XLS: IR semantics
 XLS: IR semantics
 Data types
 Functions, procs, and blocks
 Operations
 Unary bitwise operations
 Variadic bitwise operations
 Arithmetic unary operations
 Arithmetic binary operations
 Comparison operations
 Shift operations
 Extension operations
 Channel operations
 Array operations
 Tuple operations
 Bitvector operations
 Controloriented operations
 Sequencing operations
 Other sideeffecting operations
 RTLlevel operations
The XLS IR is a pure datafloworiented IR that has the staticsingleassignment property, but is specialized for generating circuitry. The aim is to create effective circuit designs through a "lifted" understanding of the highlevel operations and their semantics, instead of trying to reverse all relevant properties via dependence analysis, which often cannot take advantage of high level knowledge that the designer holds in their mind at design time.
This document describes the semantics of the XLS intermediate representation (IR) including data types, operations, and textual representation.
Data types
Bits
A vector of bits with a fixed width.
Type syntax:
bits[N]
where N
is the number of bits.
Value syntax:
 A literal decimal number. Example:
42
.  A binary number prefixed with
0b
. Example:0b10101
 A hexadecimal number:
0x
. Example:0xdeadbeef
The representation may optionally include the bit width in which case the type
is prefixed before the literal: bits[N]:$literal
. Example: bits[8]:0xab
.
Array
A onedimensional array of elements of the same type with a fixed number of elements. An array can contain bits, arrays, or tuples as elements. Empty (zeroelement) arrays are not supported.
Type syntax:
$type[N]
: an array containing N
elements of type $type
. Examples:
 Twoelement array of 8bit bits type:
bits[8][2]
 Threeelement array of tuple type:
(bits[32], bits[2])[3]
Value syntax:
[$value_1, ... , $value_N]
where $value_n
is the value of the n
th
element. Examples:
 Array of bits elements with explicit bit count:
[bits[8]:10, bits[8]:30]
 Threeelement array consisting of twoelement arrays of bits elements:
[[1, 2], [3, 4], [5, 6]]]
Tuple
An ordered set of fixed size containing elements with potentially different types. tuples can contain bits, arrays, or tuples as elements. May be empty.
Type syntax:
($type_{0}, ..., $type_{N1})
where N
is the number of elements and where
$type_n
is the type of the n
th element.
Value syntax:
($value_{0}, ..., $value_{N1})
where $value_n
is the value of the n
th
element. Examples:
 Tuple containing two bits elements:
(0b100, 0b101)
 A nested tuple containing various element types:
((1, 2), 42, [5, 6])
Token
A type used to enforce ordering between channel operations. The token type has no value and all tokens are identical. A token is purely symbolic / semantic and has no correlate in hardware.
Type syntax:
token
Functions, procs, and blocks
The XLS IR has three functionlevel abstractions each which hold a dataflow graph of XLS IR operations: functions, procs, and blocks. Names of function, procs and blocks must be unique among their respective abstractions (functions, procs, and blocks). For example, a block cannot share a name with another block but can share a name with a function.
Function
A function is a stateless abstraction with a singleoutput which is computed from zero or more input parameters. May invoke other functions.
Proc
A Proc is a stateful abstraction with an arbitrarilytyped recurrent state. Procs can communicate with other procs via channels which (abstractly) are infinitedepth FIFOs with flow control. Channel communication is handled via send and receive IR operations. Procs may invoke functions.
TODO(meheff): 2021/11/04 Expand to include more details.
Block
A Block is an RTLlevel abstraction used for code generation. It corresponds to a single Verilog module. Procs and functions are converted to blocks as part of the code generation process. Blocks may “invoke” other blocks via instantiation. A block includes explicit representations of RTL constructs: ports, registers, and instantiations. The constructs are scoped within the block.
Port
A port is a representation of an input or output to the block. These correspond
to ports on Verilog modules. Ports can be arbitrarilytyped. In the block, each
port is represented with a input_port
or output_port
operation.
Register
A register is a representation of a hardware register (flop). Registers can be
arbitrarilytyped. Each register must have a single register_write
and a
single register_read
operation for writing and reading the register
respectively.
Each register may optionally specify its reset behavior. The reset can be
specified to occur either synchronously or asynchronously and either on the
reset
signal of the associated register_write
being activehigh or
activelow. If specified the reset value must match the type of the register. If
no reset behavior is specified then the reset
argument of register_write
must be unset.
Instantiation
An instantiation is a blockscoped construct that represents a module
instantiation at the Verilog level. The instantiated object can be another
block, a FIFO (not yet supported), or a externally defined Verilog module (not
yet supported). The instantiation is integrated into the instantiating block
with instantiation_input
and instantiation_output
operations. There is a
onetoone mapping between the instantiation input/output and the ports of the
instantiated objects.
Operations
Operations share a common syntax and have both positional and keyword arguments à la Python. Positional arguments are ordered and must appear first in the argument list. Positional arguments are exclusively the identifiers of the operands. Keyword arguments are unordered and must appear after the positional arguments. Keyword arguments can include arbitrary value types.
result = operation(pos_arg_0, ..., pos_arg_N, keyword_0=value0, ..., keyword_M=valueM, ...)
Common keyword arguments
Keyword  Type  Required  Default  Description 

