X-Git-Url: https://git.martlubbers.net/?a=blobdiff_plain;f=top%2Fimp.tex;h=1efd3614331ef81edbc3d0d31976cfe1e83b7044;hb=ed3041263afe2ea88dc234a46f3b00d63493b8a8;hp=a8f451cc69b51f8a88bb1856b8c1a9789a8da1be;hpb=e36fac1dc27e8fda89f7970d4e1eb1d49d73f47f;p=phd-thesis.git diff --git a/top/imp.tex b/top/imp.tex index a8f451c..1efd361 100644 --- a/top/imp.tex +++ b/top/imp.tex @@ -4,18 +4,530 @@ \begin{document} \input{subfileprefix} - \chapter{Implementation}% \label{chp:implementation} \begin{chapterabstract} - This chapter shows the implementation of the \gls{MTASK} system. - It is threefold: first it shows the implementation of the byte code compiler for \gls{MTASK}'s \gls{TOP} language, then is details of the implementation of \gls{MTASK}'s \gls{TOP} engine that executes the \gls{MTASK} tasks on the microcontroller, and finally it shows how the integration of \gls{MTASK} tasks and \glspl{SDS} is implemented both on the server and on the device. + \noindent This chapter shows the implementation of the \gls{MTASK} system by: + \begin{itemize} + \item shows the implementation of the byte code compiler for \gls{MTASK}'s \gls{TOP} language; + \item gives details of the implementation of \gls{MTASK}'s \gls{TOP} engine that executes the \gls{MTASK} tasks on the microcontroller; + \item and explains how the integration of \gls{MTASK} tasks and \glspl{SDS} is implemented both on the server and on the device. + \end{itemize} \end{chapterabstract} -\section{Byte code compiler} -IFL19 paper, bytecode instructieset~\cref{chp:bytecode_instruction_set} +Microcontrollers usually have flash-based program memory which wears out fairly quick. +For example, the atmega328p in the \gls{ARDUINO} UNO is rated for 10000 write cycles. +While this sounds like a lot, if new tasks are sent to the device every minute or so, a lifetime of not even seven days is guaranteed. +Hence, for dynamic applications, generating code at run-time for interpretation on the device is necessary. +This byte code is then interpreted on microcontrollers with very little memory and processing power and thus save precious write cycles of the program memory. +precious write cycles of the program memory. + +In order to provide the device with the tools to interpret the byte code, it is programmed with a \gls{RTS}, a customisable domain-specific \gls{OS} that takes care of the execution of tasks but also low-level mechanisms such as the communication, multi tasking, and memory management. +Once the device is programmed with the \gls{MTASK} \gls{RTS}, it can continuously receive new tasks. + +\subsection{Instruction set} +The instruction set is a fairly standard stack machine instruction set extended with special \gls{TOP} instructions. +\Cref{lst:instruction_type} shows the \gls{CLEAN} type representing the instruction set of which \cref{tbl:instr_task} gives detailed semantics. +Type synonyms are used to provide insight on the arguments of the instructions. +One notable instruction is the \cleaninline{MkTask} instruction, it allocates and initialises a task tree node and pushes a pointer to it on the stack. + +\begin{lstClean}[caption={The type housing the instruction set.},label={lst:instruction_type}] +:: ArgWidth :== UInt8 :: ReturnWidth :== UInt8 +:: Depth :== UInt8 :: Num :== UInt8 +:: SdsId :== UInt8 :: JumpLabel =: JL UInt16 + +//** Datatype housing all instructions +:: BCInstr + //Return instructions + //Jumps + = BCJumpF JumpLabel | BCJump JumpLabel | BCLabel JumpLabel | BCJumpSR ArgWidth JumpLabel + | BCReturn ReturnWidth ArgWidth | BCTailcall ArgWidth ArgWidth JumpLabel + //Arguments + | BCArgs ArgWidth ArgWidth + //Task node creation and refinement + | BCMkTask BCTaskType | BCTuneRateMs | BCTuneRateSec + //Task value ops + | BCIsStable | BCIsUnstable | BCIsNoValue | BCIsValue + //Stack ops + | BCPush String255 | BCPop Num | BCRot Depth Num | BCDup | BCPushPtrs + //Casting + | BCItoR | BCItoL | BCRtoI | ... + // arith + | BCAddI | BCSubI | ... + ... + +//** Datatype housing all task types +:: BCTaskType + = BCStableNode ArgWidth | ArgWidth + // Pin io + | BCReadD | BCWriteD | BCReadA | BCWriteA | BCPinMode + // Interrupts + | BCInterrupt + // Repeat + | BCRepeat + // Delay + | BCDelay | BCDelayUntil //* Only for internal use + // Parallel + | BCTAnd | BCTOr + //Step + | BCStep ArgWidth JumpLabel + //Sds ops + | BCSdsGet SdsId | BCSdsSet SdsId | BCSdsUpd SdsId JumpLabel + // Rate limiter + | BCRateLimit + ////Peripherals + //DHT + | BCDHTTemp UInt8 | BCDHTHumid UInt8 + ... +\end{lstClean} + +\subsection{Compiler} +The bytecode compiler interpretation for the \gls{MTASK} language is implemented as a monad stack containing a writer monad and a state monad. +The writer monad is used to generate code snippets locally without having to store them in the monadic values. +The state monad accumulates the code, and stores the stateful data the compiler requires. +\Cref{lst:compiler_state} shows the data type for the state, storing: +function the compiler currently is in; +code of the main expression; +context (see \todo{insert ref to compilation rules step here}); +code for the functions; +next fresh label; +a list of all the used \glspl{SDS}, either local \glspl{SDS} containing the initial value (\cleaninline{Left}) or lifted \glspl{SDS} (see \cref{sec:liftsds}) containing a reference to the associated \gls{ITASK} \gls{SDS}; +and finally there is a list of peripherals used. + +\begin{lstClean}[label={lst:compiler_state},caption={\Gls{MTASK}'s byte code compiler type}] +:: BCInterpret a :== StateT BCState (WriterT [BCInstr] Identity) a +:: BCState = + { bcs_infun :: JumpLabel + , bcs_mainexpr :: [BCInstr] + , bcs_context :: [BCInstr] + , bcs_functions :: Map JumpLabel BCFunction + , bcs_freshlabel :: JumpLabel + , bcs_sdses :: [Either String255 MTLens] + , bcs_hardware :: [BCPeripheral] + } +:: BCFunction = + { bcf_instructions :: [BCInstr] + , bcf_argwidth :: UInt8 + , bcf_returnwidth :: UInt8 + } +\end{lstClean} + +Executing the compiler is done by providing an initial state. +After compilation, several post-processing steps are applied to make the code suitable for the microprocessor. +First, in all tail call \cleaninline{BCReturn}'s are replaced by \cleaninline{BCTailCall} to implement tail call elimination. +Furthermore, all byte code is concatenated, resulting in one big program. +Many instructions have commonly used arguments so shorthands are introduced to reduce the program size. +For example, the \cleaninline{BCArg} instruction is often called with argument \qtyrange{0}{2} and can be replaced by the \cleaninline{BCArg0}--\cleaninline{BCArg2} shorthands. +Furthermore, redundant instructions (e.g.\ pop directly after push) are removed as well in order not to burden the code generation with these intricacies. +Finally the labels are resolved to represent actual program addresses instead of freshly generated identifiers. +After the byte code is ready, the lifted \glspl{SDS} are resolved to provide an initial value for them. +The result---byte code, \gls{SDS} specification and perpipheral specifications---are the result of the process, ready to be sent to the device. + +\section{Compilation rules} +This section describes the compilation rules, the translation from abstract syntax to byte code. +The compilation scheme consists of three schemes\slash{}functions. +When something is surrounded by double vertical bars, e.g.