Notes on Verilog support in Yosys¶
Unsupported Verilog-2005 Features¶
The following Verilog-2005 features are not supported by Yosys and there are currently no plans to add support for them:
- Non-synthesizable language features as defined in
IEC 62142(E):2005 / IEEE Std. 1364.1(E):2002
The
tri
,triand
andtrior
net typesThe
config
anddisable
keywords and library map files
Verilog Attributes and non-standard features¶
The
full_case
attribute on case statements is supported (also the non-standard// synopsys full_case
directive)The
parallel_case
attribute on case statements is supported (also the non-standard// synopsys parallel_case
directive)The
// synopsys translate_off
and// synopsys translate_on
directives are also supported (but the use of` `ifdef .. `endif `
is strongly recommended instead).The
nomem2reg
attribute on modules or arrays prohibits the automatic early conversion of arrays to separate registers. This is potentially dangerous. Usually the front-end has good reasons for converting an array to a list of registers. Prohibiting this step will likely result in incorrect synthesis results.The
mem2reg
attribute on modules or arrays forces the early conversion of arrays to separate registers.The
nomeminit
attribute on modules or arrays prohibits the creation of initialized memories. This effectively putsmem2reg
on all memories that are written to in aninitial
block and are not ROMs.The
nolatches
attribute on modules or always-blocks prohibits the generation of logic-loops for latches. Instead all not explicitly assigned values default to x-bits. This does not affect clocked storage elements such as flip-flops.The
nosync
attribute on registers prohibits the generation of a storage element. The register itself will always have all bits set to ‘x’ (undefined). The variable may only be used as blocking assigned temporary variable within an always block. This is mostly used internally by Yosys to synthesize Verilog functions and access arrays.The
nowrshmsk
attribute on a register prohibits the generation of shift-and-mask type circuits for writing to bit slices of that register.The
onehot
attribute on wires mark them as one-hot state register. This is used for example for memory port sharing and set by the fsm_map pass.The
blackbox
attribute on modules is used to mark empty stub modules that have the same ports as the real thing but do not contain information on the internal configuration. This modules are only used by the synthesis passes to identify input and output ports of cells. The Verilog backend also does not output blackbox modules on default.read_verilog
, unless called with-noblackbox
will automatically set the blackbox attribute on any empty module it reads.The
noblackbox
attribute set on an empty module preventsread_verilog
from automatically setting the blackbox attribute on the module.The
whitebox
attribute on modules triggers the same behavior asblackbox
, but is for whitebox modules, i.e. library modules that contain a behavioral model of the cell type.The
lib_whitebox
attribute overwriteswhitebox
whenread_verilog
is run in-lib
mode. Otherwise it’s automatically removed.The
dynports
attribute is used by the Verilog front-end to mark modules that have ports with a width that depends on a parameter.The
hdlname
attribute is used by some passes to document the original (HDL) name of a module when renaming a module. It should contain a single name, or, when describing a hierarchical name in a flattened design, multiple names separated by a single space character.The
keep
attribute on cells and wires is used to mark objects that should never be removed by the optimizer. This is used for example for cells that have hidden connections that are not part of the netlist, such as IO pads. Setting thekeep
attribute on a module has the same effect as setting it on all instances of the module.The
keep_hierarchy
attribute on cells and modules keeps theflatten
command from flattening the indicated cells and modules.The
gate_cost_equivalent
attribute on a module can be used to specify the estimated cost of the module as a number of basic gate instances. See the help message of commandkeep_hierarchy
which interprets this attribute.The
init
attribute on wires is set by the frontend when a register is initialized “FPGA-style” withreg foo = val
. It can be used during synthesis to add the necessary reset logic.The
top
attribute on a module marks this module as the top of the design hierarchy. Thehierarchy
command sets this attribute when called with-top
. Other commands, such asflatten
and various backends use this attribute to determine the top module.The
src
attribute is set on cells and wires created by to the string<hdl-file-name>:<line-number>
