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diff --git a/manual/APPNOTE_011_Design_Investigation.tex b/manual/APPNOTE_011_Design_Investigation.tex index 1a0c0095..9a8f604b 100644 --- a/manual/APPNOTE_011_Design_Investigation.tex +++ b/manual/APPNOTE_011_Design_Investigation.tex @@ -49,34 +49,34 @@ \def\FIXME{{\color{red}\bf FIXME}} -\lstset{basicstyle=\ttfamily,frame=trBL,xleftmargin=2em,xrightmargin=1em,numbers=left} +\lstset{basicstyle=\ttfamily,frame=trBL,xleftmargin=0.7cm,xrightmargin=0.2cm,numbers=left} \begin{document} \title{Yosys Application Note 011: \\ Interactive Design Investigation} -\author{Clifford Wolf \\ November 2013} +\author{Clifford Wolf \\ December 2013} \maketitle \begin{abstract} Yosys \cite{yosys} can be a great environment for building custom synthesis -flows \cite{glaserwolf}. It can also be an excellent tool for teaching and -learning Verilog based RTL synthesis. In both applications it is of great -importance to be able to analyze the designs it produces easily. +flows. It can also be an excellent tool for teaching and learning Verilog based +RTL synthesis. In both applications it is of great importance to be able to +analyze the designs it produces easily. This Yosys application note covers the generation of circuit diagrams with the Yosys {\tt show} command, the selection of interesting parts of the circuit -using the {\tt select} command, and briefly discusses advanced commands for -investigating the actual behavior of circuits. +using the {\tt select} command, and briefly discusses advanced investigation +commands for evaluating circuits and solving SAT problems. \end{abstract} \section{Installation and Prerequisites} -This Application Note is based on GIT Rev. {\tt \FIXME} from \FIXME{} of -Yosys \cite{yosys}. The {\tt README} file covers how to install Yosys. The +This Application Note is based on the Yosys \cite{yosys} GIT Rev. {\tt \FIXME} from +\FIXME{}. The {\tt README} file covers how to install Yosys. The {\tt show} command requires a working installation of GraphViz \cite{graphviz} for generating the actual circuit diagrams. Yosys must be build with Qt -support in order to activate the built-in SVG viewer. Alternatively an -external viewer can be used. +support for the built-in SVG viewer. Alternatively an external viewer can be +used, if Qt is not available. \section{Overview} @@ -86,7 +86,7 @@ Sec.~\ref{intro_show} introduces the {\tt show} command and explains the symbols used in the circuit diagrams generated by it. Sec.~\ref{navigate} introduces additional commands used to navigate in the -design and select portions of the design and print additional information on +design, select portions of the design, and print additional information on the elements in the design that are not contained in the circuit diagrams. Sec.~\ref{poke} introduces commands to evaluate the design and solve SAT @@ -137,8 +137,8 @@ Subsequent calls to {\tt show} re-use the {\tt yosys-svgviewer} instance \subsection{A simple circuit} -Fig.~\ref{example_src} shows a simple synthesis script and Verilog file that -demonstrates the usage of {\tt show} in a simple setting. Note that {\tt show} +Fig.~\ref{example_src} shows a simple synthesis script and a Verilog file that +demonstrate the usage of {\tt show} in a simple setting. Note that {\tt show} is called with the {\tt -pause} option, that halts execution of the Yosys script until the user presses the Enter key. The {\tt show -pause} command also allows the user to enter an interactive shell to further investigate the @@ -148,8 +148,8 @@ So this script, when executed, will show the design after each of the three synthesis commands. The generated circuit diagrams are shown in Fig.~\ref{example_out}. The first diagram (from top to bottom) shows the design directly after being -read by the Verilog front-end. Input and output ports are visualized using -octagonal shapes. Cells are visualized as rectangles with inputs on the left +read by the Verilog front-end. Input and output ports are displayed as +octagonal shapes. Cells are displayed as rectangles with inputs on the left and outputs on the right side. The cell labels are two lines long: The first line contains a unique identifier for the cell and the second line contains the cell type. Internal cell types are prefixed with a dollar sign. The Yosys manual @@ -157,7 +157,7 @@ contains a chapter on the