| Overview |
| ======== |
| This is SLICC, a domain specific language to specify cache coherence protocol |
| we have developed in Multifacet group. |
| |
| It is developed by Milo Martin <milo@cs.wisc.edu> |
| This document is prepared by Min Xu <mxu@cae.wisc.edu> while I am learning the |
| system. With minor correctness updates by Brad Beckmann <beckmann@cs.wisc.edu> |
| |
| It can be used to generate C++ code that works with RUBY cache simulator as |
| well as generate HTML and other document to describe the target protocol. |
| |
| Some user document is available in doc directory. |
| |
| Tech details |
| ============ |
| SLICC take a text input with similar syntax to C++ language and use the lexer |
| and parser in parser directory to construct a Abstract Syntax Tree (AST) |
| internally. After having done this first pass, the AST is traversed to fill |
| several interval table, such as symbol table, type table, etc. Finally the code |
| is generated by traversing the tree once again. |
| |
| Note, by Milo's good coding habit, almost all C++ class define their private |
| copy/assignment constructor. This prevents accidentally copying/assigning an |
| object by its address. |
| |
| The AST basically looks like a hierarchical representation of the text input. |
| At the highest level, it has the "Machine", each Machine has several "states" |
| and "events" and "actions" and "transistions". |
| |
| Since the language is domain specific, many assumptions of the target system is |
| hardcoded in SLICC. For example, ruby would expect the generated code for each |
| system node, has the following components: |
| processor(sequencer, not generated?) |
| cache |
| directory (with memory block value, only when compiled with tester) |
| network interface (NI) |
| |
| Directory generator/ contains routines to generate HTML/MIF format output. |
| fileio.[Ch] has a routine to conditionally write a file only when the original |
| content of the file is different from what is going to be written, this avoid |
| re-make those file after regenerate the protocol. html_gen.[Ch] contains the |
| symbol name munge and index page generation. mif_gen.[Ch] contains the entire |
| MIF output generation routine, mainly a table buildup. |
| |
| Directory symbol/ contains classes to represent symbols in the slicc input |
| file. Base class is "Symbol". Derived subclasses are "Action Event Func State |
| StateMachine Transition Type Var". "Symbol" has knowledge about its locations |
| in the source file and short name, long name. "SymbolTable" is a list of |
| symbols and g_sym_table is the global SymbolTable of the slicc system. |
| One can query a SymbolTable by symbol's id. Also SymbolTable is responsible for |
| keeping track of Symbol's declaration in correct scope. The current |
| implementation uses a stack which dynamically determine the scope of symbol |
| lookups. Global scope is at bottom of the stack (vector[0]). SymbolTable is |
| also the main place to write out the generated C++/HTML/MIF files. |
| SymbolTable::writeNodeFiles() is one of the place to look for hardcoded C++ |
| code for node.[Ch]. And Type.[Ch] is the place where generating enumeration and |
| Message/NetworkMessage declaration and implementation. Func.[Ch] is used to |
| generate function of the class Chip. StateMachine.[Ch] wrap the whole thing |
| up by putting States, Actions, Events together. It actually has a two dimension |
| table like the one represented in the HTML output. Actions are indexed with |
| the initial state and observed event. After the tabel being built, the |
| StateMachine class can write out Transitions/Controller/wakeup_logic into C++ |
| outputs. Finally, in symbol directory, Var.[Ch] seem to incomplete? |
| |
| Demystify all those "predefined" external types, like "Address". Where are |
| they defined? They are in ../protocol/RubySlicc-*.sm and |
| ../protocol/RubySlicc_interfaces.slicc is include in the slicc invocation |
| command in ../ruby/Makefile. |
| |
| Another myth: "trigger" method is hardcoded in ast/InPortDeclAST.C and |
| ast/FuncCallExprAST.C. The function is similar to inlined function in the |
| output generated code, so you cannot find any occurance of string "trigger" in |
| the generated code. "trigger" also increment a counter that is checked every |
| time a transition is done. In one ruby cycle, only TRANSITIONS_PER_RUBY_CYCLE |
| number of transitions can be done. ast/FuncCallExprAST.C also contains some |
| code for function "error" and "assert" and "DEBUG_EXPR", all in the same |
| manner. Ruby always issues transitions from the first port while there is any. |
| Stalled transition in Ruby does not consume a sub-cycle. This models the |
| hardware that probe all port in parallel, pick one transition from the highest |
| priority queue if the transistion was not stalled by any resources constraint. |
| |
| Another note: scheduleEvent() call of ruby make sure a consumer is woken up at |
| specified cycle, and only once per cycle. |
| |
| Action z_stall, where is it? It is hardcoded in symbols/StateMachine.C. In |
| function StateMachine::printCSwitch(), z_stall cause the generated code return |
| TransitionResult_ProtocolStall. Also the HTML output for z_stall has to be |
| consequently hardcoded. I am not sure that's really a good idea or not. :-) |
| |
| Question: How comes there is no "for" loop statement in slicc? |
| Answer: Been there, done that. That is easy to add, first of all. But unbound |
| loop make slicc eventually un-synthesizable. We want to avoid that. If you want |
| to loop through a bounded array do something, make the action done in a |
| external interface in RubySlicc_Util.h. Inside, you just pass the vector as |
| parameter to the external interface to achieve the same effects. |
| |
| Another bad thing of using loop statement like for is that we can not determine |
| how many buffer space to allocate before the transition. With a vector, if it |
| easy to understand we can always allocate the worst case number of hardware |
| resources. |
| |
| Question: Wait! It seems statement check_allocate does nothing! |
| Answer: No, it does call areNSoltsAvailable() function of the object before any |
| statement is executed in one action. It does *NOT* generate code in its |
| original place in the code, instead, it scan the body of the action code and |
| determine how many slots are needed to allocated before hand. So the |
| transaction is all done or nothing done. I had tried to make all actions return |
| boolean values and the false return cause a transition to abort with |
| ResourceStall. But it is later on deemed to be too flexible in its semantics. |
| We should never introduce control flow inside the transitions, so that each |
| transition is either "all" or "nothing". Just that simple. BTW, if you call |
| check_allocate twice, areNSoltsAvailable(2) is generated, three times generates |
| areNSoltsAvailable(3), etc. |