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XLS Style Guide

The Google style guides recommend enforcing local consistency where stylistic choices are not predefined. This file notes some of the choices we make locally in the XLS project, with the relevant Google style guides (C++, Python) as their bases.

C++

  • Align the pointer or reference modifier token with the type; e.g. Foo& foo = ... instead of Foo &foo = ..., and Foo* foo = ... instead of Foo *foo= ....
  • Use /*parameter_name=*/value style comments if you choose to annotate arguments in a function invocation. clang-tidy recognizes this form, and provides a Tricorder notification if parameter_name is mismatched against the parameter name of the callee.
  • Prefer int64_t over int to avoid any possibility of overflow.
  • Always use Status or StatusOr for any error that a user could encounter.
  • Other than user-facing errors, use Status only in exceptional situations. For example, Status is good to signal that a required file does not exist but not for signaling that constant folding did not constant fold an expression.

    See how heavyweight is StatusOr for more details on thinking about the costs involved.

  • Internal errors for conditions that should never be false can use CHECK, but may also use Status or StatusOr.
  • Prefer to brace single-statement blocks. Because the XLS_ASSIGN_OR_RETURN macro expands into multiple statements, this can cause problems when using unbraced single-statement blocks. Instead of XLS developers needing to think about individual cases of single statement blocks, we brace all single statement blocks.
  • Prefer using XLS_ASSIGN_OR_RETURN / XLS_RETURN_IF_ERROR when appropriate, but when binding a StatusOr wrapped value prefer to name it thing_or so that it can be referenced without the wrapper as thing; e.g.
    absl::StatusOr<Thing> thing_or = f();
    if (!thing_or.ok()) {
      // ... handling of the status via thing_or.status() and returning ...
    }
    const Thing& thing = thing_or.value();
    
  • Prefer CHECK to DCHECK, except that DCHECK can be used to verify conditions that it would be too expensive to verify in production, but that are fast enough to include outside of production.
  • Use QCHECK and LOG(QFATAL) during program startup when verifying startup parameters (i.e., flags); prefer CHECK and LOG(FATAL) in all other circumstances, as the Q variants suppress atexit handling (including --cpu_profile).
  • Follow the C++ style guide for capitalization guidelines; however, in the somewhat ambiguous case of I/O (short for Input/Output, which we use often), the slash counts as internal spacing and therefore the capitalization we use is IO, as in WrapIO or StreamingIOReader.
  • Prefer to use the XLS_FRIEND_TEST macro vs friending manually-mangled test names.

    At times it can be useful to test unit test a private/protected member of a class, and the XLS_FRIEND_TEST macro makes this possible. Note that the test case must live outside an unnamed namespace in the test file for the "friending" to work properly.

  • For simple const accessors, for the sake of consistency in the code base, and a weak preference towards the benefits of information hiding, prefer to return view types over the apparent type of the member; e.g.
    class MyClass {
     public:
      // This return type is preferrable to `const std::vector<uint64_t>&`.
      absl::Span<const uint64_t> values() const { return values_; }
    
     private:
      std::vector<uint64_t> values_;
    };
    
  • Follow the style guide's decision to avoid RTTI. In practice, this means down_cast<> should be used instead of dynamic_cast<>. However, the style guide says to avoid hand-implementing RTTI-like workarounds. The DSLX and XLScc frontends are places where avoiding RTTI would require implementing workarounds that end up looking a lot like RTTI, so dynamic_cast<> is common and accepted for those parts of the codebase. Elsewhere, especially with IR Node types, down_cast<> should be used instead.
  • Prefer std::string_view to absl::string_view. absl::string_view mainly differs from std::string_view in construction from nullptr, which our usage/callers do not depend upon. This decision lets us switch over to the more consistent end-state sooner. Although the style guide recommends we prefer absl::string_view for now, the rationale for why does not really apply to us and their target end state is clear.
  • XLS code is often written in a functional (i.e. separating functions from the [ideally immutable] structs they operate on) and layered style, which leads to _utils.h style translation units that layer on and compose functionality. Prefer the suffix _utils.h for these, vs _helpers.h or other alternatives.
  • Static member functions should be used sparingly, generally only for factories that call a private constructor. We prefer to document implementations with a /* static */ comment as a reminder to readers (and writers that there is no this available). Comments are not an ideal way to mark this kind of information, but there should be a small number of these functions and as factories it is unlikely the static qualifier will be dropped in the future to put the comments out of sync.

Functions

  • Short or easily-explained argument lists (as defined by the developer) can be explained inline with the rest of the function comment. For more complex argument lists, the following pattern should be used:
    // <Function description>
    // Args:
    //   arg1: <arg1 description>
    //   arg2: <arg2 description>
    //   ...
    

IR nodes

Class Hierarchy and OOP Design

A frequently asked question about XLS's design is how the IR class hierarchy gels with Google style guide recommendations. This section is intended to provide a rationale for "tagging" leaf node types and for using node->Is<NodeType>() and switch (node->op()) to form categories of node types instead of a class inheritance taxonomy.

