DSLX Tutorial: What is a Proc?
Up to this point, our tutorials have described stateless, non-communicating, combinational modules. To add state or communication with other actors, we need to venture into the exciting land of procs!
Procs, short for "communicating sequential processes", are the means by which DSLX models sequential and stateful modules. DSLX's semantics are based on Kahn process networks, or KPNs. A pure KPN is made up of independent computing units communicating with each other in timing-insensitive ways; whenever possible, we try to make it easy to express your desired behavior in this style.1 In XLS, our independent computing units are called processes, or procs. Each proc has a fixed set of I/O interfaces (aka channels, usually FIFO queues), a fixed amount of memory (aka state), and the ability to carry out a bounded amount of computation on their state & inputs whenever they activate.
We can think of each proc as activating as often as it can; e.g., in hardware, up to once per clock cycle. Each activation proceeds as the information & resources for it to run become available. For example, suppose the proc is designed to:
- read a number from each of channels A and B,
- add the numbers together, and
- write the result to channel C.
The first activation will wait until it can read the first numbers from each of channels A and B, then write their sum as the first value on channel C. The second activation will read the second number from each of channels A and B, then write their sum as the second value on channel C... and so on.
Note
XLS guarantees that, for each I/O operation X, the first activation's action (X0) will happen before the second activation's action (X1), and so on. Therefore, the first activation will read the first numbers on channels A and B, and write the first number on channel C. However, if a proc includes multiple I/O operations - even on the same channel - and these operations need to happen in a specific order (whether in the same activation or between activations), there is no ordering guarantee by default. Instead, you can express that using tokens, as we'll discuss later.
Channels
In general, channels provide possible I/O operations each activation can carry out. A proc can receive from an input channel or send on an output channel any number of times per activation (though for now, only once per channel2). There are multiple types of channels, which can make things rather more complicated; for now, we'll discuss the standard streaming channel type.
By default, receives are considered to be blocking operations; if no data is available on the channel, the activation will stall3 until the data becomes available. (This is part of how the defaults ensure that the results are timing-insensitive, making it easier for you to use XLS to produce more optimized hardware.) Once the data is available, the receiving proc will take the value from the channel, removing it if the channel is a FIFO queue, and proceed.
Sends, by contrast, are considered to be non-blocking operations; by default, XLS models channels as if they used infinite-depth queues, so sends can always complete.
Note
In reality, when compiled to RTL, hardware FIFOs have finite depth. By default, XLS allows for backpressure on finite-depth channels. If the channel is not able to receive new data when the send should trigger, the activation will stall until the channel is ready. This always produces correct behavior in the absence of deadlocks, but can introduce deadlocks in RTL that did not exist at higher levels.
Using these, we can implement our first example, which reads from two channels and writes the sum to a third:
pub proc adder {
A: chan<u32> in;
B: chan<u32> in;
C: chan<u32> out;
// The initial value of the proc's state (empty in this case).
init { () }
// The interface used by anything that spawns this proc, which will need to
// configure its inputs & outputs.
config (A: chan<u32> in, B: chan<u32> in, C: chan<u32> out) {
(A, B, C)
}
// The description of how this proc actually acts when running.
next(st: ()) {
let (tok_A, data_A) = recv(join(), A);
let (tok_B, data_B) = recv(join(), B);
let sum = data_A + data_B;
let tok = join(tok_A, tok_B);
send(tok, C, sum);
}
}
You might be surprised at the extra values being passed to and returned from our
recv and send operations. These are tokens, which we will discuss in more
detail below; they are used to establish ordering between operations where data
dependencies aren't sufficient.4
More generally, sends and receives can also be conditional; we can decide whether or not to carry out each operation based on a computed predicate. For example, we can write a proc that:
- reads a value x from channel A, and
- if that input is zero: * reads a value y from channel B, and * writes y to channel C.
- otherwise: * writes x to channel C.
Therefore, each activation reads one value from channel A, reads either one or zero values from channel B, and writes one value to channel C. We can implement this as follows:
pub proc fallback {
A: chan<u32> in;
B: chan<u32> in;
C: chan<u32> out;
init { () }
config (A: chan<u32> in, B: chan<u32> in, C: chan<u32> out) {
(A, B, C)
}
next(st: ()) {
let (tok, data_A) = recv(join(), A);
let (tok, data_B) = recv_if(tok, B, data_A != u32:0, u32:0);
let val = if data_A == u32:0 {
data_B
} else {
data_A
};
send(tok, C, val);
}
}
Note
You might be surprised that we didn't simply put the data_B conditional
receive inside the correct branch of our if expression. DSLX does not
currently allow receives to happen in conditional expressions, so we need to
control them separately.
