Intrinsics

Intrinsics provide an implementation-specific way to extend the FIRRTL language with new operations.

Intrinsics are currently implemented as intmodule’s.

Motivation ¶

Intrinsics provide a way to add functionality to FIRRTL without having to extend the FIRRTL language. This allows a fast path for prototyping new operations to rapidly respond to output requirements. Intrinsics maintain strict definitions and type checking.

Supported Intrinsics ¶

circt_sizeof ¶

Returns the size of a type. The input port is not read from and may be any type, including uninferred types.

circt_isX ¶

Tests if the value is a literal x. FIRRTL doesn’t have a notion of ‘x per-se, but x can come in to the system from external modules and from SV constructs.
Verification constructs need to explicitly test for ‘x.

circt_plusargs_value ¶

Tests and extracts a value from simulator command line options with SystemVerilog $value$plusargs. This is described in SystemVerilog 2012 section 21.6.

We do not currently check that the format string substitution flag matches the type of the result.

circt_plusargs_test ¶

Tests simulator command line options with SystemVerilog $test$plusargs. This is described in SystemVerilog 2012 section 21.6.

circt_clock_gate ¶

Enables and disables a clock safely, without glitches, based on a boolean enable value. If the enable input is 1, the output clock produced by the clock gate is identical to the input clock. If the enable input is 0, the output clock is a constant zero.

The enable input is sampled at the rising edge of the input clock; any changes on the enable before or after that edge are ignored and do not affect the output clock.

circt_chisel_assert ¶

Generate a clocked SV assert statement, with optional formatted error message.

Example output:

wire _GEN = ~enable | cond;
assert__label: assert property (@(posedge clock) _GEN) else $error("message");

circt_chisel_ifelsefatal ¶

Generate a particular Verilog sequence that’s similar to an assertion.

Has legacy special behavior and should not be used by new code.

This intrinsic also accepts the label and guard parameters which are recorded but not used in the normal emission.

Example SV output:

`ifndef SYNTHESIS
  always @(posedge clock) begin
    if (enable & ~cond) begin
      if (`ASSERT_VERBOSE_COND_)
        $error("message");
      if (`STOP_COND_)
        $fatal;
    end
  end // always @(posedge)
`endif // not def SYNTHESIS

circt_chisel_assume ¶

Generate a clocked SV assume statement, with optional formatted error message.

Example SV output:

assume__label: assume property (@(posedge clock) ~enable | cond) else $error("message");	

circt_chisel_cover ¶

Generate a clocked SV cover statement.

Example SV output:

cover__label: cover property (@(posedge clock) enable & cond);

circt_unclocked_assume ¶

Generate a SV assume statement whose predicate is used in a sensitivity list of the enclosing always block.

Example SV output:

ifdef USE_FORMAL_ONLY_CONSTRAINTS
 `ifdef USE_UNR_ONLY_CONSTRAINTS
   wire _GEN = ~enable | pred;
   always @(edge _GEN)
     assume_label: assume(_GEN) else $error("Conditional compilation example for UNR-only and formal-only assert");
 `endif // USE_UNR_ONLY_CONSTRAINTS
endif // USE_FORMAL_ONLY_CONSTRAINTS

circt_dpi_call ¶

Call a DPI function. clock is optional and if clock is not provided, the callee is invoked when input values are changed. If provided, the dpi function is called at clock’s posedge. The result values behave like registers and the DPI function is used as a state transfer function of them.

enable operand is used to conditionally call the DPI since DPI call could be quite more expensive than native constructs. When enable is low, results of unclocked calls are undefined and evaluated into X. Users are expected to gate result values by another enable to model a default value of results.

For clocked calls, a low enable means that its register state transfer function is not called. Hence their values will not be modify in that clock.

DPI Intrinsic ABI ¶

Function Declaration:

• Imported DPI function must be a void function that has input arguments which correspond to operand types, and an output argument which correspond to a result type.
• Output argument must be a last argument.

Types:

• Operand and result types must be passive.
• A vector is lowered to an unpacked open array type, e.g. a: Vec<4, UInt<8>> to byte a [].
• A bundle is lowered to a packed struct.
• Integer types are lowered into into 2-state types.
• Small integer types (< 64 bit) must be compatible to C-types and arguments are passed by values. Users are required to use specific integer types for small integers shown in the table below. Large integers are lowered to bit and passed by a reference.

Example SV output:

node result = intrinsic(circt_dpi_call<isClocked = 1, functionName="dpi_func"> : UInt<64>, clock, enable, uint_8_value, uint_32_value, uint_8_vector)

import "DPI-C" function void dpi_func(
  input  byte    in_0,
         int     in_1,
         byte    in_2[],
  output longint out_0
);

...

logic [63:0] _dpi_func_0;
reg   [63:0] _GEN;
always @(posedge clock) begin
  if (enable) begin
    dpi_func(in1, in2, _dpi_func_0);
    _GEN <= _dpi_func_0;
  end
end