The base type Node encapsulates an element that takes input operands and produces an output, along with some metadata like type, name, and references to source locations. Each IR node (e.g. add, send, concat, etc.) extends directly from Node.

Each Node defines op() and Is<NodeType>() methods which are more performant alternatives to C++'s RTTI. For example,

if (node->Is<Send>()) {
  return node->As<Send>()->channel_id();  // As<Send>() is down_cast<Send*>()
}

or

switch (node->op()) {
  case Op::kSend:
    return down_cast<Send*>(node)->channel_id();
  // case Op:: ...
}

are common patterns on IR nodes.

In many contexts, this code would be a cry for better abstractions- some reasonable ideas include:

  1. A virtual std::optional<int64_t> Node::channel_id() (or absl::StatusOr<int64_t>) implementation.
  2. A subclass or mixin trait like ChannelNode that extends Node for Send to derive.
  3. A visitor that implements similar functionality outside of the class hierarchy.

These ideas are generally not a good fit for IR nodes. The first idea's main problem is that there are a lot of node types and the base class will become huge if it needs to contain every property of each type of node. Furthermore, the base class will be difficult to reason about without more structure- e.g. does a node with a channel_id sometimes, always, or never also have a predicate?

The second idea seems to address the problems of the first, but it is not clear how to design a useful type hierarchy for IR nodes. The subset of nodes we care about is very context dependent. The examples above invite a ChannelNode abstraction, but other places in the code might care about unary vs. n-ary ops, or ops that produce bare values vs. tuples, or some other way to group nodes. We can find ourselves facing the first idea's complexity explosion if we make a mixin trait for each pass, and there aren't good ways to inject mixin traits for each compilation unit.

The third idea of using a visitor (or, similarly, a typeclass) is used in the XLS codebase at times, but mostly where there's some well-defined behavior for most kinds of nodes. If you want to pluck out an ad-hoc subset of nodes, you need to make a new kind of visitor for that subset and it ends up being similar to the code above. In the limit, you might need all arbitrary combinations which will lead to too many visitor types to maintain centrally.

Using node->Is<NodeType>() or switch (node->op()) are concise and readable ways for the common task of operating on a new category of nodes. The typical OOP tools we'd often use instead don't map well to the needs of an IR, so we discourage adding to the base type or type hierarchy of IR nodes. We encourage gathering categories that are reused in node_util.h.

It's also worth noting that node->op(), node->Is<NodeType>(), node->As<NodeType>(), and down_cast<NodeType*>(node) are more performant than C++ RTTI and dynamic_cast<>. C++ RTTI is not designed to be cheap and if we used dynamic_cast<> instead of our own tags + down_cast<>, we expect that would perform significantly worse. Performance is not the primary rationale for the design decision discussed above, but the knock-on performance effects further support the decision.

Passing Node Types

  • Unlike most data, IR elements should be passed as non-const pointers, even when expected to be const (which would usually indicate passing them as const references). Experience has shown that IR elements often develop non-const usages over time. Consider the case of IR analysis passes - those passes themselves rarely need to mutate their input data, but they build up data structures whose users often need to mutate their contents. In addition, treating elements as pointers makes equality comparisons more straightforward (avoid taking an address of a reference) and helps avoid accidental copies (assigning a reference to local, etc.). Non-const pointer usage propagates outwards such that the few cases where a const reference could actually be appropriate become odd outliers, so our guidance is that IR elements should uniformly be passed as non-const pointers.
  • A corollary to the above is that nullptr is generally not a valid input to functions taking IR elements. When an IR element is optional, we recommend explicitly using std::optional<T*>. We deviate from the style guide here because for IR elements T* sometimes means const T&, T&, or just T in addition to T*, but which is not apparent from the signature. However, using nullptr for IR element types is OK when the usage is fully encapsulated.

Protocol buffers

  • Prefer to use proto3 specifications in all new protocol buffer files.

FAQ

How heavyweight is StatusOr?

What follows is the general guidance on how absl::StatusOr is used -- it is used extensively throughout the XLS code base as an error-style indicator object wrapper, so it is important to understand the mental model used for its cost.

Consider cost wise that: a) creating an ok StatusOr is cheap, b) creating a non-ok StatusOr is expensive (that is, imagine the non-ok Status within a StatusOr is the expensive part).

The implication being: if there's an API where "not found" is a reasonable outcome, prefer std::optional<> as a return value to indicate that / go with the grain of cost.

Something like a filesystem API would be a classic example -- where you shouldn't be rooting around looking for files that aren't there -- so a not-found absl::StatusOr result would be fine to use.

A good potential mental model is to imagine the program may run with logging of a traceback for every non-ok status that is created. (This is related to a debugging capability in Google internally called --util_status_save_stack_trace that captures backtraces when error Statuses are created.) Ideally, with such a logging flag turned on, the screen wouldn't fill up with "non error tracebacks", only tracebacks from events where something really went wrong.