path: root/verilog.tex

\section{Verilog}\label{sec:verilog}

%\JW{I'm not sure the Verilog syntax figure adds enough value to be worthy of inclusion in the body -- perhaps it could be demoted to an appendix. Instead, I think it would be interesting to see a bit more about how the semantics works. Not the whole gamut of inference rules or anything, but a little bit about how the rules work and what the appropriate notion of state is.}

This section describes the Verilog semantics that were chosen for the target language, including the changes that were made to the semantics to be a better fit as an HLS target.

The Verilog standard is quite large, and not all Verilog features are needed to be able to describe hardware.  Many Features are only useful for simulation and do not affect the hardware itself.  We can therefore restrict the Verilog semantics to the synthesisable subset of Verilog~\cite{05_ieee_stand_veril_regis_trans_level_synth}.  In addition to that, HLS dictates which Verilog constructs are generated, meaning the Verilog subset that has to be modelled by the semantics can be reduced even further to only support the constructs that are needed.  Supporting a smaller subset in the semantics also means that there is less chance that the standard is misunderstood, and that the semantics actually model how the Verilog is simulated and synthesised by existing tools and how it is dictated by the standard.

The Verilog semantics are based on the semantics proposed by \citet{loow19_verif_compil_verif_proces}, which was used to create a formal translation from a logic representation encoded in the HOL4~\cite{slind08_brief_overv_hol4} theorem prover into an equivalent Verilog design.  These semantics are quite practical as they restrict themselves to a small subset of Verilog, which can nonetheless be used to model all hardware constructs one would want to design.  The main features that are supported by the syntax and semantics are module items, such as variable declarations and always blocks, statements and expressions.  As hardware designs normally describe events that will be executed periodically for an infinite amount of time, the top-level of the semantics can be described using small-step semantics, whereas the execution of one small step is then described using big-step semantics.

The semantics of Verilog differ from regular programming languages, as it is used to describe hardware directly, which is inherently parallel, instead of describing an algorithm, which is often done sequentially.  The main construct in Verilog is the always block, which is construct that contains statements.  A module can contain multiple always blocks, which all run in parallel.  These always blocks further contain statements such as if-statements or assignments to variables.  Each always block also contains a list of events at which it should trigger, which could either contain signals that are assigned to other signals in that always block, or a different signal such as a clock which will trigger the always block periodically, only the latter are actually supported in our target semantics.  An example of a rule in the semantics for executing an always block in the semantics shown below, where $\Sigma$ is the state of the registers in the module and $s$ is the statement inside the always block:

\begin{equation*}
  \inferrule[Always]{(\Sigma, s)\longrightarrow_{\text{stmnt}} \Sigma'}{(\Sigma, \yhkeyword{always @(posedge clk) } s) \longrightarrow_{\text{always}} \Sigma'}
\end{equation*}

\noindent which shows that assuming the statement $s$ in the always block runs with state $\Sigma$ and produces the new state $\Sigma'$, the always block will result in the same final state.  As only clocked always blocks are supported, and one step in the semantics correspond to one clock cycle, it means that this rule is run once per clock cycle which is what it is defined to do.

Two types of assignments are supported in always blocks: nonblocking and blocking assignment.  Nonblocking assignment modifies the signal at the end of the timestep and atomically.  Blocking assignment, on the other hand, assigns the variable directly in the always block for later signals to pick up.  To model both these assignments, the state $\Sigma$ has to be split into two parts: $\Gamma$, containing the current values of all variables, and $\Delta$, containing the values that will be assigned to the variables at the end of the clock cycle, we can therefore say that $\Sigma = (\Gamma, \Delta)$.  The nonblocking assignment can therefore be expressed as the following:

\begin{equation*}
  \inferrule[Nonblocking Reg]{(\Gamma, e) \longrightarrow_{\text{expr}} v}{((\Gamma, \Delta), d\ \yhkeyword{ <= } e) \longrightarrow_{\text{stmnt}} (\Gamma, \Delta [d \mapsto v])}\\
\end{equation*}

\noindent where assuming that $\longrightarrow_{\text{expr}}$ evaluates an expression $e$ to a value $v$, then the nonblocking assignment $d\ \yhkeyword{ <= } e$ will update the future state of the variable $d$ with value $v$.  Finally, at the end of the clock cycle, the registers need to be updated with their future values in $\Delta$, which can be shown by the rule in the semantics that runs a whole module, where $m$ is the module definition, $m_{i}$ is a list of variable declarations or always blocks inside $m$, $\sigma$ is the register that holds the current state the module is in, who's value should correspond to the program counter in 3AC and finally \textit{sf} is the current stack frame that has to be kept track on to uniquely identify the current state.