\ $\stacksize{a_i}$, it denotes the number of stack cells required to store it. + +Some schemes have a \emph{context} $r$ as an argument which contains information about the location of the arguments in scope. +More information is given in the schemes requiring such arguments. + +\newcommand{\cschemeE}[2]{\mathcal{E}\llbracket#1\rrbracket~#2} +\newcommand{\cschemeF}[1]{\mathcal{F}\llbracket#1\rrbracket} +\newcommand{\cschemeS}[3]{\mathcal{S}\llbracket#1\rrbracket~#2~#3} +\begin{table} + \centering + \begin{tabularx}{\linewidth}{l X} + \toprule + Scheme & Description\\ + \midrule + $\cschemeE{e}{r}$ & Produces the value of expression $e$ given the context $r$ and pushes it on the stack. + The result can be a basic value or a pointer to a task.\\ + $\cschemeF{e}$ & Generates the bytecode for functions.\\ + $\cschemeS{e}{r}{w} $ & Generates the function for the step continuation given the context $r$ and the width $w$ of the left-hand side task value.\\ + \bottomrule + \end{tabularx} +\end{table} + +\subsection{Expressions} +Almost all expression constructions are compiled using $\mathcal{E}$. +The argument of $\mathcal{E}$ is the context (see \cref{ssec:functions}). +Values are always placed on the stack; tuples and other compound data types are unpacked. +Function calls, function arguments and tasks are also compiled using $\mathcal{E}$ but their compilations is explained later. + +\begin{align*} + \cschemeE{\text{\cleaninline{lit}}~e}{r} & = \text{\cleaninline{BCPush (bytecode e)}};\\ + \cschemeE{e_1\mathbin{\text{\cleaninline{+.}}}e_2}{r} & = \cschemeE{e_1}{r}; + \cschemeE{e_2}{r}; + \text{\cleaninline{BCAdd}};\\ + {} & \text{\emph{Similar for other binary operators}}\\ + \cschemeE{\text{\cleaninline{Not}}~e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCNot}};\\ + {} & \text{\emph{Similar for other unary operators}}\\ + \cschemeE{\text{\cleaninline{If}}~e_1~e_2~e_3}{r} & = + \cschemeE{e_1}{r}; + \text{\cleaninline{BCJmpF}}\enskip l_{else}; \mathbin{\phantom{=}} \cschemeE{e_2}{r}; \text{\cleaninline{BCJmp}}\enskip l_{endif};\\ + {} & \mathbin{\phantom{=}} \text{\cleaninline{BCLabel}}\enskip l_{else}; \cschemeE{e_3}{r}; \mathbin{\phantom{=}} \text{\cleaninline{BCLabel}}\enskip l_{endif};\\ + {} & \text{\emph{Where $l_{else}$ and $l_{endif}$ are fresh labels}}\\ + \cschemeE{\text{\cleaninline{tupl}}~e_1~e_2}{r} & = + \cschemeE{e_1}{r}; + \cschemeE{e_2}{r};\\ + {} & \text{\emph{Similar for other unboxed compound data types}}\\ + \cschemeE{\text{\cleaninline{first}}~e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCPop}}\enskip w;\\ + {} & \text{\emph{Where $w$ is the width of the left value and}}\\ + {} & \text{\emph{similar for other unboxed compound data types}}\\ + \cschemeE{\text{\cleaninline{second}}\enskip e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCRot}}\enskip w_1\enskip (w_1+w_2); + \text{\cleaninline{BCPop}}\enskip w_2;\\ + {} & \text{\emph{Where $w_1$ is the width of the left and, $w_2$ of the right value}}\\ + {} & \text{\emph{similar for other unboxed compound data types}}\\ +\end{align*} + +Translating $\mathcal{E}$ to \gls{CLEAN} code is very straightforward, it basically means executing the monad. +Almost always, the type of the interpretation is not used, i.e.\ it is a phantom type. +To still have the functions return the correct type, the \cleaninline{tell`}\footnote{\cleaninline{tell` :: [BCInstr] -> BCInterpret a}} helper is used. +This function is similar to the writer monad's \cleaninline{tell} function but is casted to the correct type. +\Cref{lst:imp_arith} shows the implementation for the arithmetic and conditional expressions. +Note that $r$, the context, is not an explicit argument but stored in the state. + +\begin{lstClean}[caption={Interpretation implementation for the arithmetic and conditional classes.