Kroening et al, v2c a verilog to c translator

v2c – A Verilog to C Translator
Rajdeep Mukherjee1, Michael Tautschnig2, and Daniel Kroening1
1 University of Oxford, UK
2 Queen Mary, University of London
{rajdeep.mukherjee,kroening}@cs.ox.ac.uk, mt@eecs.qmul.ac.uk
Abstract. We present v2c, a tool for translating Verilog to C. The tool accepts
synthesizable Verilog as input and generates a word-level C program as an output,
which we call the software netlist. The generated program is cycle-accurate and
bit precise. The translation is based on the synthesis semantics of Verilog. There
are several use cases for v2c, ranging from hardware property verification, co-
verification to simulation and equivalence checking. This paper gives details of
the translation and demonstrates the utility of the tool.
1 Introduction
At the bit level, formal property verification for hardware is scalable to circuits up to
the block level but runs out of capacity for SoC-level or full-chip designs. Verification
at the word level promises more efficient reasoning, and thus better scalability. How-
ever, unlike the AIGER format that is used to represent bit-level netlists, there is no
standard format to represent circuits at the word level. In this paper, we argue that hard-
ware circuits given in Verilog can be represented at the word level by encoding them
as C programs, which we call a software netlist. To this end, we present a Verilog to
C translator which we name v2c. Given a Verilog RTL design, v2c applies the synthe-
sis semantics to automatically generate an equivalent C program. The tool is available
online at http://www.cprover.org/hardware/v2c/.
The primary motivation for the transition from bit level to word level is to gain scal-
ability [5,6]. The exploitation of high-level structures for better reasoning is a standard
goal in hardware verification. We propose to take one further step: the automatic transla-
tion of hardware circuits to a software netlist model in C allows us to leverage advanced
software verification techniques such as abstract interpretation and loop acceleration,
which have never been applied in conventional bit-level hardware verification.
Verilog and C share many common operators. However, Verilog offers a number of
additional operators like part-select, bit-select from vectors, concatenation and reduc-
tion operators, which are not available in C. Additionally, Verilog statements like the
initial block, the always block, the generate statement, procedural assignment (blocking,
non-blocking) and continuous assignment are not supported in C. Further, Verilog of-
fers 4-valued data-types. These non-trivial constructs, combined with parallelism, make
the translation of Verilog to C challenging.
Supported by ERC project 280053 and the SRC task 2269.001.

Although SoC designs are increasingly written at a higher level of abstraction [3,4],
there is still a significant body of existing design IP blocks that are written in VHDL or
Verilog. Our tool v2c allows rapid generation of software netlist models from hardware
IPs given in Verilog RTL. Other tools like VTOC [2] or Verilator 3 also generate C/C++
code; however, VTOC was not obtainable; the code generated by Verilator is suitable
for simulation only and is not amenable to formal analysis.
2 v2c – The Verilog RTL to C Translator
to C Expressions
Hardware Design
(In Verilog)
Software-netlist
(in C)
Preprocessing, parsing
and type-checking
Rule-based
Vectored Assignment
Lexical Analysis
Dependency
Analysis
Translation
Convert Assignments
Fig. 1. Translation stages in v2c
Figure 1 illustrates the translation steps of v2c. The front-end phase performs macro
preprocessing, parses Verilog and checks the types. The front-end supports the 1364-
2005 IEEE Standard for Verilog HDL. It generates a type-annotated parse-tree, which
is passed to the translation phase. During the translation phase, the tool applies the
synthesis semantics and performs a rule-based translation following the Verilog mod-
ule hierarchy. The rule-based translation produces vectored assignments by mapping
bit-operations to equivalent shift and mask operations and performs a global depen-
dency analysis to determine inter-module and intra-modular dependencies. The trans-
lation phase is followed by the code-generation phase, where the intermediate vectored
expressions and translated module items are converted into C expressions. Note that
we refrain from any optimizations or abstractions to obtain a correct and trustworthy
output.
Software Netlist: A software netlist SN is a four-tuple L,A,l0,le , where L is the finite
set of locations for modeling the program counter in the corresponding sequential code,
l0 ∈ L is the initial location, le ∈ L is the error location and A ⊆ L×M ×L is the control
flow automation. The edges in A are labelled with a quantifier-free first-order formula
M over program variables, which encode an assignment or an assume statement. The
formula M is defined by five-tuples In,Out,Seq,Comb,Asgn , where In, Out, Seq,
Comb are input, output, sequential/state-holding and combinational/stateless signals,
respectively. Asgn is a finite set of assignments to Out, Seq and Comb where
– Asgn ::= CAsgn|SAsgn
– CAsgn ::= (Vc = bvExpr)|(Vc = bool), Vc ∈ Comb Out
3 http://www.veripool.org/wiki/verilator