internal cell library used in Yosys. Constants are shown as ellipses with the constant value as label. The syntax {\tt <bit\_width>'<bits>} is used for for constants that are not 32-bit wide -and/or contain bits that are not 0 or 1 (but {\tt x} or {\tt z}). Ordinary +and/or contain bits that are not 0 or 1 (i.e. {\tt x} or {\tt z}). Ordinary 32-bit constants are written using decimal numbers. Single-bit signals are shown as thin arrows pointing from the driver to the @@ -178,7 +178,7 @@ The {\tt proc} command transforms the process from the first diagram into a multiplexer and a d-type flip-flip, which brings us to the 2nd diagram. The Rhombus shape to the right is a dangling wire. (Wire nodes are only shown -if they are dangling or have "`public"' names, for example names assigned from +if they are dangling or have ``public'' names, for example names assigned from the Verilog input.) Also note that the design now contains two instances of a {\tt BUF}-node. This are artefacts left behind by the {\tt proc}-command. It is quite usual to see such artefacts after calling commands that perform changes @@ -186,9 +186,9 @@ in the design, as most commands only care about doing the transformation in the least complicated way, not about cleaning up after them. The next call to {\tt clean} (or {\tt opt}, which includes {\tt clean} as one of its operations) will clean up this artefacts. This operation is so common in Yosys scripts that it -can simply be abbreviated by using the {\tt ;;} token, which doubles as +can simply be abbreviated with the {\tt ;;} token, which doubles as separator for commands. Unless one wants to specifically analyze this artefacts -left behind some operations, it is therefore recommended to call {\tt clean} +left behind some operations, it is therefore recommended to always call {\tt clean} before calling {\tt show}. \medskip @@ -224,20 +224,20 @@ circuit is a half-adder built from simple CMOS gates.)} As has been indicated by the last example, Yosys is can manage signal vectors (aka. multi-bit wires or buses) as native objects. This provides great advantages when analyzing circuits that operate on wide integers. But it also introduces -some additional complexity when the individual bits of of a signal vector need -to be accessed. The example show in Fig.~\ref{splice_dia} and \ref{splice_src} +some additional complexity when the individual bits of of a signal vector +are accessed. The example show in Fig.~\ref{splice_dia} and \ref{splice_src} demonstrates how such circuits are visualized by the {\tt show} command. The key elements in understanding this circuit diagram are of course the boxes with round corners and rows labeled {\tt <MSB\_LEFT>:<LSB\_LEFT> -- <MSB\_RIGHT>:<LSB\_RIGHT>}. Each of this boxes has one signal per row on one side and a common signal for all rows on the -other side. The {\tt <MSB>:<LSB>} tuples specify which bits are broken out from the signals -and are connected. So The top row of the box connecting the signals {\tt a} and {\tt b} indicates +other side. The {\tt <MSB>:<LSB>} tuples specify which bits of the signals are broken out +and connected. So the top row of the box connecting the signals {\tt a} and {\tt x} indicates that the bit 0 (i.e. the range 0:0) from signal {\tt a} is connected to bit 1 (i.e. the range 1:1) of signal {\tt x}. Lines connecting such boxes together and lines connecting such boxes to cell -ports have slightly different look to emphasise that they are not actual signal +ports have a slightly different look to emphasise that they are not actual signal wires but a necessity of the graphical representation. This distinction seems like a technicality, until one wants to debug a problem related to the way Yosys internally represents signal vectors, for example when writing custom @@ -258,11 +258,11 @@ Verilog file containing blackbox modules. There are two ways to load cell descriptions into Yosys: First the Verilog file for the cell library can be passed directly to the {\tt show} command using the {\tt -lib <filename>} option. Secondly it is possible to load cell libraries into the design with -the {\tt read\_verilog -lib <filename>} command. The later option has the great +the {\tt read\_verilog -lib <filename>} command. The 2nd method has the great advantage that the library only needs to be loaded once and can then be used in all subsequent calls to the {\tt