pos 
SourceLocation 
no  The source location associated with this operation. The syntax is a triplet of commaseparated integer values: Fileno,Lineno,Colno 
Unary bitwise operations
Performs a bitwise operation on a single bitstyped operand.
Syntax
result = identity(operand)
result = not(operand)
Types
Value  Type 

operand 
bits[N] 
result 
bits[N] 
Operations
Operation  Opcode  Semantics 

identity 
Op::kIdentity 
result = operand 
not 
Op::kNot 
result = ~operand 
Variadic bitwise operations
Performs a bitwise operation on oneormore identicallytyped bits operands. If only a single argument is provided the operation is a noop.
Syntax
result = and(operand_{0}, ..., operand_{N1})
result = or(operand_{0}, ..., operand_{N1})
result = xor(operand_{0}, ..., operand_{N1})
Types
Value  Type 

operand_{i} 
bits[N] 
result 
bits[N] 
Operations
Operation  Opcode  Semantics 

and 
Op::kAnd 
result = lhs & rhs & ... 
or 
Op::kOr 
result = lhs \ rhs \ ... 
xor 
Op::kXor 
result = lhs ^ rhs ^ ... 
Arithmetic unary operations
Performs an arithmetic operation on a single bitstyped operand.
Syntax
result = neg(operand)
Types
Value  Type 

operand 
bits[N] 
result 
bits[N] 
Operations
Operation  Opcode  Semantics 

neg 
Op::kNeg 
result = operand 
Arithmetic binary operations
Performs an arithmetic operation on a pair of bits operands. Unsigned operations are prefixed with a 'u', and signed operations are prefixed with a 's'.
Syntax
result = add(lhs, rhs)
result = smul(lhs, rhs)
result = umul(lhs, rhs)
result = sdiv(lhs, rhs)
result = smod(lhs, rhs)
result = sub(lhs, rhs)
result = udiv(lhs, rhs)
result = umod(lhs, rhs)
result = smulp(lhs, rhs)
result = umulp(lhs, rhs)
Types
Currently signed and unsigned multiply, as wells as their partial product variants, support arbitrary width operands and result. For all other arithmetic operations the operands and the result are the same width. The expectation is that all arithmetic operations will eventually support arbitrary widths.
Operations
Operation  Opcode  Semantics 

add 
Op::kAdd 
result = lhs + rhs 
sdiv 
Op::kSDiv 
result = $signed(lhs) / $signed(rhs) * ** 
smod 
Op::kSMod 
result = $signed(lhs) % $signed(rhs) * *** 
smul 
Op::kSMul 
result = $signed(lhs) * $signed(rhs) 
smulp 
Op::kSMulp 
result[0] + result[1] = $signed(lhs) * $signed(rhs) **** 
sub 
Op::kSub 
result = lhs  rhs 
udiv 
Op::kUDiv 
result = lhs / rhs * ** 
umod 
Op::kUMod 
result = lhs % rhs * 
umul 
Op::kUMul 
result = lhs * rhs 
umulp 
Op::kUMulp 
result[0] + result[1] = lhs * rhs **** 
* Synthesizing division or modulus can lead to failing synthesis and/or problems with timing closure. It is usually best not to rely on this Verilog operator in practice, but instead explicitly instantiate a divider of choice.
** Division rounds toward zero. For unsigned division this is the same as truncation. If the divisor is zero, unsigned division produces a maximal positive value. For signed division, if the divisor is zero the result is the maximal positive value if the dividend is nonnegative or the maximal negative value if the dividend is negative.
*** The sign of the result of modulus matches the sign of the left operand. If the right operand is zero the result is zero.
**** The partial product multiply variants return a twoelement tuple with
both elements having the same type. The outputs are not fully constrained; the
operations are free to return any values that sum to the product lhs * rhs
.
Comparison operations
Performs a comparison on a pair of identicallytyped bits operands. Unsigned operations are prefixed with a 'u', and signed operations are prefixed with a 's'. Produces a result of bits[1] type.
Syntax
result = eq(lhs, rhs)
result = ne(lhs, rhs)
result = sge(lhs, rhs)
result = sgt(lhs, rhs)
result = sle(lhs, rhs)
result = slt(lhs, rhs)
result = uge(lhs, rhs)
result = ugt(lhs, rhs)
result = ule(lhs, rhs)
result = ult(lhs, rhs)
Types
Value  Type 