– SAsgn ::= (Vs = bvExpr)|(Vs = bool), Vs ∈ Seq
– bvExpr ::= bvconst|bvvar|ITE(cond,bv1 ...bvn)|bvop(bv1 ...bvn),cond ∈ bool,bvi ∈
{bvconst,bvvar}
– bool ::= true|false|¬b|b1 ∧b2|b1 ∨b2|bvrel{b1 ...bn}
2.1 Translating Verilog Module Items
Data Model: The data model in Verilog is significantly different from C. Each bit of
a C integer value can have only two states, namely 0 and 1. Bits in Verilog HDL can
take one of four values, namely 0, 1, X and Z. A value of 0 represents low voltage and
value of 1 represents high voltage. Further, the values X and Z represent an unknown
logic state and a high impedance value, respectively. The simplest synthesis semantics
for X is treating it as a “don’t-care” assignment, which allows the synthesis to choose
a 0 or 1 to further improve logic minimization. v2c treats X and Z values to be non-
deterministic.
Registers, wires, parameters and constants: Verilog supports structural data types
called nets, which are wire and reg. The value of wire variables changes continuously
as the input value changes. By contrast, the reg types hold their values until another
value is assigned to them. A structure containing all state holding elements of a module
is declared in C to store the register variables. Wires are declared as local variables
in C. Verilog parameters are constants, which are frequently used to specify the width of
variables. Parameters are declared as constants in C. Verilog also allows the definition
of translation unit constants using the ‘define construct, e.g., ‘define STATE
2’b00;, which is the same as the #define preprocessor directives in C.
Variable declaration: Variables of specific bit-width (register, wire) in Verilog are
translated to the next largest native data type in C such as char, short int, long,
long long, etc.
Always and Initial blocks: Always blocks are the concurrent statements, which ex-
ecute when a variable in the sensitivity list changes. The statements enclosed inside
the always block within begin...end constructs are executed in parallel or sequentially
depending on whether it is a non-blocking or blocking statement, respectively. The be-
haviour of an initial block is the same as that of an always block, except that they are
executed exactly once, before the execution of any always block. Figure 2 and Figure 3
demonstrate the translation of Verilog always blocks. All code snippets are partial due
to space limitations.
Module Hierarchy with Input/Output Port: The communication between modules
takes place through ports. Ports can be input only, output only and inout. Figure 2 gives
an example of a Verilog module hierarchy on the left and the translated code block in C
on the right. The output ports are passed as reference to reflect the changes in the parent
module. The generated C code preserves the module hierarchy of the RTL. Structurally

