SCons build system

This page provides some tips on using M5's build system.

M5's build system uses SCons, a powerful replacement for make (and autoconf, and ccache). SCons uses standard Python to describe the software build process, enabling some very sophisticated & complex build procedures.

This page is not an attempt to fully document the build system. Because many people aren't familiar with SCons, and because gem5's build system in some areas takes advantage of the complexity that SCons enables, this page tries to collect useful pointers that might not otherwise be obvious.

SCons tips

• As with make, you can build multiple targets by specifying them all on a single command line.
• As with make, the "-j" option can be used to enable parallel builds (useful on multiprocessor hosts).

For example, the following command will build both the optimized Alpha syscall emulation and debug MIPS syscall emulation targets using up to 4 concurrent processes:

% scons -j 4 build/ALPHA/m5.opt build/MIPS/m5.debug

• You can print out all the generic SCons options using "scons -H".
• If you specify a directory rather than a file, SCons will build all of the targets it knows about under that directory. Thus "scons build/ALPHA" will build all of the various ALPHA binaries and run the regression tests on them (since the regression test targets are in the subdirectory build/ALPHA/tests).
• The --debug=explain and --debug=tree options are very useful for figuring out why SCons does (or doesn't) rebuild a target when you change a source file.

gem5 build system tips

• There are a number of command-line options you can set of the form "option=value", for example:

% scons CC=gcc44 USE_MYSQL=False build/ALPHA/gem5.opt

Almost all of these options are sticky, that is, if you specify them once, they remain in force for all future builds in that particular target (e.g., ALPHA) until they are changed explicitly in a future invocation. (The only non-sticky option is update_ref, which, when set, causes the results of any regression tests to overwrite the reference outputs in preparation for being committed as the new reference outputs.)

• You can print out all the gem5-specific build options using "scons -h".
• You can define your own configurations beyond ALPHA, ARM, etc. These names are just the names of files in the build_opts directory containing option settings. You can generate a new configuration by specifying an predefined configuration (using default=) and overriding other options on the command line, e.g.:

% scons --default ALPHA USE_MYSQL=False build/ALPHA_NOSQL/gem5.debug
% scons --default ALPHA CC=mycc CXX=myc++ build/ALPHA_MYCOMPILERS/gem5.debug

The configuration name is actually totally arbitrary, and need not include the ISA name or anything else:

% scons --default MIPS build/FOOBAR/gem5.debug

• The build options for a particular configuration are stored in the build/options directory, so you can blow away your configuration-specific build directory ("rm -rf build/ALPHA") without losing the settings of your sticky options.
• All the build state, including SCons state as well as any non-default build option settings, are stored under the build directory. Thus if you delete the entire build directory (""rm -rf build") then you are back at the state you were in when you first unpacked the source tree.
• You can build binaries in locations other than the "build" subdirectory of your source tree. This feature can be useful in some circumstances, for example if you have a central NFS-mounted copy of your source tree, but you want to build that tree on multiple different hosts using the local disk of each host. (Supporting this model is another reason why all the bulid state is kept in the build directory and not in the root of the source tree.) See the comments at the top of the m5/SConstruct file.