Build System

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Build System

M5'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 called SConscript and are found throughout the tree, usually near the files they're associated with.

Build targets

In M5, scons build targets are of the form <build dir>/<configuration>/<target>. The <build dir> part of the target is a directory path that ends in "build". Typically this is simply "build" by itself, but you can specify a directory called "build" located somewhere else instead. The <configuration> part selects a set of preset build configuration variables values which correspond to common . The possible options are the file names in the build_opts directory, and are discussed more below. The build targets can be regression tests which are explained in more detail below, or they can be different versions of the M5 binary. The name of the binary is "m5" with an extension that specifies what version to build. Currently supported versions are m5.debug, m5.opt,, and

m5.debug has optimizations turned off. This ensures that variables won't be optimized out, functions won't be unexpectedly inlined, and control flow will won't 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.

m5.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 M5 that should be focused on to improve performance.

These versions are summarized in the following table.

Binary name Optimizations Run time debugging support Profiling support
m5.debug X
m5.opt X X X X X

Command line options

Scons will recognize the following command line options specific to M5.

Option Effect
--color Turn on colorized output
--no-color Turn off colorized output
--default Override which 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
PATH Path to search for programs
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_SE/m5.debug

Configuration variables

These configuration variables are used to control the way M5 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
EFENCE Link with Electric Fence malloc debugger False
FAST_ALLOC_DEBUG Enable fast object allocator debugging False X
FAST_ALLOC_STATS Enable fast object allocator statistics False X
FULL_SYSTEM Full-system support False X
NO_FAST_ALLOC Disable fast object allocator False X
NO_HTML Do not create HTML files False
NO_VECTOR_BOUNDS_CHECKS Don't do bounds checks True X
PROTOCOL Coherence protocol for Ruby MI_example X
RUBY Build with Ruby False
SS_COMPATIBLE_FP Make floating-point results compatible with SimpleScalar False X
TARGET_ISA Target ISA alpha 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_MYSQL Use MySQL for stats output whether mysql_config was found X
USE_POSIX_CLOCK Use POSIX Clocks whether posix clocks are available on this host X
USE_SSE2 Compile for SSE2 (-msse2) to get IEEE FP on x86 hosts False

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. For instance, using the SPARC_FS configuration will turn on full system mode with the FULL_SYSTEM variable and set the TARGET_ISA to sparc.

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 M5 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 turn on RUBY support for an existing ALPHA_FS build, you could use the following command.

scons RUBY=True build/ALPHA_FS/m5.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

Adding files to the build

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.
swig Whether to use flags suitable for a swig wrapper C++ file. Defaults to False.
bin_only Whether this file should only be compiled into the main gem5 binary. Defaults to False. If set, this implies skip_lib.
skip_lib Whether this file should be excluded from the library version of gem5. Defaults to False.
SwigSource Add a swig file to the build package Package for the python version of the source.
source Relative path to the swig input file.
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 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
TraceFlag 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.