identical code often aids debugging, as identifying corresponding C/RTL operations is
easier.
Module Hierarchy with Input/Output Ports Data and Function definitions in C
module top ( in1 , in2 ) ;
input [ 3 : 0 ] in1 , in2 ;
wire [ 3 : 0 ] o1 , o2 ;
and A1 ( in1 , in2 , o1 , o2 ) ;
and A2 ( . c ( o1 ) , . d ( o2 ) , . a ( o1 ) , . b ( in2 ) ) ;
endmodule
/ / Module D e f i n i t i o n
module and1 ( a , b , c , d ) ;
input [ 3 : 0 ] a , b ;
output [ 3 : 0 ] c , d ;
reg [ 3 : 0 ] c ;
always @(∗) begin
c = a & b ;
end
assign d = 1;
endmodule
s t r u c t s t a t e e l e m e n t s a n d {
unsigned char c ; };
s t r u c t s t a t e e l e m e n t s a n d sand ;
void and ( unsigned char a , unsigned char b ,
unsigned char ∗c , unsigned char ∗d ) {
∗d = 1; sand . c = a & b ;
}
void top ( unsigned char in1 , unsigned char in2 ) {
unsigned char o1 , o2 ;
and ( in1 , in2 , &o1 , &o2 ) ;
and ( o1 , in2 , &o1 , &o2 ) ;
}
void main ( ) {
unsigned char in1 , in2 ;
top ( in1 , in2 ) ;
}
Fig. 2. Handling Module hierarchy with I/O ports
Procedural Assignments: Procedural assignments are used within Verilog always and
initial blocks and are of two types: blocking and non-blocking. Blocking assignments
are executed in sequential order. However, the effect of blocking assignments is visible
immediately, whereas the effect of non-blocking assignments is delayed until all events
triggered are processed. This form of parallelism in procedural assignments are mod-
eled in v2c by first storing the value of register variables in auxiliary variables in the
beginning of the clock cycle. Each read access to the register variables are then replaced
by these auxiliary variables. This ensures that an assignment to a register variable do
not influence subsequent procedural assignments. Figure 3 illustrates the translation of
procedural assignments (given at the top) to the equivalent C semantics (given at the
bottom).
Non-blocking assignment Blocking assignment Continuous assignment
reg [ 7 : 0 ] x , y , z ;
wire in = 1 ’ b1 ;
always @( posedge clk ) begin
x <= in ;
y <= x ;
z <= y ;
end
reg [ 7 : 0 ] x , y , z ;
wire in = 1 ’ b1 ;
x = in ;
y = x ;
z = y ;
end
wire in ;
reg a , b , t ;
wire a = in ;
wire c = b ; wire d = c ;
b <= a ;
t <= b ;
end
s t r u c t smain {
unsigned char x , y , z ; } sm ;
unsigned char xs , ys , zs ;
Bool in = 1;
/ / save r e g i s t e r v a r i a b l e s
xs=sm . x ; ys=sm . y ; zs=sm . z ;
/ / update r e g i s t e r v a r i a b l e s
sm . x = in ;
sm . y = xs ;
sm . z = ys ;
s t r u c t smain {
unsigned char x , y , z ;}sm ;
Bool in = 1;
/ / clocked block
sm . x = in ;
sm . y = sm . x ;
sm . z = sm . y ;
s t r u c t smain {
Bool a , b , t ; } sm ;
Bool in , c , d , as , bs , cs , ds , t s ;
sm . a = in ; / / continuous assign
/ / save r e g i s t e r v a r i a b l e s
as=sm . a ; bs=sm . b ; t s =sm . t ;
/ / clocked block
sm . b = as ; sm . t = bs ;
/ / continuous assignment
c = sm . b ; d = c ;
Fig. 3. Tanslation of non-blocking, blocking and continuous assignments

Continuous Assignment: The continuous assignment is used to assign a value to a
wire. Continuous assignments are concurrent statements, which are immediately trig-
gered when there is any change in any of the signals used on the right-hand side. The
translation of continuous assignments are discussed next.
2.2 Dependency Analysis
v2c performs intra-modular dependency analysis to correctly model the dependencies
between the combinatorial and sequential blocks. Let us consider the following three
cases for dependency analysis which are demonstrated with an example in Figure 3.
1. A variable, say x, appearing in continuous assignment, say A, is updated directly
by the input signal and the same variable is read inside an always block. The con-
tinuous assignment A is placed before the always block to capture any change to
the input signal and subsequently propagate the updated value of x to the translated
always block.
2. A variable, say x, assigned in a continuous assignment statement, say A, appears in
the right-hand side of another continuous assignment statement, say B. In this case,
the variable assignment A is placed before the other assignment B which reads x.
3. A variable, say x, appearing in the right-hand side of a continuous assignment,
say A, is driven by an always block. This gives an ordering where the continuous
assignment is placed after the always block to capture the updated value of x.
For designs with inter-modular combinatorial paths or combinatorial loops, the combi-
natorial signals (wire variables) may settle after several executions before the next clock
cycle. The combinatorial exchanges between modules depends on the stability condi-
tion for the combinatorial signals and thus it is necessary to execute the combinatorial
logic until the stability condition is reached. Determining such stability condition for
large circuits is hard. An alternative way to handle combinatorial exchanges between
modules is by using assumptions over the signals that encode combinatorial logic in
the respective modules following synthesis semantics. An example using the latter ap-
proach is given at http://www.cprover.org/hardware/v2c/.
Bit-precise code generation: v2c generates a bit-precise software netlist model in C.
The tool automatically handles complex bit-level operators in Verilog like bit-select or
part-select operators from a vector, concatenation operators, reduction OR and other op-
erators. v2c retains the word-level structure of the Verilog RTL and generates vectored
expressions. Figure 4 shows Verilog code (at the top) and the generated C expressions
(at the bottom), which are combinations of bit-wise and arithmetic operators like bit-
wise OR, AND, multiplication, subtraction, shifts and other C operators.
3 Equivalence of Hardware and software Netlist
We have applied v2c to a range of Verilog RTL circuits, which were obtained from
different sources. We have observed that the translation produces the correct output.