lhs 
bits[N] 
rhs 
bits[N] 
result 
bits[1] 
Operations
Operation  Opcode  Semantics 

eq 
Op::kEq 
result = lhs == rhs 
ne 
Op::kNe 
result = lhs != rhs 
sge 
Op::kSGe 
result = lhs >= rhs 
sgt 
Op::kSGt 
result = lhs > rhs 
sle 
Op::kSLe 
result = lhs <= rhs 
slt 
Op::kSLt 
result = lhs < rhs 
uge 
Op::kUGe 
result = lhs >= rhs 
ugt 
Op::kUGt 
result = lhs > rhs 
ule 
Op::kULe 
result = lhs <= rhs 
ult 
Op::kULt 
result = lhs < rhs 
Shift operations
Performs an shift operation on an input operand where the shift amount is specified by a second operand.
Syntax
result = shll(operand, amount)
result = shra(operand, amount)
result = shrl(operand, amount)
Types
The shifted operand and the result of the shift are the same width. Widths of the shift amount may be arbitrary.
Operations
Operation  Opcode  Semantics 

shll 
Op::kShll 
result = lhs << rhs * 
shra 
Op::kShra 
result = lhs >>> rhs (arithmetic shift right) ** 
shrl 
Op::kShrl 
result = lhs >> rhs * 
* Logically shifting greater than or equal to the number of bits in the lhs
produces a result of zero.
** Arithmetic right shifting greater than or equal to the number of bits in the
lhs
produces a result equal to all of the bits set to the sign of the lhs
.
Extension operations
Extends a bit value to a new (larger) target bitlength.
Syntax
result = zero_ext(x, new_bit_count=42)
result = sign_ext(x, new_bit_count=42)
Types
Value  Type 

arg 
bits[N] 
new_bit_count 
int64_t 
result 
bits[new_bit_count] 
Note: new_bit_count
should be >= N
or an error may be raised.
zero_ext
Zeroextends a value: turns its bitlength into the new target bitlength by filling zeroes in the most significant bits.
sign_ext
Signextends a value: turns its bitlength into the new target bitlength by filling in the most significant bits (MSbs) with the following policy:
 ones in the MSbs if the MSb of the original value was set, or
 zeros in the MSbs if the MSb of the original value was unset.
Channel operations
These operations send or receive data over channels. Channels are monomorphic, and each channel supports a fixed set of data types which are sent or received in a single transaction.
receive
Receives a data value from a specified channel. The type of the data value is
determined by the channel. An optional predicate value conditionally enables the
receive operation. An optional blocking
attribute determines whether the
receive operation is blocking. A blocking receive waits (or blocks) until valid
data is present at the channel. Compared to a blocking receive, a nonblocking
receive has an additional entry in its return tuple of type bits[1]
denoting
whether the data read is valid.
result = receive(tkn, predicate=<pred>, blocking=<bool>, channel_id=<ch>)
Types
Value  Type 

tkn 
token 
pred 
bits[1] 
result 
(token, T) if blocking == true else (token, T, bits[1]) 
Keyword arguments
Keyword  Type  Required  Default  Description 

predicate 
bits[1] 
no  A value is received iff predicate is true 

blocking 
bool 
no  true 
Whether the receive is blocking 
channel_id 
int64_t 
yes  The ID of the channel to receive data from 
If the predicate is false the data values in the result are zerofilled.
send
Sends data to a specified channel. The type of the data values is determined by the channel. An optional predicate value conditionally enables the send operation.
result = send(tkn, data, predicate=<pred>, channel_id=<ch>)
Types
Value  Type 

tkn 
token 
data 
T 
pred 
bits[1] 
result 
token 
The type of data
must match the type supported by the channel.
Keyword arguments
Keyword  Type  Required  Default  Description 

predicate 
bits[1] 
no  A value is sent iff predicate is true 

channel_id 
int64_t 
yes  The ID of the channel to send data to. 
Array operations
array
Constructs an array of its operands.
result = array(operand_{0}, ..., operand_{N1})
Types
Value  Type 