},label={lst:imp_arith}] +instance expr BCInterpret where + lit t = tell` [BCPush (toByteCode{|*|} t)] + (+.) a b = a >>| b >>| tell` [BCAdd] + ... + If c t e = freshlabel >>= \elselabel->freshlabel >>= \endiflabel-> + c >>| tell` [BCJumpF elselabel] >>| + t >>| tell` [BCJump endiflabel,BCLabel elselabel] >>| + e >>| tell` [BCLabel endiflabel] +\end{lstClean} + +\subsection{Functions} +Compiling functions occurs in $\mathcal{F}$, which generates bytecode for the complete program by iterating over the functions and ending with the main expression. +When compiling the body of the function, the arguments of the function are added to the context so that the addresses can be determined when referencing arguments. +The main expression is a special case of $\mathcal{F}$ since it neither has arguments nor something to continue. +Therefore, it is just compiled using $\mathcal{E}$. + +\begin{align*} + \cschemeF{main=m} & = + \cschemeE{m}{[]};\\ + \cschemeF{f~a_0 \ldots a_n = b~\text{\cleaninline{In}}~m} & = + \text{\cleaninline{BCLabel}}~f; \cschemeE{b}{[\langle f, i\rangle, i\in \{(\Sigma^n_{i=0}\stacksize{a_i})..0\}]};\\ + {} & \mathbin{\phantom{=}} \text{\cleaninline{BCReturn}}~\stacksize{b}~n; \cschemeF{m};\\ +\end{align*} + +A function call starts by pushing the stack and frame pointer, and making space for the program counter (\cref{lst:funcall_pushptrs}) followed by evaluating the arguments in reverse order (\cref{lst:funcall_args}). +On executing \cleaninline{BCJumpSR}, the program counter is set and the interpreter jumps to the function (\cref{lst:funcall_jumpsr}). +When the function returns, the return value overwrites the old pointers and the arguments. +This occurs right after a \cleaninline{BCReturn} (\cref{lst:funcall_ret}). +Putting the arguments on top of pointers and not reserving space for the return value uses little space and facilitates tail call optimization. + +\begin{figure} + \begin{subfigure}{.24\linewidth} + \centering + \includestandalone{memory1} + \caption{\cleaninline{BCPushPtrs}}\label{lst:funcall_pushptrs} + \end{subfigure} + \begin{subfigure}{.24\linewidth} + \centering + \includestandalone{memory2} + \caption{Arguments}\label{lst:funcall_args} + \end{subfigure} + \begin{subfigure}{.24\linewidth} + \centering + \includestandalone{memory3} + \caption{\cleaninline{BCJumpSR}}\label{lst:funcall_jumpsr} + \end{subfigure} + \begin{subfigure}{.24\linewidth} + \centering + \includestandalone{memory4} + \caption{\cleaninline{BCReturn}}\label{lst:funcall_ret} + \end{subfigure} + \caption{The stack layout during function calls.}% +\end{figure} + +Calling a function and referencing function arguments are an extension to $\mathcal{E}$ as shown below. +Arguments may be at different places on the stack at different times (see \cref{ssec:step}) and therefore the exact location always has to be determined from the context using \cleaninline{findarg}\footnote{\cleaninline{findarg [l`:r] l = if (l == l`) 0 (1 + findarg r l)}}. +Compiling argument $a_{f^i}$, the $i$th argument in function $f$, consists of traversing all positions in the current context. +Arguments wider than one stack cell are fetched in reverse to preserve the order. + +\begin{align*} + \cschemeE{f(a_0, \ldots, a_n)}{r} & = + \text{\cleaninline{BCPushPtrs}}; \cschemeE{a_n}{r}; \cschemeE{a_{\ldots}}{r}; \cschemeE{a_0}{r}; \text{\cleaninline{BCJumpSR}}~n~f;\\ + \cschemeE{a_{f^i}}{r} & = + \text{\cleaninline{BCArg}~findarg}(r, f, i)~\text{for all}~i\in\{w\ldots v\};\\ + {} & v = \Sigma^{i-1}_{j=0}\stacksize{a_{f^j}}~\text{ and }~ w = v + \stacksize{a_{f^i}}\\ +\end{align*} + +Translating the compilation schemes for functions to Clean is not as straightforward as other schemes due to the nature of shallow embedding.