\begin{equation*}
\inferrule[Module]{\Gamma\ !\ \sigma = \yhconstant{Some } v_{\sigma} \\ (m_{i}, \Gamma, \epsilon)\ \longrightarrow_{\vec{m}} (m_{i}, \Gamma', \Delta') \\ (\Gamma' // \Delta')\ !\ \sigma = \yhconstant{Some }
v_{\sigma}'}{\yhconstant{State } \textit{sf }\ m\ v_{\sigma}\ \Gamma \longrightarrow \yhconstant{State } \textit{sf }\ m\ v_{\sigma}'\ (\Gamma' // \Delta')}
\end{equation*}

This rule describes how the top-level definition of \texttt{State} changes when one step is performed on a module $m$.

When targeting a hardware description language such as Verilog, it is important to be consistent between simulating the hardware and the behaviour of the synthesised result on actual hardware.  In the target Verilog semantics, only clocked always blocks are supported as these ensure that there are not mismatches between simulation and synthesis, as combinational always blocks that trigger on any change of an internal signal may behave differently in simulation or synthesis based on the order of operations.  This limitation in the semantics therefore helps keep the Verilog correct and consistent.  In addition to that, only nonblocking assignment is used in signals that are used in multiple always blocks, which stops any race conditions from occurring as all the signal updates happen deterministically.  Finally, a specific order of evaluation for the always blocks is chosen, and because of the limitations listed before, choosing an order is guaranteed to have the same behaviour as executing the always blocks in any order.

\subsection{Changes to the Semantics}

Some small changes were made to the semantics proposed by \citet{loow19_verif_compil_verif_proces} to adapt the semantics to work better as a HLS target, as well as adding support for features that were needed in the target language.

The main change to the semantics is the support for two-dimensional arrays, which are needed to model RAM in Verilog.  RAM is needed to model the stack in C efficiently, without having to declare a variable for each possible stack location.  In the original semantics, RAMs, as well as inputs and outputs to the module could be modelled using a function from variable names (strings) to values, which could be modified accordingly to model inputs to the module.  This is quite an abstract description of memory, which can also be expressed as an array of bitvectors, which is the path we took instead.  This requires the addition of array operators to the semantics and correct reasoning of loads and stores to the array in different always blocks simultaneously.  Consider the following Verilog code:

\begin{minted}{verilog}
always @(posedge clk) begin
  x[0] = 23;
  x[1] <= 1;
end
\end{minted}

The code above modifies one array value using nonblocking assignment and it then modifies a second entry in the array with nonblocking assignment.  If the existing semantics used for values was used to update the array, then during the merge of values for the array, the array \texttt{x} from the nonblocking association map would replace the array from the blocking association map.  This would replace \texttt{x[0]} with it's original value and therefore behave incorrectly.  Instead, updates to arrays need to be recorded for each index in the array, and merging of the blocking and nonblocking queue needs to take into account each index separately.  

Explicit support for declaring inputs, outputs and internal variables was added to the semantics to make sure that the generated Verilog also contains the correct declarations.  This adds some guarantees to the generated Verilog and ensures that it synthesises and simulates correctly.

In addition to adding support for two-dimensional arrays, support for receiving external inputs was removed from the semantics for the case of simplicity, as these are not needed for an HLS target.  The main module in Verilog models the main function in C, and as the inputs to a C function shouldn't change during it's execution, there is no need to add support for external inputs for Verilog modules.  Finally, another simplification to the semantics that was made is to use 32 bit integers instead of arrays of booleans for the bitvector representation.  As the translation only currently has support for \texttt{int}, it is possible to simplify the semantics further and not have to handle bitvectors of an arbitrary size.

%~\NR{Isn't the 'main' addition that the PL community may not have seen the always blocks? It can be counter-intuitive to them. Also have you intentionally avoided assign statements? Finally, what is $\epsilon$ in the syntax?}\YH{Yes, I will probably have to explain always blocks some more, I'll try and add a paragraph about them.  Yes, assign statements are quite complex to model and many semantics do not model them.  They are also not needed for translation.  The $\epsilon$ is an empty skip statement, which is just a semicolon.}

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