operand_{i} 
T 
result 
T[N] 
Array can take an arbitrary number of operands including zero (which produces an empty array). The nth operand becomes the nth element of the array.
array_index
Returns a single element from an array.
Syntax
result = array_index(array, indices=[idx_{0}, ... , idx_{N1}])
Types
Value  Type 

array 
Array of at least N dimensions 
idx_{i} 
Arbitrary bits type 
result 
T 
Returns the element of array
indexed by the indices idx_{0} ... idx_{N1}
.
The array must have at least as many dimensions as number of index elements N
.
Each element idx_{i}
indexes a dimension of array
. The first element
idx_{0}
indexes the outer most dimension, the second element idx_{1}
indexes
the second outer most dimension, etc. The result type T
is the type of array
with the N
outer most dimensions removed.
Any outofbounds indices idx_{i}
are clamped to the maximum in bounds index
for the respective dimension.
The table below shows examples of the result type T
and the result expression
assuming input array operand A
.
Indices  Array type  result type T 
Result expression 

{1, 2} 
bits[3][4][5] 
bits[3] 
A[1][2] 
{10, 2} 
bits[3][4][5] 
bits[3] 
A[4][2] (first index is outofbounds and clamped at the maximum index) 
{1} 
bits[3][4][5] 
bits[3][4] 
A[1] 
{} 
bits[3][4][5] 
bits[3][4][5] 
A 
{} 
bits[32] 
bits[32] 
A 
array_slice
Returns a slice of an array.
Syntax
result = array_slice(array, start, width=<width>)
Types
Value  Type 

array 
Array 
start 
Arbitrary bits type 
result 
Array with same element_type as array and size of width 
Keyword arguments
Keyword  Type  Required  Default  Description 

width 
int64_t 
yes  Width to make the resulting array. 
Returns a copy of the segment of the input array consisting of the <width>
consecutive elements starting from start
. If any element in that segment is
outofbounds of the original array the value at the corresponding index is the
final element in the array. This is consistent behavior with respect to the
index operation.
array_update
Returns a modified copy of an array.
Syntax
result = array_update(array, value, indices=[idx_{0}, ... , idx_{N1}])
Types
Value  Type 

array 
Array of at least N dimensions 
value 
T 
idx_{i} 
Arbitrary bits type 
result 
Same type as array 
Returns a copy of the input array with the element at the given indices replaced
with the given value. If any index is out of bounds, the result is identical to
the input array
. The indexing semantics is identical to array_index
with the
exception of outofbounds behavior.
Tuple operations
tuple
Constructs a tuple of its operands.
result = tuple(operand_{0}, ..., operand_{N1})
Types
Value  Type 

operand_{i} 
T_{i} 
result 
(T_{0}, ... , T_{N1}) 
Tuple can take and arbitrary number of operands including zero (which produces an empty tuple).
tuple_index
Returns a single element from a tupletyped operand.
Syntax
result = tuple_index(operand, index=<index>)
Types
Value  Type 

operand 
(T_{0}, ... , T_{N1}) 
result 
T_{<index>} 
Keyword arguments
Keyword  Type  Required  Default  Description 

index 
int64_t 
yes  Index of tuple element to produce 
Bitvector operations
bit_slice
Slices a contiguous range of bits from a bitstyped operand.
Syntax
result = bit_slice(operand, start=<start>, width=<width>)
Types
Value  Type 

operand 
bits[N] 
result 
bits[<width>] 
Keyword arguments
Keyword  Type  Required  Default  Description 

start 
int64_t 
yes  The starting bit of the slice. start is is zeroindexed where zero is the leastsignificant bit of the operand. 

width 
int64_t 
yes  The width of the slice. 
The bitwidth of operand
must be greater than or equal to <start>
plus
<width>
.
bit_slice_update
Replaces a contiguous range of bits in a bitstyped operand at a variable start index with a given value.
Syntax
result = bit_slice_update(operand, start, update_value)
Types
Value  Type 

operand 
bits[N] 
start 
bits[I] 
update_value 
bits[M] 
result 
bits[N] 
Evaluates to operand
with the contiguous M
bits starting at index start
replaced with update_value
. Outofbound bits (which occur if start + M > N
)
are ignored. Examples:
operand 
start 
update_value 
result 

bits[16]:0xabcd 
0 
bits[8]:0xff 
bits[16]:0xabff 
bits[16]:0xabcd 
4 
bits[8]:0xff 
bits[16]:0xaffd 
bits[16]:0xabcd 
12 
bits[8]:0xff 
bits[16]:0xfbcd 
bits[16]:0xabcd 
16 
bits[8]:0xff 
bits[16]:0xabcd 
dynamic_bit_slice
Slices a contiguous range of bits from a bitstyped operand, with variable starting index but fixed width. Outofbounds slicing is supported by treating all outofbounds bits as having value 0.
Syntax
result = dynamic_bit_slice(operand, start, width=<width>)
Types
Value  Type 