\todo{deze \P{} moet ge\-\"up\-da\-ted worden} +The \cleaninline{fun} class has a single function with a single argument. +This argument is a Clean function that---when given a callable Clean function representing the mTask function---will produce \cleaninline{main} and a callable function. +To compile this, the argument must be called with a function representing a function call in mTask. +\Cref{lst:fun_imp} shows the implementation for this as Clean code. +To uniquely identify the function, a fresh label is generated. +The function is then called with the \cleaninline{callFunction} helper function that generates the instructions that correspond to calling the function. +That is, it pushes the pointers, compiles the arguments, and writes the \cleaninline{JumpSR} instruction. +The resulting structure (\cleaninline{g In m}) contains a function representing the mTask function (\cleaninline{g}) and the \cleaninline{main} structure to continue with. +To get the actual function, \cleaninline{g} must be called with representations for the argument, i.e.\ using \cleaninline{findarg} for all arguments. +The arguments are added to the context and \cleaninline{liftFunction} is called with the label, the argument width and the compiler. +This function executes the compiler, decorates the instructions with a label and places them in the function dictionary together with the metadata such as the argument width. +After lifting the function, the context is cleared again and compilation continues with the rest of the program. + +\begin{lstClean}[label={lst:fun_imp},caption={The backend implementation for functions.}] +instance fun (BCInterpret a) BCInterpret | type a where + fun def = {main=freshlabel >>= \funlabel-> + let (g In m) = def \a->callFunction funlabel (toByteWidth a) [a] + argwidth = toByteWidth (argOf g) + in addToCtx funlabel zero argwidth + >>| infun funlabel + (liftFunction funlabel argwidth + (g (retrieveArgs funlabel zero argwidth) + ) ?None) + >>| clearCtx >>| m.main + } + +argOf :: ((m a) -> b) a -> UInt8 | toByteWidth a +callFunction :: JumpLabel UInt8 [BCInterpret b] -> BCInterpret c | ... +liftFunction :: JumpLabel UInt8 (BCInterpret a) (?UInt8) -> BCInterpret () +\end{lstClean} + +\subsection{Tasks} +Task trees are created with the \cleaninline{BCMkTask} instruction that allocates a node and pushes it to the stack. +It pops arguments from the stack according to the given task type. +The following extension of $\mathcal{E}$ shows this compilation scheme (except for the step combinator, explained in \cref{ssec:step}). + +\begin{align*} + \cschemeE{\text{\cleaninline{rtrn}}~e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCMkTask BCStable}}_{\stacksize{e}};\\ + \cschemeE{\text{\cleaninline{unstable}}~e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCMkTask BCUnstable}}_{\stacksize{e}};\\ + \cschemeE{\text{\cleaninline{readA}}~e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCMkTask BCReadA}};\\ + \cschemeE{\text{\cleaninline{writeA}}~e_1~e_2}{r} & = + \cschemeE{e_1}{r}; + \cschemeE{e_2}{r}; + \text{\cleaninline{BCMkTask BCWriteA}};\\ + \cschemeE{\text{\cleaninline{readD}}~e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCMkTask BCReadD}};\\ + \cschemeE{\text{\cleaninline{writeD}}~e_1~e_2}{r} & = + \cschemeE{e_1}{r}; + \cschemeE{e_2}{r}; + \text{\cleaninline{BCMkTask BCWriteD}};\\ + \cschemeE{\text{\cleaninline{delay}}~e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCMkTask BCDelay}};\\ + \cschemeE{\text{\cleaninline{rpeat}}~e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCMkTask BCRepeat}};\\ + \cschemeE{e_1\text{\cleaninline{.\|\|.}}e_2}{r} & = + \cschemeE{e_1}{r}; + \cschemeE{e_2}{r}; + \text{\cleaninline{BCMkTask BCOr}};\\ + \cschemeE{e_1\text{\cleaninline{.