show} command. -In addition to that the 2nd diagram was generated after {\tt splitnet -ports} +In addition to that, the 2nd diagram was generated after {\tt splitnet -ports} was run on the design. This command splits all signal vectors into individual signal bits, which is often desirable when looking at gate-level circuits. The {\tt -ports} option is required to also split module ports. Per default the @@ -279,15 +279,15 @@ plotting multiple modules in one run. In {\tt yosys-svgviewer} the left mouse button is per default bound to move the diagram (and the mouse wheel can be used for zooming in and out). However, in some cases one wants to copy text from the diagram. In this cases the -View->Interactive checkbox must be activated. This switch the rendering back-end -to one that supports interaction with the SVG file, such as selecting text. +View->Interactive checkbox must be activated. This switches the rendering back-end +in a mode that supports interaction with the SVG file, such as selecting text. In densely connected circuits it is sometimes hard to keep track of the individual signal wires. For this cases it can be useful to call {\tt show} with the {\tt -colors <integer>} argument, which randomly assigns colors to the -nets. The integer (> 0) is used as seed value for the random number -generation. Sometimes it is necessary it try some values to find an assignment -of colors that works. +nets. The integer (> 0) is used as seed value for the random color +assignments. Sometimes it is necessary it try some values to find an assignment +of colors that looks good. The command {\tt help show} prints a complete listing of all options supported by the {\tt show} command. @@ -295,15 +295,15 @@ by the {\tt show} command. \section{Navigating the design} \label{navigate} -Plotting circuit diagrams for entire modules in the design brings us only so -far. For complex modules the generated circuit diagrams are just stupidly big +Plotting circuit diagrams for entire modules in the design brings us only helps +in simple cases. For complex modules the generated circuit diagrams are just stupidly big and are no help at all. In such cases one first has to select the relevant portions of the circuit. -In addition to {\it what\/} to display one only needs to carefully decide +In addition to {\it what\/} to display one also needs to carefully decide {\it when\/} to display it, with respect to the synthesis flow. In general -it is a good idea to troubleshoot a circuit in the earliest state where -a problem can be reproduces. So if for example internal state before calling +it is a good idea to troubleshoot a circuit in the earliest state in which +a problem can be reproduced. So if, for example, the internal state before calling the {\tt techmap} command already fails to verify, it is better to troubleshoot the coarse-grain version of the circuit before {\tt techmap} than the gate-level circuit after {\tt techmap}. @@ -313,7 +313,7 @@ circuit after {\tt techmap}. Note: It is generally recommended to verify the internal state of a design by writing it to a Verilog file using {\tt write\_verilog -noexpr} and using the simulation models from {\tt simlib.v} and {\tt simcells.v} from the Yosys data -directory (see {\tt yosys-config -{}-datdir}). +directory (as printed by {\tt yosys-config -{}-datdir}). \subsection{Interactive Navigation} @@ -407,9 +407,9 @@ module-context and not design-context. \label{seladd} \end{figure} -When a module is selected using {\tt cd} command, all commands (with a few +When a module is selected using the {\tt cd} command, all commands (with a few exceptions, such as the {\tt read\_*} and {\tt write\_*} commands) operate -only on the selected module. So this can also be useful for synthesis scripts +only on the selected module. This can also be useful for synthesis scripts where different synthesis strategies should be applied to different modules in the design. @@ -441,7 +441,7 @@ dump t:\$add} will print information on all {\tt \$add} cells in the active module. Whenever a command has {\tt [selection]} as last argument in its usage help, this means that it will use the engine behind the {\tt select} command to evaluate additional arguments and use the resulting selection instead of -the selection performed by the last {\tt select} command. +the selection created by the last {\tt select} command. Normally the {\tt