operand 
bits[N] 
start 
bits[M] 
result 
bits[<width>] 
start
can be of arbitrary bit width. It will be interpreted as an unsigned
integer.
Keyword arguments
Keyword  Type  Required  Default  Description 

width 
int64_t 
yes  The width of the slice. 
concat
Concatenates and arbitrary number of bitstyped operands.
result = concat(operand{0}, ..., operand{n1})
Types
Value  Type 

operand_{i} 
bits[N_{i}] 
result 
bits[Sum(N_{i})] 
This is equivalent to the verilog concat operator: result = {arg0, ..., argN}
reverse
Reverses the order of bits of its operand.
result = reverse(operand)
Types
Value  Type 

operand 
bits[N] 
result 
bits[N] 
decode
Implements a binary decoder.
result = decode(operand, width=<width>)
Types
Value  Type 

operand 
bits[N] 
result 
bits[M] 
The result width M
must be less than or equal to 2**N
where N
is the
operand width.
Keyword arguments
Keyword  Type  Required  Default  Description 

width 
int64_t 
yes  Width of the result 
decode
converts the binaryencoded operand value into a onehot result. For an
operand value of n
interpreted as an unsigned number the n
th result bit and
only the n
th result bit is set. The width of the decode
operation may be
less than the maximum value expressible by the input (2**N
 1). If the
encoded operand value is larger than the number of bits of the result the result
is zero.
encode
Implements a binary encoder.
result = encode(operand, width=<width>)
Types
Value  Type 

operand 
bits[N] 
result 
bits[M] 
The result width M
must be equal to \(\(\lceil \log_{2} N \rceil\)\).
encode
converts the onehot operand value into a binaryencoded value of the
"hot" bit of the input. If the n
th bit and only the n
th bit of the operand
is set the result is equal the value n
as an unsigned number.
If multiple bits of the input are set the result is equal to the logical or of
the results produced by the input bits individually. For example, if bit 3 and
bit 5 of an encode
input are set the result is equal to 3  5 = 7.
If no bits of the input are set the result is zero.
one_hot
Produces a bits value with exactly one bit set. The index of the set bit depends upon the input value.
This operation can be thought of like a command: "canonicalize/normalize this
input value to be onehot", where lsb_prio
indicates which side of the
bitvector wins in the case that there are multiple input bits set. Contrast this
with the decode
operation which instead converts a binaryencoded
operand value into a onehot value.
(Note: once this operation has been applied to a value, the optimizer knows that the output has the onehot property [i.e. exactly one bit is set], which helps the optimizer know that onehotselect operations that use the output value as a selector are selecting between distinct values.)
Syntax
result = one_hot(input, lsb_prio=true)
result = one_hot(input, lsb_prio=false)
Types
Value  Type 

input 
bits[N] 
result 
bits[N+1] 
Keyword arguments
Keyword  Type  Required  Default  Description 

lsb_prio 
bool 
yes  Whether the least significant bit (LSb) has priority. 
For lsb_prio=true
: result bit i
for 0 <= i < N
is set in result
iff bit
i
is set in the input and all lower bits j
for j < i
are not set in the
input.
For lsb_prio=false
: result bit i
for N1 >= i >= 0
is set in result
iff
bit i
is set in the input and all higher (more significant) bits j
for j >
i
are not set in the input.
For both lsb_prio=true
and lsb_prio=false
, result bit N
(the most
significant bit in the output) is only set if no bits in the input are set.
Examples:
one_hot(0b0011, lsb_prio=true)
=>0b00001
 note that an extra MSb has been appended to the output to potentially represent the "all zeros" case.one_hot(0b0111, lsb_prio=false)
=>0b00100
.one_hot(0b00, lsb_prio=false)
=>0b100
.one_hot(0b00, lsb_prio=true)
=>0b100
 note the output forone_hot
is the same for the allzeros case regardless of whetherlsb_prio
is true or false.
This operation is useful for constructing match or switch operation semantics where a condition is matched against an ordered set of cases and the first match is chosen. It is also useful for onehot canonicalizing, e.g. as a prelude to counting leading/trailing zeros.
Controloriented operations
For context note that, in XLS, operations are evaluated eagerly in a very
general sense: all "branches" of computation may be evaluated in full before the
result is selected via an operation such as one_hot_sel
or sel
. This model
is amenable to pipelinelike hardware execution, where operations tend to be
fixed in some spatial area and operations execute a single function, while
interconnect is used for reconfiguration purposes.
Towards this eagerevaluationcapable model, operations used within a function
are generally not Turingcomplete: operations such as counted_for
require a
finite bound so that they could be implemented using a finite amount of pipeline
area. Operations such as dynamic_counted_for
are an exception, where that
operation will only be possible to use in a timemultiplexed code generation
mode, such as the XLS sequential emitter, where arbitrary iteration to some
dynamic bound is likely to be possible.
param
A parameter to the current IR function, which can be used as an operand for operations within the function.
Syntax
Parameters have a special syntactic form distinct from other nodes, where they are listed directly in the function signature with their type.
fn f(x: bits[32]) > bits[32] {
ret identity.2 = identity(x, id=2)
}
Types
Value  Type 