&&.}}e_2}{r} & = + \cschemeE{e_1}{r}; + \cschemeE{e_2}{r}; + \text{\cleaninline{BCMkTask BCAnd}};\\ +\end{align*} + +This simply translates to Clean code by writing the correct \cleaninline{BCMkTask} instruction as exemplified in \cref{lst:imp_ret}. + +\begin{lstClean}[caption={The backend implementation for \cleaninline{rtrn}.},label={lst:imp_ret}] +instance rtrn BCInterpret +where + rtrn m = m >>| tell` [BCMkTask (bcstable m)] +\end{lstClean} + +\subsection{Step combinator}\label{ssec:step} +The \cleaninline{step} construct is a special type of task because the task value of the left-hand side may change over time. +Therefore, the continuation tasks on the right-hand side are \emph{observing} this task value and acting upon it. +In the compilation scheme, all continuations are first converted to a single function that has two arguments: the stability of the task and its value. +This function either returns a pointer to a task tree or fails (denoted by $\bot$). +It is special because in the generated function, the task value of a task can actually be inspected. +Furthermore, it is a lazy node in the task tree: the right-hand side may yield a new task tree after several rewrite steps (i.e.\ it is allowed to create infinite task trees using step combinators). +The function is generated using the $\mathcal{S}$ scheme that requires two arguments: the context $r$ and the width of the left-hand side so that it can determine the position of the stability which is added as an argument to the function. +The resulting function is basically a list of if-then-else constructions to check all predicates one by one. +Some optimization is possible here but has currently not been implemented. + +\begin{align*} + \cschemeE{t_1\text{\cleaninline{>>*.}}t_2}{r} & = + \cschemeE{a_{f^i}}{r}, \langle f, i\rangle\in r; + \text{\cleaninline{BCMkTask}}~\text{\cleaninline{BCStable}}_{\stacksize{r}}; \cschemeE{t_1}{r};\\ + {} & \mathbin{\phantom{=}} \text{\cleaninline{BCMkTask}}~\text{\cleaninline{BCAnd}}; \text{\cleaninline{BCMkTask}}~(\text{\cleaninline{BCStep}}~(\cschemeS{t_2}{(r + [\langle l_s, i\rangle])}{\stacksize{t_1}}));\\ +\end{align*} + +\begin{align*} + \cschemeS{[]}{r}{w} & = + \text{\cleaninline{BCPush}}~\bot;\\ + \cschemeS{\text{\cleaninline{IfValue}}~f~t:cs}{r}{w} & = + \text{\cleaninline{BCArg}} (\stacksize{r} + w); + \text{\cleaninline{BCIsNoValue}};\\ + {} & \mathbin{\phantom{=}} \cschemeE{f}{r}; + \text{\cleaninline{BCAnd}};\\ + {} & \mathbin{\phantom{=}} \text{\cleaninline{BCJmpF}}~l_1;\\ + {} & \mathbin{\phantom{=}} \cschemeE{t}{r}; + \text{\cleaninline{BCJmp}}~l_2;\\ + {} & \mathbin{\phantom{=}} \text{\cleaninline{BCLabel}}~l_1; + \cschemeS{cs}{r}{w};\\ + {} & \mathbin{\phantom{=}} \text{\cleaninline{BCLabel}}~l_2;\\ + {} & \text{\emph{Where $l_1$ and $l_2$ are fresh labels}}\\ + {} & \text{\emph{Similar for \cleaninline{IfStable} and \cleaninline{IfUnstable}}}\\ +\end{align*} + +First the context is evaluated. +The context contains arguments from functions and steps that need to be preserved after rewriting. +The evaluated context is combined with the left-hand side task value by means of a \cleaninline{.&&.} combinator to store it in the task tree so that it is available after a rewrite. +This means that the task tree is be transformed as follows: + +\begin{lstClean} +t1 >>= \v1->t2 >>= \v2->t3 >>= ... +//is transformed to +t1 >>= \v1->rtrn v1 .&&. t2 >>= \v2->rtrn (v1, v2) .&&. t3 >>= ... +\end{lstClean} + +The translation to \gls{CLEAN} is given in \cref{lst:imp_seq}. + +\begin{lstClean}[caption={Backend implementation for the step class.