State
Each proc can also have its own state elements, each of which is a piece of data of any allowed type. Each state element has an initial value, which is the value seen by the first activation; beyond that, activation N sets the state elements to values that can be read by activation N+1. This can include setting them back to whatever value activation N-1 assigned to them, effectively leaving them unchanged. It's worth noting that activation N+1 is allowed to start before the state from activation N has fully resolved; it can stall if it needs to read from the state, waiting until it can confirm that the previous activation has set the state element that it needs.
For example, we can design a proc to implement a saturating accumulator, which reads a value, adds the result to an accumulator (storing the maximum value if the result would overflow), and returns the updated accumulator value:
pub proc saturating_accumulator {
ch_in: chan<u32> in;
result: chan<u32> out;
init { u32:0 }
config (ch_in: chan<u32> in, result: chan<u32> out) {
(ch_in, result)
}
next(accumulated: u32) {
let (tok, data) = recv(join(), ch_in);
let sum = (data as u33) + (accumulated as u33);
let new_val = if sum > all_ones!<u32>() as u33 {
all_ones!<u32>()
} else {
sum as u32
};
send(tok, result, new_val);
// The last expression is the value the next activation will receive as its
// state.
new_val
}
}
State and Throughput
If we try to generate pipelined hardware for this proc (as discussed in the pipelining documentation) and cannot fit the saturating addition in a single stage, then XLS will notice that it can take more than one cycle for each activation to determine the next state after reading the current state. This means this example will not be able to achieve full throughput; i.e., it might be possible for activations to stall internally, waiting on the state from the previous activation, even though all input channels are full and no output channel is providing backpressure. By default, XLS assumes you're expecting full throughput, and will emit an error explaining this failure in terms of the worst-case throughput for the proc (the number of cycles that can elapse between two activations with no externally-caused stalls)5; the error message will also include what worst-case throughput is possible with your design as written, and how to let XLS know if this is acceptable for your use case.
Error: INVALID_ARGUMENT: Impossible to schedule proc <NAME> as specified; cannot achieve full throughput. Try `--worst_case_throughput=5`
On the other hand, we could also design a proc that compares the newest input value to the previous value, clamps the difference to the range [-5, 5], and sends the clamped difference:
pub proc clamped_diff {
ch_in: chan<s32> in;
result: chan<s32> out;
init { s32:0 }
config (ch_in: chan<s32> in, result: chan<s32> out) {
(ch_in, result)
}
next(prev: s32) {
let (tok, val) = recv(join(), ch_in);
let diff = (val as s33) - (prev as s33);
let clamped_diff = if diff > s33:5 {
s32:5
} else if diff < s33:-5 {
s32:-5
} else {
diff as s32
};
send(tok, result, clamped_diff);
val
}
}
Even if we can't fit both the subtraction and the clamping into a single pipeline stage, XLS can still pipeline this example to achieve full throughput; the new state value can be determined immediately on reading the value, so the next activation can read that state before the first activation is done computing its result.
Tokens
For computation operations, XLS can tell which operations depend on which others, and schedule them in hardware respecting these dependencies. Sometimes, though, a dependency might be external to the proc. For example, suppose our proc needs to send a message on channel A, then wait for a response on channel B. If we wait for the response before we send the message, our hardware will end up deadlocked.
To make sure XLS knows that the message needs to be sent first, we use a
token; this lets us express the dependency between these operations even
though the send does not produce any actual data that the recv can use.
Every operation where ordering effects can be important (generally because
they're visible at the interface), including I/O operations, returns a token and
can accept a token. For instance, to express the dependency we wrote above, we
can write:
// ...
let request_tok = send(join(), A, msg);
// ...
let (response_tok, response) = recv(request_tok, B);
// ...
Since the recv depends on the token produced by the send, we know that the
receive operation should not be allowed to go off until the send operation has
completed.
Of course, it's possible for an operation to need to happen after more than
one predecessor. For a simple example, maybe our request needs to be sent in
multiple parts. For this case, we have the join(tok...) function, which takes
any number of tokens and returns a single token that depends on all of them. (In
fact, we've been using that already - we wrote join() in our previous examples
whenever we needed a token that depended on nothing!) In context, this might
look like:
// ...
let request_part_1_tok = send(join(), A, request_part_1);
// ...
let request_part_2_tok = send(join(), A, request_part_2);
// ...
let request_part_3_tok = send(join(), A, request_part_3);
// ...
let full_request_tok = join(request_part_1_tok, request_part_2_tok, request_part_3_tok);
let (response_tok, response) = recv(full_request_tok, B);
// ...
In this example, our send()s can happen in any order, but we know that the
recv() will not block until after all the send()s have finished.
Cross-Activation Tokens
There are also contexts where we need to specify ordering constraints between activations. As usual, when we want to communicate some context to the next activation, we can use a state element; in this case, we can pass a token as a state element.