name 
str 
type 
type 
sel
Selects between operands based on a selector value.
This behaves as if the selector
indexes into the values given in cases
,
providing default
if it is indexing beyond the given cases
.
Syntax
result = sel(selector, cases=[case_{0}, ... , case_{N1}], default=<default>)
A default value must be provided iff the selector
is not the correct width
for the cases
array. That is, if the number of cases is less than
\(2^{bitwidth(selector)}\) then a default value must be specified (because it must
be well defined what happens when the selector takes on values outside the case
range). If the selector is exactly the correct bitwidth a default value must not
be provided.
Types
Value  Type 

selector 
bits[M] 
case_{i} 
T 
default 
T 
result 
T 
one_hot_sel
Selects between operands based on a onehot selector, OR
ing all selected
cases if more than one case is selected.
See one_hot
for an example of the onehot selector invariant. Note that when
the selector is not onehot, this operation is still well defined.
Note that when one_hot
operations are used to precondition the selector
operand to one_hot_sel
, the XLS optimizer will try to determine when they are
unnecessary and subsequently eliminate them.
Syntax
result = one_hot_sel(selector, cases=[case_{0}, ... , case_{N1}])
Types
Value  Type 

selector 
bits[N] 
case_{i} 
T 
result 
T 
The result is the logical OR of all cases case_{i}
for which the corresponding
bit i
is set in the selector. When selector is onehot this performs a select
operation.
priority_sel
Selects between operands based on a selector, choosing the highestpriority case if more than one case is selected. Each bit in the selector corresponds to a case, with the least significant bit corresponding to the first case and having the highest priority. If there are no bits in the selector set, no case is selected and the default value of 0 is chosen.
See one_hot
for an example of the onehot selector invariant. Note that when
the selector is not onehot, this operation is still well defined.
Note that when one_hot
operations are used to precondition the selector
operand to priority_sel
, the XLS optimizer will try to determine when they are
unnecessary and subsequently eliminate them.
Syntax
result = priority_sel(selector, cases=[case_{0}, ... , case_{N1}])
Types
Value  Type 

selector 
bits[N] 
case_{i} 
T 
result 
T 
The result is the first case case_{i}
for which the corresponding bit i
is
set in the selector. If the selector is known to be onehot, then the
priority_sel()
operation is equivalent to a one_hot_sel()
.
invoke
Invokes a function. The return value for the invoked function is the result value.
Syntax
result = invoke(operand_{0}, ... , operand_{N1}, to_apply=<to_apply>)
Types
Value  Type 

init 
T 
result 
T 
Keyword arguments
Keyword  Type  Required  Default  Description 

to_apply 
string 
yes  Name of the function to use as the loop body 
map
Applies a function to the elements of an array and returns the result as an array.
Syntax
result = map(operand, to_apply=<to_apply>)
Types
Value  Type 

operand 
array[T] 
result 
array[U] 
Keyword arguments
Keyword  Type  Required  Default  Description 

to_apply 
string 
yes  Name of the function to apply to each element of the operand 
dynamic_counted_for
Invokes a dynamictrip count loop.
Syntax
result = counted_for(init, trip_count, stride, body=<body>, invariant_args=<inv_args>)
Types
Value  Type 

init 
T 
trip_count 
bits[N], treated as unsigned 
stride 
bits[M], treated as signed , 
result 
T 
Keyword arguments
Keyword  Type  Required  Default  Description 

