Compiling M5

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gem5 runs on Linux and Mac OS X, but should be easily portable to other Unix-like OSes. At times in the past gem5 has worked on OpenBSD and Microsoft Windows (under Cygwin), but these platforms are not regularly tested. Cygwin in particular is no longer actively supported; if you must run on a Windows host, we recommend installing Linux (e.g., Ubuntu Server) under a VM and running gem5 there. Free virtualization solutions such as VirtualBox and VMware Player work well for this usage.

Cross-endian support has been mostly added, however this has not been extensively tested. Running a program with syscall emulation is supported regardless of host/target endianess, however full-system simulation may have cross-endian issues (ALPHA full-system is known not to work on big endian machines).

To build gem5, you need to ensure you have all the dependencies installed.

Possible Targets

gem5 can build many binaries each for a different guest architecture. The currently available architectures are ALPHA, ARM, MIPS, POWER, SPARC, and X86. In addition there is a NULL architecture.

For each possible architecture and mode, several different executables can be built:

  • gem5.debug - A binary used for debugging without any optimizations. Since no optimizations are done this binary is compiled the fastest, however since no optimizations are done it executes very slowly.
  • gem5.opt - A binary with debugging and optimization. This binary executes much faster than the debug binary and still provides all the debugging facility of the debug version. However when debugging source code it can be more difficult to use that the debug target.
  • - This binary is like the opt target, however it also includes profiling support suitable for use with gprof.
  • gem5.perf - Similar to prof, this target is aimed for CPU and heap profiling using the google perftools.
  • - This binary is the fastest binary and all debugging support is removed from the binary (including trace support). By default it also uses Link Time Optimization


Starting in the root of the source tree, you can build gem5 using a command of the form:

% scons build/<arch>/gem5.<binary>

where the items between <> are the architectures, modes and binaries listed above. For example:

% cd gem5
% scons build/ALPHA/gem5.debug

scons: Reading SConscript files ...
Checking for C header file fenv.h... yes
Building in /tmp/gem5/build/ALPHA
Options file /tmp/gem5/build/options/ALPHA not found,
  using defaults in build_opts/ALPHA
Compiling in ALPHA with MySQL support.
scons: done reading SConscript files.
scons: Building targets ...
g++ -o build/ALPHA/base/ -c -pipe -fno-strict-aliasing
    -Wall -Wno-sign-compare -Werror -Wundef -g3 -gdwarf-2 -O0
    -DTHE_ISA=ALPHA_ISA -DDEBUG -Iext/dnet -I/usr/include/python2.4
    -Ibuild/libelf/include -I/usr/include/mysql -Ibuild/ALPHA

If your output looked like the above, then congratulations, you've compiled gem5! The final binary is located at the path you specified in the argument to scons, e.g., build/ALPHA/gem5.debug. For more build options and further details about the build system, see the SCons build system page.

Installing full system files

If you want to run the full-system version (including the full-system regression tests), you will also need to download the full-system files (disk images and binaries) from the Download page.

The path to these files is determined in configs/common/ There are a couple of default paths hard-coded into this script; you can place the system files at one of those paths, edit to change those paths, or override the paths in that file by setting your M5_PATH environment variable. If this is not done correctly you will see an error like ImportError: Can't find a path to system files. when you first attempt to run the simulator in full-system mode.

Note that the default path, /dist/m5/system, is designed for environments where you have root (sudo) access (to create /dist) and want the files in a place where they can be shared by multiple users. If both of these are true, you can follow this example to put the system files at the default location:

% sudo mkdir -p /dist/m5/system
% cd /dist/m5/system
% sudo tar vxfj <path>/m5_system_2.0b3.tar.bz2
% sudo mv m5_system_2.0b3/* . ; sudo rmdir m5_system_2.0b3/
% sudo chgrp -R <grp> /dist  # where <grp> is a group that contains all the m5 users

In most cases, it's simplest to put the files wherever is convenient and then set M5_PATH to point to them.

Testing your build

Once you've compiled gem5, you can verify that the build worked by running regression tests. Regression tests are also run via scons. The command to run all tests for a particular is constructed as follows:

% scons build/<target>/tests/<binary>

For example, to run the regression tests on ALPHA/gem5.opt, type:

% scons build/ALPHA/tests/opt

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 further subdivided into three categories ("quick", "medium", 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/quick

(Note that currently the "medium" category is empty; all of the tests are "quick" or "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.

Compiling with runtime checks

GCC and clang both have the ability to compile in 'sanitizers' that ensure certain invariants at runtime. These invariants can check for memory leaks, invalid memory accesses, and other undefined behaviors. Currently, AddressSanitizer and UndefinedBehaviorSanitizer are supported with the flags --with-asan and --with-ubsan respectively. When ASAN is enabled, tcmalloc must be disabled (--without-tcmalloc), because it confuses ASAN's shadow memory.