Compiling a SLICC protocol

The SLICC file

Now that we have finished implementing the protocol, we need to compile it. You can download the complete SLICC files below:

• MSI-cache.sm
• MSI-dir.sm
• MSI-msg.sm

Before building the protocol, we need to create one more file: MSI.slicc. This file tells the SLICC compiler which state machine files to compile for this protocol. The first line contains the name of our protocol. Then, the file has a number of include statements. Each include statement has a file name. This filename can come from any of the protocol_dirs directories. We declared the current directory as part of the protocol_dirs in the SConsopts file (protocol_dirs.append(str(Dir('.').abspath))). The other directory is src/mem/protocol/. These files are included like C++h header files. Effectively, all of the files are processed as one large SLICC file. Thus, any files that declare types that are used in other files must come before the files they are used in (e.g., MSI-msg.sm must come before MSI-cache.sm since MSI-cache.sm uses the RequestMsg type).

protocol "MSI";
include "RubySlicc_interfaces.slicc";
include "MSI-msg.sm";
include "MSI-cache.sm";
include "MSI-dir.sm";

You can download the fill file here

Compiling a protocol with SCons

Most SCons defaults (found in build_opts/) specify the protocol as MI_example, an example, but poor performing protocol. Therefore, we cannot simply use a default build name (e.g., X86 or ARM). We have to specify the SCons options on the command line. The command line below will build our new protocol with the X86 ISA.

scons build/X86_MSI/gem5.opt --default=X86 PROTOCOL=MSI SLICC_HTML=True

This command will build gem5.opt in the directory build/X86_MSI. You can specify any directory here. This command line has two new parameters: --default and PROTOCOL. First, --default specifies which file to use in build_opts for defaults for all of the SCons variables (e.g., ISA, CPU_MODELS). Next, PROTOCOL overrides any default for the PROTOCOL SCons variable in the default specified. Thus, we are telling SCons to specifically compile our new protocol, not whichever protocol was specified in build_opts/X86.

There is one more variable on this command line to build gem5: SLICC_HTML=True. When you specify this on the building command line, SLICC will generate the HTML tables for your protocol. You can find the HTML tables in <build directory>/mem/protocol/html. By default, the SLICC compiler skips building the HTML tables because it impacts the performance of compiling gem5, especially when compiling on a network file system.

After gem5 finishes compiling, you will have a gem5 binary with your new protocol! If you want to build another protocol into gem5, you have to change the PROTOCOL SCons variable. Thus, it is a good idea to use a different build directory for each protocol, especially if you will be comparing protocols.

When building your protocol, you will likely encounter errors in your SLICC code reported by the SLICC compiler. Most errors include the file and line number of the error. Sometimes, this line number is the line after the error occurs. In fact, the line number can be far below the actual error. For instance, if the curly brackets do not match correctly, the error will report the last line in the file as the location.