invariant_args 
array of  yes  Names of the invariant operands as the loop body  
body 
string 
yes  Name of the function to use as the loop body 
dynamic_counted_for
invokes the function body
trip_count
times, passing
loopcarried data that starts with value init
. The induction variable is
incremented by stride
after each iteration.
 The first argument passed to
body
is the induction variable  presently, the induction variable always starts at zero and increments bystride
after every trip.  The second argument passed to
body
is the loopcarry data. The return type ofbody
must be the same as the type of theinit
loop carry data. The value returned from the last trip is the result of thecounted_for
expression.  All subsequent arguments passed to
body
are passed frominvariant_args
; e.g. if there are two members ininvariant_args
those values are passed as the third and fourth arguments.
Therefore body
should have a signature that matches the following:
body(i, loop_carry_data, [invariant_arg0, invariant_arg1, ...])
Note that we currently inspect the body
function to see what type of induction
variable (i
above) it accepts in order to pass an i
value of that type.
trip_count
must have fewer bits than i
and stride
should have fewer than
or equal number of bits to i
.
Code generation support for dynamic_counted_for
is limited because the
pipeline generator cannot handle an unknown trip count.
counted_for
Invokes a fixedtrip count loop.
Syntax
result = counted_for(init, trip_count=<trip_count>, stride=<stride>, body=<body>, invariant_args=<inv_args>)
Types
Value  Type 

init 
T 
result 
T 
Keyword arguments
Keyword  Type  Required  Default  Description 

trip_count 
int64_t 
yes  Trip count of the loop (number of times that the loop body will be executed)  
stride 
int64_t 
no  1  Stride of the induction variable 
invariant_args 
array of  yes  Names of the invariant operands as the loop body  
body 
string 
yes  Name of the function to use as the loop body 
counted_for
invokes the function body
trip_count
times, passing
loopcarried data that starts with value init
.
 The first argument passed to
body
is the induction variable  presently, the induction variable always starts at zero and increments bystride
after every trip.  The second argument passed to
body
is the loopcarry data. The return type ofbody
must be the same as the type of theinit
loop carry data. The value returned from the last trip is the result of thecounted_for
expression.  All subsequent arguments passed to
body
are passed frominvariant_args
; e.g. if there are two members ininvariant_args
those values are passed as the third and fourth arguments.
Therefore body
should have a signature that matches the following:
body(i, loop_carry_data[, invariant_arg0, invariant_arg1, ...])
Note that we currently inspect the body
function to see what type of induction
variable (i
above) it accepts in order to pass an i
value of that type.
Sequencing operations
Some operations in XLS IR are sensitive to sequence order, similar to
channel operations, but are not themselves
channelrelated. Tokens are used to determine the possible sequencing of these
effects, and after_all
can be used to join together tokens as a sequencing
merge point for concurrent threads of execution described by different tokens.
after_all
Used to construct partial orderings among channel operations.
result = after_all(operand_{0}, ..., operand_{N1})
Types
Value  Type 

operand_{i} 
token 
result 
token 
after_all
can consume an arbitrary number of token operands including zero.
Other sideeffecting operations
Aside from channels operations such as send
and receive
several other
operations have sideeffects. Care must be taken when adding, removing, or
transforming these operations, e.g., in the optimizer.
assert
Raises an error at software runtime (DSLX/IR interpretation, JIT execution, RTL simulation) if the given condition evaluates to false. The operation takes a literal string attribute which is included in the error message. This is a softwareonly operation and has no representation in the generated hardware. Tokens are used to connect the operation to the graph and order with respect to other sideeffecting operations.
result = assert(tkn, condition, message=<string>)
result = assert(tkn, condition, message=<string>, label=<string>)
Types
Value  Type 

tkn 
token 
condition 
bits[1] 
result 
token 
Keyword arguments
Keyword  Type  Required  Default  Description 

message 
string 
yes  Message to include in raised error  
label 
optional string 
yes  Label to associate with the assert statement in the generated (System)Verilog 
cover
Records the number of times the given condition evaluates to true. Just like
assert
, this is a softwareonly construct and is not emitted in a final
hardware design.
result = cover(condition, label=<string>)
Types
Value  Type 

condition 
bits[1] 
result 
() 
Keyword arguments
Keyword  Type  Required  Default  Description 

label 
string 
yes  Name associated with the counter. 
gate
Gates an arbitrarilytyped value based on a condition.
The result of the operation is the data operand if the condition is true,
otherwise the result is a zero value of the type of the data operand (i.e., the
value is gated off). A helpful mnemonic is to think of this as analogous to an
AND
gate: if the condition is true
, the value passes through, otherwise it's
zeroed.
This operation can reduce switching and may be used in power optimizations. This is intended for use in operand gating for power reduction, and the compiler may ultimately use it to perform registerlevel loadenable gating.
The operation is considered sideeffecting to prevent removal of the operation when the gated result (condition is false) is not observable. The 'sideeffect' of this operation is the effect it can have on power consumption.
Despite being 'sideeffecting' this operation is special cased to still be eligible for total removal by various passes. This will only be done in cases where the gate is redundant, for example the condition is known to be false or the data is known to be zero.
result = gate(condition, data)
Types
Value  Type 