select} command overwrites a previous selection. The commands {\tt select -add} and {\tt select -del} can be used to add @@ -480,9 +480,9 @@ select -list} command to list the current selection.) In many cases simply adding more and more stuff to the selection is an ineffective way of selecting the interesting part of the design. Special -arguments can be used to differently combine the elements on the stack. +arguments can be used to combine the elements on the stack. For example the {\tt \%i} arguments pops the last two elements from -the stack, intersects them, and pushed the result back on the stack. So the +the stack, intersects them, and pushes the result back on the stack. So the following command will select all {\$add} cells that have the {\tt foo} attribute set: @@ -498,7 +498,7 @@ can be used to mark portions of code for analysis.) Selecting {\tt a:sumstuff} in this module will yield the circuit diagram shown in Fig.~\ref{sumprod_00}. As only the cells themselves are selected, but not the temporary wire {\tt \$1\_Y}, the two adders are shown as two disjunct -parts. This can be very useful for global signal like clock and reset signals: just +parts. This can be very useful for global signals like clock and reset signals: just unselect them using a command such as {\tt select -del clk rst} and each cell using them will get its own net label. @@ -520,8 +520,8 @@ all cells and signals that are used to generate the signal {\tt sum}. The {\tt \ action can be used to select the input cones of all object in the top selection in the stack maintained by the {\tt select} command. -As the {\tt \%x} action, this commands broadens the selection by one "`step"'. But -this time to operation inly works against the direction of data flow. That means, +As the {\tt \%x} action, this commands broadens the selection by one ``step''. But +this time the operation only works against the direction of data flow. That means, wires only select cells via output ports and cells only select wires via input ports. Fig.~\ref{select_prod} show the sequence of diagrams generated by the following @@ -558,13 +558,13 @@ action, or we only want to follow certain cell types and/or ports. This can be a patterns that can be appended to the {\tt \%ci} action. Lets consider the design from Fig.~\ref{memdemo_src}. It serves no purpose other than being a non-trivial -circuit for demonstrating the usage of {\tt \%ci} pattern. We synthesize the circuit using {\tt proc; +circuit for demonstrating some of the advanced Yosys features. We synthesize the circuit using {\tt proc; opt; memory; opt} and change to the {\tt memdemo} module with {\tt cd memdemo}. If we type {\tt show} now we see the diagram shown in Fig.~\ref{memdemo_00}. \begin{figure}[b!] \lstinputlisting{APPNOTE_011_Design_Investigation/memdemo.v} -\caption{Demo circuit for demonstrating cell/port pattern in {\tt \%ci} actions} +\caption{Demo circuit for demonstrating some advanced Yosys features} \label{memdemo_src} \end{figure} @@ -600,8 +600,8 @@ an include or exclude pattern, followed by an optional comma separated list of cell types, followed by an optional comma separated list of port names in square brackets. -Since we know that the only cell considered in this case we could as well only -specify the port names: +Since we know that the only cell considered in this case is a {\tt \$dff} cell, +we could as well only specify the port names: \begin{verbatim} show y %ci2:+[Q,D] @@ -628,14 +628,14 @@ show y %ci2:-[CLK] %ci2 From this we would learn that the next cell is a {\tt \$mux} cell and we would add additional pattern to narrow the selection on the path we are interested. In the end we would end up -with a commands such as +with a command such as \begin{verbatim} show y %ci2:+$dff[Q,D] %ci*:-$mux[S]:-$dff \end{verbatim} in which the first {\tt \%ci} jumps over the initial d-type flip-flop and the -2nd action selects the entire input cone without going multiplexer select +2nd action selects the entire input cone without going over multiplexer select inputs and flip-flop cells. The diagram produces by this command is shown in Fig.