For example, suppose we need to write a serialization interface that takes in a
complex struct and produces a sequence of four 32-bit values. If the input
channel receives inputs A and B, we need to make sure to produce [A1, A2,
A3, A4, B1, B2, B3, B4] on the output channel. We can pass a token within each
activation to make sure that the sends are properly sequenced, but we also need
to pass it to the next activation to make sure it doesn't start serializing
B until A is finished. (i.e., we want to prevent orders like [A1, A2, B1,
...].) Using tokens in our state, we can write:
Note
This example will not work as written until DSLX supports multiple I/O operations per channel per activation.
pub proc serialize {
ch_in: chan<complex_struct> in;
result: chan<u32> out;
init { join() }
config (ch_in: chan<complex_struct> in, result: chan<u32> out) {
(ch_in, result)
}
next (tok: token) {
let (input_tok, val) = recv(join(), ch_in);
// ... (calculate val1)
let tok = send(tok, result, val1);
// ... (calculate val2)
let tok = send(tok, result, val2);
// ... (calculate val3)
let tok = send(tok, result, val3);
// ... (calculate val4)
send(tok, result, val4)
}
}
Timing-Sensitive Operations
Warning
Using timing-sensitive operations means that your circuit's behavior can depend on the exact details of scheduling. It is possible to write correct XLS code under these constraints, but the logic must work no matter how operations are pipelined, and should (within reason) be designed to be correct even if the number of stages in the pipeline varies. Testing/DV is substantially more difficult for timing-sensitive procs. As such, similar to how unsafe operations work in Rust, we recommend that you avoid using timing-sensitive operations except where strictly necessary. It's good practice to keep them confined to small well-understood procs that implement certain necessary behaviors.
Using streaming channels as documented above, there are some circuits that are simply impossible to implement; for example, you cannot implement an arbiter, a process that listens on multiple channels and forwards the highest-priority message sending at any given time, while using only blocking reads. All unimplementable circuits are timing-sensitive; their results depend on when the inputs arrive on each channel, not just on the order of arrival. This makes it much harder to write a provably correct design; however, there are still times these circuits are needed!
For these times, XLS does include some timing-sensitive operations.
In particular, XLS has a non-blocking receive operation,
recv_non_blocking(tok, ch, default). This attempts to read from the channel
ch. If the channel's queue is empty at the time of the read, it returns \
(tok, default, false). Otherwise, it acts like a normal receive,
removing the leading element
data from the ch queue and returning (tok, data, true).
We can use this operation to implement a simple arbiter, combining two channels
(data0_in and data1_in) into one result by letting the higher-priority
data (from data0_in) through whenever it is ready with no delay, and sending
the lower-priority data (from data1_in) only when there's no higher-priority
message to send:
struct Message {
value1: u32,
value2: u64,
value3: bool
}
pub proc priority_arbiter {
data0_in: chan<Message> in;
data1_in: chan<Message> in;
result: chan<(u1, Message)> out;
init { () }
config (data0_in: chan<Message> in, data1_in: chan<Message> in, result: chan<(u1, Message)> out) {
(data0_in, data1_in, result)
}
next (st: ()) {
let (tok, data0_msg, data0_valid) = recv_non_blocking(join(), data0_in, zero!<Message>());
let (tok, data1_msg, data1_valid) = recv_if_non_blocking(tok, data1_in, !data0_valid, zero!<Message>());
let (source, to_send) = if (data0_valid) {
(u1:0, data0_msg)
} else {
(u1:1, data1_msg)
};
send_if(tok, result, data0_valid || data1_valid, (source, to_send));
}
}
-
For when it's necessary, we also expose a limited number of ways to express timing-sensitive computations, which we'll discuss later. ↩
-
At the moment, DSLX only supports channels where we can prove that at most one I/O operation can fire per channel per activation. Work is in progress to allow the user to opt into multiple I/O operations per channel per activation, and we expect this support to land soon. It will be opt-in, however, since this introduces the extra overhead of an arbiter to sequence the operations, and also adds potential backpressure since each channel can still accept at most one datum per clock cycle! ↩
-
When generating pipelined hardware, no later activation can proceed past the stalled stage. This may cause activations at earlier stages to stall as well, since the resources for them to run are not yet available. Activations at later stages can proceed as normal. Once the data becomes available, the pipeline will resume all stalled operations. See the pipelining documentation for more details. ↩
-
For now,
sendandrecvneed to take and return tokens even when data dependencies are sufficient to express the ordering requirements. We do have work in progress to create a syntax where tokens are only used where necessary; watch this space! ↩ -
This is technically an inverse throughput. The throughput of a proc is properly defined as the number of activations that occur per cycle... but rather than saying that your proc has WCT 1/2, it's more natural to think in inverse throughput - the number of cycles per activation - and write WCT 2. ↩