condition 
bits[1] 
data 
T 
result 
T 
RTLlevel operations
These IR operations correspond to RTLlevel constructs in the emitted Verilog. These operations are added and used in the code generation process and may only appear in blocks (not procs or functions).
input_port
Corresponds to an input port on a Verilog module.
Syntax
result = input_port()
Types
Value  Type 

result 
T 
An input_port operation can be an arbitrary type.
output_port
Corresponds to an output port on a Verilog module. The value sent to the output port is the data operand.
Syntax
result = output_port(data)
Types
Value  Type 

data 
T 
result 
T 
register_read
Reads a value from a register.
The register is defined on the block.
Syntax
result = register_read(register=<register_name>)
Types
Value  Type 

result 
T 
The type T
of the result of the operation is the type of the register.
Keyword arguments
Keyword  Type  Required  Default  Description 

register 
string 
yes  Name of the register to read 
register_write
Writes a value to a register.
The write to the register may be conditioned upon an optional loadenable and/or reset signal. The register is defined on the block.
If reset
is given the register
associated with this read must have a
reset behavior set.
If the reset
value matches the resetactive value of the register then the
reset_value
of the register is written and the data
is ignored.
If the load_enable
argument is present the register will only be written if
the argument evaluates to 1
, remaining unchanged otherwise (i.e. if present it
is equivalent to register_write.REG(sel(load_enable, {register_read.REG,
data}))
).
The reset
and load_enable
arguments affect the value written according to
the following table.
 Register
reset  reset
value  load_enable
value  new value 
: behavior : : : :
        
 active_low == false
 false
/ 0
 not present  data

 active_low == false
 true
/ 1
 not present  reset_value

 active_low == true
 false
/ 0
 not present  reset_value

 active_low == true
 true
/ 1
 not present  data

 active_low == false
 false
/ 0
 true
/ 1
 data

 active_low == false
 true
/ 1
 true
/ 1
 reset_value

 active_low == true
 false
/ 0
 true
/ 1
 reset_value

 active_low == true
 true
/ 1
 true
/ 1
 data

 active_low == false
 false
/ 0
 false
/ 0
 No change 
 active_low == false
 true
/ 1
 false
/ 0
 reset_value

 active_low == true
 false
/ 0
 false
/ 0
 reset_value

 active_low == true
 true
/ 1
 false
/ 0
 No change 
 not present  not present  true
/ 1
 data

 not present  not present  false
/ 0
 No change 
Syntax
result = register_write(data, load_enable=<load_enable>, reset=<reset>, register=<register_name>)
Types
Value  Type 

data 
T 
load_enable 
bits[1] (optional) 
reset 
bits[1] (optional) 
result 
() (empty tuple) 
Keyword arguments
Keyword  Type  Required  Default  Description 

register 
string 
yes  Name of the register to write 
The type T
of the data operand must be the same as the the type of the
register.
instantiation_input
Corresponds to a single input port of an instantiation.
An instantiation is a blockscoped construct that represents a module
instantiation at the Verilog level. Each instantation_input
operation
corresponds to a particular port of the instantiated object, so generally a
single instantiation can have multiple associated instantiation_input
operations (one for each input port).
Syntax
result = instantiation_input(data, instantiation=<instantiation>, port_name=<port_name>)
Types
Value  Type 

data 
T 
result 
() 
Keyword arguments
Keyword  Type  Required  Default  Description 

instantiation 
string 
yes  Name of the instantiation.  
port_name 
string 
yes  Name of the associated port of the instantiation. 
The type T
of the data operand must be the same as the the type of the
associated input port of the instantiated object.
instantiation_output
Corresponds to a single output port of an instantiation.
An instantiation is a blockscoped construct that represents a module
instantiation at the Verilog level. Each instantation_output
operation
corresponds to a output particular port of the instantiated object, so generally
a single instantiation can have multiple associated instantiation_output
operations (one for each output port).
Syntax
result = instantiation_output(instantiation=<instantiation>, port_name=<port_name>)
Types
Value  Type 

result 
T 
Keyword arguments
Keyword  Type  Required  Default  Description 

instantiation 
string 
yes  Name of the instantiation.  
port_name 
string 
yes  Name of the associated port of the instantiation. 
The type T
of the result is type of the associated output port of the
instantiated object.