~\ref{memdemo_01}. @@ -647,7 +647,7 @@ previously also accepts this advanced syntax. This actions for traversing the circuit graph, combined with the actions for boolean operations such as intersection ({\tt \%i}) and difference ({\tt \%d}) -are a powerful tool for extracting the relevant portions of the circuit under +are powerful tools for extracting the relevant portions of the circuit under investigation. See {\tt help select} for a complete list of actions available in selections. @@ -672,7 +672,7 @@ sets up relevant selections, so they can easily be recalled, for example when Yosys needs to be re-run after a design or source code change. The {\tt history} command can be used to list all recent interactive commands. -A feature that can be useful to create such a script from the commands used in +This feature can be useful for creating such a script from the commands used in an interactive session. \section{Advanced investigation techniques} @@ -688,7 +688,7 @@ if the circuit under investigation is encapsulated in a separate module. Fig.~\ref{submod} shows how the {\tt submod} command can be used to split the circuit from Fig.~\ref{memdemo_src} and \ref{memdemo_00} into its components. -The {\tt -name} option can is used to specify the name of the new module and +The {\tt -name} option is used to specify the name of the new module and also the name of the new cell in the current module. \begin{figure}[t] @@ -766,7 +766,8 @@ commands can be applied. \begin{figure}[b] \lstinputlisting{APPNOTE_011_Design_Investigation/primetest.v} -\caption{A simple miter circuit for testing if a number is prime} +\caption{A simple miter circuit for testing if a number is prime. But it has a +problem (see main text and Fig.~\ref{primesat}).} \label{primetest} \end{figure} @@ -869,48 +870,51 @@ corresponding input values. For Example: \end{verbatim} } -Note that the {\tt sat} command support signal names in both arguments +Note that the {\tt sat} command supports signal names in both arguments to the {\tt -set} option. In the above example we used {\tt -set s1 s2} to constraint {\tt s1} and {\tt s2} to be equal. When more complex constraints are needed, a wrapper circuit must be constructed that checks the constraints and signals if the constraint was met using an extra output port, which then can be forced to a value using the {\tt -set} option. (Such a circuit that contains the circuit under test -plus additional constraint checking circuitry is called a {\tt miter\/} +plus additional constraint checking circuitry is called a {\it miter\/} circuit.) Fig.~\ref{primetest} shows a miter circuit that is supposed to be used as a prime number test. If {\tt ok} is 1 for all input values {\tt a} and {\tt b} for a given {\tt p}, then {\tt p} is prime, or at least that is the idea. -The Yosys shell session shown in Fig.~\ref{primesat} demonstrate that SAT +The Yosys shell session shown in Fig.~\ref{primesat} demonstrates that SAT solvers can even find the unexpected solutions to a problem: Using integer -overflow there actually is a way of "`factorizing"' 31. A solution would of -course be to perform the test in 32 bits, for example by replacing {\tt -p != a*b} in the miter with {\tt p != \{16'd0,a\}*b}. But as 31 fits well into -8 bits, we can also simply force the upper 8 bits of {\tt a} and {\tt b} -to zero, as is done in the second command in Fig.~\ref{primesat}. +overflow there actually is a way of ``factorizing'' 31. The clean solution +would of course be to perform the test in 32 bits, for example by replacing +{\tt p != a*b} in the miter with {\tt p != \{16'd0,a\}*b}, or by using a +temporary variable for the 32 bit product {\tt a*b}. But as 31 fits well into +8 bits (and as the purpose of this document is to show off Yosys features) +we can also simply force the upper 8 bits of {\tt a} and {\tt b} to zero for +the {\tt sat} call, as is done in the second command in Fig.~\ref{primesat} +(line 31). The {\tt -prove} option used in this example works similar to {\tt -set}, but tries to find a case in which the two arguments are not equal. If such a case is -not found, the property proven to hold for all inputs that satisfy the other +not found, the property is proven to hold for all inputs that satisfy the other constraints. It might be worth noting, that SAT solvers are not particularly efficient at factorizing large numbers. But if a small factorization problem occurs as part of a larger circuit problem, the Yosys SAT solver is perfectly capable -of solving it. This can, for example, be an issue when using SAT solvers -to prove the correct behavior of ALU circuits. +of solving it. \subsection{Solving sequential