by the HDL front-end and is then carried through the synthesis. When entities are combined, a new |-separated string is created that contains all the strings from the original entities.The
defaultvalue
attribute is used to store default values for module inputs. The attribute is attached to the input wire by the HDL front-end when the input is declared with a default value.The
parameter
andlocalparam
attributes are used to mark wires that represent module parameters or localparams (when the HDL front-end is run in-pwires
mode).Wires marked with the
hierconn
attribute are connected to wires with the same name (formatcell_name.identifier
) when they are imported from sub-modules byflatten
.The
clkbuf_driver
attribute can be set on an output port of a blackbox module to mark it as a clock buffer output, and thus preventclkbufmap
from inserting another clock buffer on a net driven by such output.The
clkbuf_sink
attribute can be set on an input port of a module to request clock buffer insertion by theclkbufmap
pass.The
clkbuf_inv
attribute can be set on an output port of a module with the value set to the name of an input port of that module. When theclkbufmap
would otherwise insert a clock buffer on this output, it will instead try inserting the clock buffer on the input port (this is used to implement clock inverter cells that clock buffer insertion will “see through”).The
clkbuf_inhibit
is the default attribute to set on a wire to prevent automatic clock buffer insertion byclkbufmap
. This behaviour can be overridden by providing a custom selection toclkbufmap
.The
invertible_pin
attribute can be set on a port to mark it as invertible via a cell parameter. The name of the inversion parameter is specified as the value of this attribute. The value of the inversion parameter must be of the same width as the port, with 1 indicating an inverted bit and 0 indicating a non-inverted bit.The
iopad_external_pin
attribute on a blackbox module’s port marks it as the external-facing pin of an I/O pad, and preventsiopadmap
from inserting another pad cell on it.The module attribute
abc9_lut
is an integer attribute indicating toabc9
that this module describes a LUT with an area cost of this value, and propagation delays described usingspecify
statements.The module attribute
abc9_box
is a boolean specifying a black/white-box definition, with propagation delays described usingspecify
statements, for use byabc9
.The port attribute
abc9_carry
marks the carry-in (if an input port) and carry-out (if output port) ports of a box. This information is necessary forabc9
to preserve the integrity of carry-chains. Specifying this attribute onto a bus port will affect only its most significant bit.The module attribute
abc9_flop
is a boolean marking the module as a flip-flop. This allowsabc9
to analyse its contents in order to perform sequential synthesis.The frontend sets attributes
always_comb
,always_latch
andalways_ff
on processes derived from SystemVerilog style always blocks according to the type of the always. These are checked for correctness inproc_dlatch
.The cell attribute
wildcard_port_conns
represents wildcard port connections (SystemVerilog.*
). These are resolved to concrete connections to matching wires inhierarchy
.In addition to the
(* ... *)
attribute syntax, Yosys supports the non-standard{* ... *}
attribute syntax to set default attributes for everything that comes after the{* ... *}
statement. (Reset by adding an empty{* *}
statement.)In module parameter and port declarations, and cell port and parameter lists, a trailing comma is ignored. This simplifies writing Verilog code generators a bit in some cases.
Modules can be declared with
module mod_name(...);
(with three dots instead of a list of module ports). With this syntax it is sufficient to simply declare a module port as ‘input’ or ‘output’ in the module body.When defining a macro with
\`define
, all text between triple double quotes is interpreted as macro body, even if it contains unescaped newlines. The triple double quotes are removed from the macro body. For example:
`define MY_MACRO(a, b) """
assign a = 23;
assign b = 42;
"""
The attribute
via_celltype
can be used to implement a Verilog task or function by instantiating the specified cell type. The value is the name of the cell type to use. For functions the name of the output port can be specified by appending it to the cell type separated by a whitespace. The body of the task or function is unused in this case and can be used to specify a behavioral model of the cell type for simulation. For example:
module my_add3(A, B, C, Y);
parameter WIDTH = 8;
input [WIDTH-1:0] A, B, C;
output [WIDTH-1:0] Y;
...
endmodule
module top;
...
(* via_celltype = "my_add3 Y" *)
(* via_celltype_defparam_WIDTH = 32 *)
function [31:0] add3;
input [31:0] A, B, C;
begin
add3 = A + B + C;
end
endfunction
...