},label={lst:imp_seq}] +instance step BCInterpret where + (>>*.) lhs cont + //Fetch a fresh label and fetch the context + = freshlabel >>= \funlab->gets (\s->s.bcs_context) + //Generate code for lhs + >>= \ctx->lhs + //Possibly add the context + >>| tell` (if (ctx =: []) [] + //The context is just the arguments up till now in reverse + ( [BCArg (UInt8 i)\\i<-reverse (indexList ctx)] + ++ map BCMkTask (bcstable (UInt8 (length ctx))) + ++ [BCMkTask BCTAnd] + )) + //Increase the context + >>| addToCtx funlab zero lhswidth + //Lift the step function + >>| liftFunction funlab + //Width of the arguments is the width of the lhs plus the + //stability plus the context + (one + lhswidth + (UInt8 (length ctx))) + //Body label ctx width continuations + (contfun funlab (UInt8 (length ctx))) + //Return width (always 1, a task pointer) + (Just one) + >>| modify (\s->{s & bcs_context=ctx}) + >>| tell` [BCMkTask (instr rhswidth funlab)] + +toContFun :: JumpLabel UInt8 -> BCInterpret a +toContFun steplabel contextwidth + = foldr tcf (tell` [BCPush fail]) cont +where + tcf (IfStable f t) + = If ((stability >>| tell` [BCIsStable]) &. f val) + (t val >>| tell` []) + ... + stability = tell` [BCArg (lhswidth + contextwidth)] + val = retrieveArgs steplabel zero lhswidth +\end{lstClean} + +\subsection{\texorpdfstring{\Glspl{SDS}}{Shared data sources}} +The compilation scheme for \gls{SDS} definitions is a trivial extension to $\mathcal{F}$ since there is no code generated as seen below. + +\begin{align*} + \cschemeF{\text{\cleaninline{sds}}~x=i~\text{\cleaninline{In}}~m} & = + \cschemeF{m};\\ +\end{align*} + +The \gls{SDS} access tasks have a compilation scheme similar to other tasks (see \cref{ssec:scheme_tasks}). +The \cleaninline{getSds} task just pushes a task tree node with the \gls{SDS} identifier embedded. +The \cleaninline{setSds} task evaluates the value, lifts that value to a task tree node and creates an \gls{SDS} set node. + +\begin{align*} + \cschemeE{\text{\cleaninline{getSds}}~s}{r} & = + \text{\cleaninline{BCMkTask}} (\text{\cleaninline{BCSdsGet}} s);\\ + \cschemeE{\text{\cleaninline{setSds}}~s~e}{r} & = + \cschemeE{e}{r}; + \text{\cleaninline{BCMkTask BCStable}}_{\stacksize{e}};\\ + {} & \mathbin{\phantom{=}} \text{\cleaninline{BCMkTask}} (\text{\cleaninline{BCSdsSet}} s);\\ +\end{align*} + +While there is no code generated in the definition, the byte code compiler is storing the \gls{SDS} data in the \cleaninline{bcs_sdses} field in the compilation state. +The \glspl{SDS} are typed as functions in the host language so an argument for this function must be created that represents the \gls{SDS} on evaluation. +For this, an \cleaninline{BCInterpret} is created that emits this identifier. +When passing it to the function, the initial value of the \gls{SDS} is returned. +This initial value is stored as a byte code encoded value in the state and the compiler continues with the rest of the program. + +Compiling \cleaninline{getSds} is a matter of executing the \cleaninline{BCInterpret} representing the \gls{SDS}, which yields the identifier that can be embedded in the instruction. +Setting the \gls{SDS} is similar: the identifier is retrieved and the value is written to put in a task tree so that the resulting task can remember the value it has written. +Lifted SDSs are compiled in a very similar way.\todo{deze \P{} moet naar integration?} +The only difference is that there is no initial value but an iTasks SDS when executing the Clean function. +A lens on this SDS converting \cleaninline{a} from the \cleaninline{Shared a} to a \cleaninline{String255}---a bytecode encoded version---is stored in the state. +The encoding and decoding itself is unsafe when used directly but the type system of the language and the abstractions make it safe. +Upon sending the mTask task to the device, the initial values of the lifted SDSs are fetched to complete the SDS specification. + +% VimTeX: SynIgnore on +\begin{lstClean}[caption={Backend implementation for the SDS classes.