SAT problems} -\begin{figure}[t] +\begin{figure}[t!] \begin{lstlisting}[basicstyle=\ttfamily\scriptsize] yosys [memdemo]> sat -seq 6 -show y -show d -set-init-undef \ - -set-at 4 y 1 -set-at 5 y 2 -set-at 6 y 3 + -max_undef -set-at 4 y 1 -set-at 5 y 2 -set-at 6 y 3 +6. Executing SAT pass (solving SAT problems in the circuit). Full command line: sat -seq 6 -show y -show d -set-init-undef - -set-at 4 y 1 -set-at 5 y 2 -set-at 6 y 3 + -max_undef -set-at 4 y 1 -set-at 5 y 2 -set-at 6 y 3 Setting up time step 1: Final constraint equation: { } = { } @@ -947,6 +951,7 @@ Import show expression: \y Import show expression: \d Solving problem with 10322 variables and 27881 clauses.. +SAT model found. maximizing number of undefs. SAT solving finished - model found: Time Signal Name Dec Hex Bin @@ -974,7 +979,7 @@ SAT solving finished - model found: 5 \d -- -- 001x 5 \y 2 2 0010 ---- -------------------- ---------- ---------- --------------- - 6 \d 1 1 0001 + 6 \d -- -- xxxx 6 \y 3 3 0011 \end{lstlisting} \caption{Solving a sequential SAT problem in the {\tt memdemo} module from Fig.~\ref{memdemo_src}.} @@ -990,8 +995,8 @@ Fig.~\ref{memdemo_sat} show the solution to this question, as produced by the following command: \begin{verbatim} - sat -seq 6 -show y -show d -set-init-undef \ - -set-at 4 y 1 -set-at 5 y 2 -set-at 6 y 3 + sat -seq 6 -show y -show d -set-init-undef \ + -max_undef -set-at 4 y 1 -set-at 5 y 2 -set-at 6 y 3 \end{verbatim} The {\tt -seq 6} option instructs the {\tt sat} command to solve a sequential @@ -1004,13 +1009,17 @@ all registers to the undef ({\tt x}) state. The way the {\tt x} state is treated in Verilog will ensure that the solution will work for any initial state. +The {\tt -max\_undef} option instructs the {\tt sat} command to find a solution +with a maximum number of undefs. This way we can see clearly which inputs bits +are relevant to the solution. + Finally the three {\tt -set-at} options add constraints for the {\tt y} signal to play the 1, 2, 3 sequence, starting with time step 4. It is not surprising that the solution sets {\tt d = 0} in the first step, as this is the only way of setting the {\tt s1} and {\tt s2} registers to a known -value. The other options are a bit more difficult to work out manually, but -the SAT solver finds the correct solution in an instant. +value. The input values for the other steps are a bit harder to work out +manually, but the SAT solver finds the correct solution in an instant. \medskip @@ -1027,14 +1036,14 @@ many cases it is sufficient to simply display circuit diagrams, maybe use some additional commands to narrow the scope of the circuit diagrams to the interesting parts of the circuit. But some cases require more than that. For this applications Yosys provides commands that can be used to further inspect the behavior of the -circuit, either by evaluating which outputs are generated from certain inputs -({\tt eval}) or by evaluation which inputs and initial conditions can result +circuit, either by evaluating which output values are generated from certain input values +({\tt eval}) or by evaluation which input values and initial conditions can result in a certain behavior at the outputs ({\tt sat}). The SAT command can even be used to prove (or disprove) theorems regarding the circuit, in more advanced cases with the additional help of a miter circuit. -This features can be powerful tools, for the circuit designer using Yosys as a -utility for building circuits, and the software developer using Yosys as a +This features can be powerful tools for the circuit designer using Yosys as a +utility for building circuits and the software developer using Yosys as a framework for new algorithms alike. \begin{thebibliography}{9} @@ -1043,13 +1052,6 @@ framework for new algorithms alike. Clifford Wolf. The Yosys Open SYnthesis Suite. \url{http://www.clifford.at/yosys/} -\bibitem{glaserwolf} -Johann Glaser. Clifford Wolf. Methodology and Example-Driven Interconnect -Synthesis for Designing Heterogeneous Coarse-Grain Reconfigurable -Architectures. In: Jan Haase (Editor). {\it Models, Methods, and Tools for Complex Chip Design. -Lecture Notes in Electrical Engineering. Volume 265, 2014, pp 201-221.\/} -\href{http://dx.doi.org/10.1007/978-3-319-01418-0_12}{DOI 10.1007/978-3-319-01418-0\_12} - \bibitem{graphviz} Graphviz - Graph Visualization Software. \url{http://www.graphviz.org/} |