endmodule
The
wiretype
attribute is added by the verilog parser for wires of a typedef’d type to indicate the type identifier.Various
enum_value_{value}
attributes are added to wires of an enumerated type to give a map of possible enum items to their values.The
enum_base_type
attribute is added to enum items to indicate which enum they belong to (enums – anonymous and otherwise – are automatically named with an auto-incrementing counter). Note that enums are currently not strongly typed.A limited subset of DPI-C functions is supported. The plugin mechanism (see
help plugin
) can be used to load .so files with implementations of DPI-C routines. As a non-standard extension it is possible to specify a plugin alias using the<alias>:
syntax. For example:
module dpitest;
import "DPI-C" function foo:round = real my_round (real);
parameter real r = my_round(12.345);
endmodule
$ yosys -p 'plugin -a foo -i /lib/libm.so; read_verilog dpitest.v'
Sized constants (the syntax
<size>'s?[bodh]<value>
) support constant expressions as<size>
. If the expression is not a simple identifier, it must be put in parentheses. Examples:WIDTH'd42
,(4+2)'b101010
The system tasks
$finish
,$stop
and$display
are supported in initial blocks in an unconditional context (only if/case statements on expressions over parameters and constant values are allowed). The intended use for this is synthesis-time DRC.There is limited support for converting
specify
..endspecify
statements to special$specify2
,$specify3
, and$specrule
cells, for use in blackboxes and whiteboxes. Useread_verilog -specify
to enable this functionality. (By default these blocks are ignored.)The
reprocess_after
internal attribute is used by the Verilog frontend to mark cells with bindings which might depend on the specified instantiated module. Modules with such cells will be reprocessed during thehierarchy
pass once the referenced module definition(s) become available.The
smtlib2_module
attribute can be set on a blackbox module to specify a formal model directly using SMT-LIB 2. For such a module, thesmtlib2_comb_expr
attribute can be used on output ports to define their value using an SMT-LIB 2 expression. For example:
(* blackbox *)
(* smtlib2_module *)
module submod(a, b);
input [7:0] a;
(* smtlib2_comb_expr = "(bvnot a)" *)
output [7:0] b;
endmodule
Non-standard or SystemVerilog features for formal verification¶
Support for
assert
,assume
,restrict
, andcover
is enabled whenread_verilog
is called with-formal
.The system task
$initstate
evaluates to 1 in the initial state and to 0 otherwise.The system function
$anyconst
evaluates to any constant value. This is equivalent to declaring a reg asrand const
, but also works outside of checkers. (Yosys also supportsrand const
outside checkers.)The system function
$anyseq
evaluates to any value, possibly a different value in each cycle. This is equivalent to declaring a reg asrand
, but also works outside of checkers. (Yosys also supportsrand
variables outside checkers.)The system functions
$allconst
and$allseq
can be used to construct formal exist-forall problems. Assumptions only hold if the trace satisfies the assumption for all$allconst/$allseq
values. For assertions and cover statements it is sufficient if just one$allconst/$allseq
value triggers the property (similar to$anyconst/$anyseq
).Wires/registers declared using the
anyconst/anyseq/allconst/allseq
attribute (for example(* anyconst *) reg [7:0] foobar;
) will behave as if driven by a$anyconst/$anyseq/$allconst/$allseq
function.The SystemVerilog tasks
$past
,$stable
,$rose
and$fell
are supported in any clocked block.The syntax
@($global_clock)
can be used to create FFs that have no explicit clock input ($ff
cells). The same can be achieved by using@(posedge <netname>)
or@(negedge <netname>)
when<netname>
is marked with the(* gclk *)
Verilog attribute.
Supported features from SystemVerilog¶
When read_verilog
is called with -sv
, it accepts some language features
from SystemVerilog:
The
assert
statement from SystemVerilog is supported in its most basic form. In module context:assert property (<expression>);
and within an always block:assert(<expression>);
. It is transformed to an$assert
cell.The
assume
,restrict
, andcover
statements from SystemVerilog are also supported. The same limitations as with theassert
statement apply.The keywords
always_comb
,always_ff
andalways_latch
,logic
andbit
are supported.Declaring free variables with
rand
andrand const
is supported.Checkers without a port list that do not need to be instantiated (but instead behave like a named block) are supported.
SystemVerilog packages are supported. Once a SystemVerilog file is read into a design with
read_verilog
, all its packages are available to SystemVerilog files being read into the same design afterwards.- typedefs are supported (including inside packages)
type casts are currently not supported
- enums are supported (including inside packages)
but are currently not strongly typed
- packed structs and unions are supported
arrays of packed structs/unions are currently not supported
structure literals are currently not supported
- multidimensional arrays are supported
array assignment of unpacked arrays is currently not supported
array literals are currently not supported
SystemVerilog interfaces (SVIs) are supported. Modports for specifying whether ports are inputs or outputs are supported.
Assignments within expressions are supported.