},label={lst:comp_sds}] +:: Sds a = Sds Int +instance sds BCInterpret where + sds def = {main = freshsds >>= \sdsi-> + let sds = modify (\s->{s & bcs_sdses=put sdsi + (Left (toByteCode t)) s.bcs_sdses}) + >>| pure (Sds sdsi) + (t In e) = def sds + in e.main} + getSds f = f >>= \(Sds i)-> tell` [BCMkTask (BCSdsGet (fromInt i))] + setSds f v = f >>= \(Sds i)->v >>| tell` + ( map BCMkTask (bcstable (byteWidth v)) + ++ [BCMkTask (BCSdsSet (fromInt i))]) +\end{lstClean} +% VimTeX: SynIgnore off + +\section{\texorpdfstring{\Glsxtrlong{RTS}}{Run time system}} + +The \gls{RTS} is designed to run on systems with as little as \qty{2}{\kibi\byte} of \gls{RAM}. +Aggressive memory management is therefore vital. +Not all firmwares for microprocessors support heaps and---when they do---allocation often leaves holes when not used in a \emph{last in first} out strategy. +Therefore the \gls{RTS} uses a chunk of memory in the global data segment with its own memory manager tailored to the needs of \gls{MTASK}. +The size of this block can be changed in the configuration of the \gls{RTS} if necessary. +On an \gls{ARDUINO} UNO ---equipped with \qty{2}{\kibi\byte} of \gls{RAM}--- this size is about \qty{1500}{\byte}. + +In memory, task data grows from the bottom up and an interpreter stack is located directly on top of it growing in the same direction. +As a consequence, the stack moves when a new task is received. +This never happens within execution because communication is always processed before execution. +Values in the interpreter are always stored on the stack, even tuples. +Task trees grow from the top down as in a heap. +This approach allows for flexible ratios, i.e.\ many tasks and small trees or few tasks and big trees. + +The event loop of the \gls{RTS} is executed repeatedly and consists of three distinct phases. +\todo{plaa\-tje van me\-mo\-ry hier} +\todo{pseu\-do\-code hier van de ex\-e\-cu\-tie} + +\subsection{C code generation} + +%TODO evt subsubsections verwijderen +\subsection{Communication} +In the first phase, the communication channels are processed. +The messages announcing \gls{SDS} updates are applied immediately, the initialization of new tasks is delayed until the next phase. + +\subsection{Execution} +The second phase consists of executing tasks. +The \gls{RTS} executes all tasks in a round robin fashion. +If a task is not initialized yet, the bytecode of the main function is interpreted to produce the initial task tree. +The rewriting engine uses the interpreter when needed, e.g.\ to calculate the step continuations. +The rewriter and the interpreter use the same stack to store intermediate values. +Rewriting steps are small so that interleaving results in seemingly parallel execution. +In this phase new task tree nodes may be allocated. +Both rewriting and initialization are atomic operations in the sense that no processing on SDSs is done other than SDS operations from the task itself. +The host is notified if a task value is changed after a rewrite step. -\section{Run time system} +\subsection{Memory management} +The third and final phase is memory management. +Stable tasks, and unreachable task tree nodes are removed. +If a task is to be removed, tasks with higher memory addresses are moved down. +For task trees---stored in the heap---the \gls{RTS} already marks tasks and task trees as trash during rewriting so the heap can be compacted in a single pass. +This is possible because there is no sharing or cycles in task trees and nodes contain pointers pointers to their parent. \input{subfilepostamble} \end{document}