default default build/ALPHA/gem5.debug

Configuration variables

These configuration variables are used to control the way gem5 is built. Some are global, affecting all configurations in a build directory, and some only affect the configuration being built. Unlike the command line options, these variables retain their value between invocations of scons.


Variable Description Default
CC C Compiler CC environment variable or value determined by scons
CXX C++ Compiler CXX environment variable or value determined by scons
BATCH Use batch pool for build and tests False
BATCH_CMD Batch pool submission command qdo
M5_BUILD_CACHE Cache built objects in this directory False
EXTRAS Add extra directories to the compilation

Per Configuration

Variable Description Default Exported as config/*.hh
CP_ANNOTATE Enable critical path annotation capability False X
CPU_MODELS CPU Models AtomicSimpleCPU,InOrderCPU,O3CPU,TimingSimpleCPU
PROTOCOL Coherence protocol for Ruby MI_example X
SS_COMPATIBLE_FP Make floating-point results compatible with SimpleScalar False X
USE_CHECKER Use checker for detailed CPU models False X
USE_FENV Use <fenv.h> IEEE mode control whether fenv.h was found on this host X
USE_POSIX_CLOCK Use POSIX Clocks whether posix clocks are available on this host X

Setting configuration variable values

The first way you set configuration variable values is through the configuration name you choose as part of the build target. This file is loaded from build_opts and contains preset values for some of these variables which configures the build as the file name suggests.

It is important to note that the values in the file corresponding to the configuration you picked are -default- values and are only used if no directory already exists with its own values already in place. Those files are for defining reasonable starting points to configure gem5 to behave the way you want it to, and are not intended to actively configure a particular build.

If you want to change a value after your build and configuration directory is already created, or if you want to override a value as it's created, you can specify the new values on the command line. The syntax is similar to setting environment variables at a shell prompt, but these go after the scons command. For example, to build MESI_CMP_directory protocol for an existing ALPHA build, you could use the following command.

scons PROTOCOL=MESI_CMP_directory build/ALPHA/gem5.opt

It's often a good idea to add --help to the scons command line which will print out all of the configuration variables and what their values are. This way you can make sure everything is set up like you want, and that you don't have any typos in any variable names. If everything is as you expect, you can remove --help to actually start the build.

Running regressions / Testing Your Build

gem5's regression system is built into SCons. That ensures that the gem5 binary is automatically rebuilt if necessary, and that tests are only rerun if they might have different results.

The regression targets are all found under "tests" in the build directory. The components of the path to a tests output files determine which test to run and how to run it. These components are as follows:

tests/<gem5 binary extension>/<test category>/<test name>/<architecture>/<operating system>/<configuration>/

You can leave out components of the path farther down, and scons will automatically build all available tests that match the components that were specified. To run all "quick" tests on the "opt" version of ALPHA configuration of gem5, you can run the following command:

scons build/ALPHA/tests/opt/quick

The regression framework is integrated into the scons build process, so the command above will (re)build ALPHA/gem5.opt if necessary before running the tests. Also thanks to scons's dependence tracking, tests will be re-run only if the binary has been rebuilt since the last time the test was run. If the previous test run is still valid (as far as scons can tell), only a brief pass/fail message will be printed out based on the result of that previous test, rather than the full output and statistics diff that is printed when the test is actually executed.

Regression tests are subdivided into two categories, "quick" and "long", based on runtime. You can run only the tests in a particular category by adding that category name to the target path, e.g.:

% scons build/ALPHA/tests/opt/long

Specific tests can be run by appending the test name:

% scons build/ALPHA/tests/opt/quick/fs/10.linux-boot

For more details, see Regression Tests.

A few of the "quick" category of tests should run without any additional setup. Some tests depend on EIO support which is provided in the "encumbered" repository and which has to be built into gem5 using the EXTRAS mechanism. Other tests depend on system files like particular disk images and kernels to be found in particular places on your system. These files are frequently available, and with a little effort and those files you should be able to get those tests running. Other tests, typically based on SPEC benchmarks and not part of "quick", require input files which have a restrictive license and can't be distributed by us. You won't be able to run these tests unless you have a license and can get these files somehow.

Adding source files and debug flags

Files are added to the build by declaring them inside SConscript files as instances of certain python classes. The build system knows how to handle those files based on what particular class was used. For instance, to add a C++ source file to the build, you could add the following line to the SConscript in the same directory as


The build system finds and processes SConscript files automatically, so you can create one near the files your adding or extend one that's already there. The following table shows what types of source files there are and what they're for.

Source file type Description Parameters
Name Description
PySource Add a python source file to the named package package The name of the package
source The name of the python source file
SimObject Add a SimObject python file as a python source object and add it to a list of sim object modules source The relative path to the python file defining the SimObject.
Source Add a c/c++ source file to the build source Relative path to source file
Werror Whether to compile with -Werror. Defaults to True.
main This file is part of main() for the gem5 binary. Defaults to False.
skip_lib Whether this file should be excluded from the library version of gem5. Defaults to False.
UnitTest Add a unit test to the build sources A list or tuple of relative paths to the source files for the unit test.

A similar mechanism is used to define debug/trace flags. These look very similar but don't actually refer to a file. A compound trace flag is a flag which controls a group of normal trace flags.

Trace flag type Description Parameters
Name Description
DebugFlag Add an individual trace flag to the build. name The name of the new trace flag.
desc A description for this flag. This is optional but recommended.
CompoundFlag Add a compound trace flag to the build. name The name of the new compound trace flag.
flags A list or tuple of the names of trace flags which this new compound trace flag will control.
desc A description for this flag. This is optional but recommended.


The EXTRAS scons variable can be used to build additional directories of source files into gem5 by setting it to a colon delimited list of paths to these additional directories. EXTRAS is a handy way to build on top of the gem5 code base without mixing your new source with the upstream source. You can then manage your new body of code however you need to independently from the main code base.