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No longer maintained.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