Bit-select Part-select (SystemVerilog) Concatenation
wire [ 7 : 0 ] in1 , in2 ;
reg [ 7 : 0 ] out1 , out2 ;
out1 [ 7 : 5 ] = in1 [ 4 : 2 ] ;
out2 [ 6 ] = in2 [ 4 ] ;
reg [ 3 1 : 0 ] in , out ;
for ( i =0; i <=3; i ++) begin
out [8∗ i +: 8]= in [8∗ i +: 8 ] ;
end
wire [ 7 : 0 ] in1 , in2 ;
reg [ 9 : 0 ] out ;
out = {in2 [ 5 : 2 ] , in1 [ 6 : 1 ] } ;
s t r u c t smain {
unsigned char out1 , out2 ; } sm ;
sm . out1 = sm . out1 & 0 x1f |
( ( ( in1 & 0x1c)>>2)<<5);
sm . out2 = (sm . out2 & 0 xbf ) |
( ( ( in2 & 0x10)>>4)<<6);
s t r u c t smain {
unsigned i n t in , out ; } sm ;
for ( i =0; i <=3; i ++) {
x=8∗ i +(8 −1); y=8∗ i ;
sm . out =(sm . out &!(2ˆ31 −2ˆy ) )
| ( sm . in &(2ˆ31 −2ˆy ) ) ; }
s t r u c t smain {
unsigned char out ; } sm ;
sm . out = ( ( ( in2 >> 2)
& 0xF ) << 6 ) |
( ( in1 >> 1) & 0x3F ) ;
Fig. 4. Handling Bit-select, part-select from vectors and concatenation operator
While we do not have a formal proof of correctness, experiments have shown that for
property verification, valid safety properties are proven to be k-inductive for the same
value of k in the hardware and software netlist models. Conversely, for unsafe designs,
the bug is found in the same cycle for both the models.
4 Implementation
We have implemented v2c in C++ on top of the CPROVER framework [1]. We make a
pre-compiled static-binary for Linux available at http://www.cprover.org/hardware/v2c/.
We also provide several benchmarks in Verilog and the corresponding software netlist
models in C, which can be used for simulation, property verification or equivalence
checking. Currently, v2c does not support multi-clock designs, transparent latches and
designs with combinatorial loops.
5 Conclusion and Future Work
This paper presents a tool for translating Verilog RTL to C. The generated software
netlist can be used as word-level representation for hardware circuits in Verilog RTL.
This design representation allows us to leverage advanced software verification tech-
niques for hardware verification. In the future, we plan to handle combinatorial feed-
back between modules and also support a richer subset of SystemVerilog assertions for
property specification.
References
1. Clarke, E., Kroening, D., Lerda, F.: A tool for checking ANSI-C programs. In: TACAS. LNCS,
vol. 2988, pp. 168–176. Springer (2004)
2. Greaves, D.J.: A Verilog to C compiler. In: RSP. pp. 122–127. IEEE Computer Society (2000)
3. Keating, M.: The Simple Art of SoC Design. Springer (2011)
4. Liu, L., Vasudevan, S.: Scaling input stimulus generation through hybrid static and dynamic
analysis of RTL. ACM TODAES 20(1), 4:1–4:33 (2014)
5. Mukherjee, R., Kroening, D., Melham, T.: Hardware verification using software analyzers. In:
ISVLSI (2015)
6. Mukherjee, R., Schrammel, P., Kroening, D., Melham, T.: Unbounded safety verification for
hardware using software analyzers. In: DATE (2016)

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