Dienstag, März 06, 2018

TableGen #4: Resolving variables

This is the fourth part of a series; see the first part for a table of contents.

It's time to look at some of the guts of TableGen itself. TableGen is split into a frontend, which parses the TableGen input, instantiates all the records, resolves variable references, and so on, and many different backends that generate code based on the instantiated records. In this series I'll be mainly focusing on the frontend, which lives in lib/TableGen/ inside the LLVM repository, e.g. here on the GitHub mirror. The backends for LLVM itself live in utils/TableGen/, together with the command line tool's main() function. Clang also has its own backends.

Let's revisit what kind of variable references there are and what kind of resolving needs to be done with an example:
class Foo<int src> {
  int Src = src;
  int Offset = 1;
  int Dst = !add(Src, Offset);

multiclass Foos<int src> {
  def a : Foo<src>;
  let Offset = 2 in
  def b : Foo<src>;

foreach i = 0-3 in
defm F#i : Foos<i>;
This is actually broken in older LLVM by one of the many bugs, but clearly it should work based on what kind of features are generally available, and with my patch series it certainly does work in the natural way. We see four kinds of variable references:
  • internally within a record, such as the initializer of Dst referencing Src and Offset
  • to a class template variable, such as Src being initialized by src
  • to a multiclass template variable, such as src being passed as a template argument for Foo
  • to a foreach iteration variable
As an aside, keep in mind that let in TableGen does not mean the same thing as in the many functional programming languages that have a similar construct. In those languages let introduces a new variable, but TableGen's let instead overrides the value of a variable that has already been defined elsewhere. In the example above, the let-statement causes the value of Offset to be changed in the record that was instantiated from the Foo class to create the b prototype inside multiclass Foos.

TableGen internally represents variable references as instances of the VarInit class, and the variables themselves are simply referenced by name. This causes some embarrassing issues around template arguments which are papered over by qualifying the variable name with the template name. If you pass the above example through a sufficiently fixed version of llvm-tblgen, one of the outputs will be the description of the Foo class:
class Foo<int Foo:src = ?> {
  int Src = Foo:src;
  int Offset = 1;
  int Dst = !add(Src, Offset);
  string NAME = ?;
As you can see, Foo:src is used to refer to the template argument. In fact, the template arguments of both classes and multiclasses are temporarily added as variables to their respective prototype records. When the class or prototype in a multiclass is instantiated, all references to the template argument variables are resolved fully, and the variables are removed (or rather, some of them are removed, and making that consistent is one of the many things I set out to clean up).

Similarly, references to foreach iteration variables are resolved when records are instantiated, although those variables aren't similarly qualified. If you want to learn more about how variable names are looked up, TGParser::ParseIDValue is a good place to start.

The order in which variables are resolved is important. In order to achieve the flexibility of overriding defaults with let-statements, internal references among record variables must be resolved after template arguments.

Actually resolving variable references used to be done by the implementations of the following virtual method of the Init class hierarchy (which represents initializers, i.e. values and expressions):
virtual Init *resolveReferences(Record &R, const RecordVal *RV) const;
This method recursively resolves references in the constituent parts of the expression and then performs constant folding, and returns the resulting value (or the original value if nothing could be resolved). Its interface is somewhat magical: R represents the "current" record which is used as a frame of reference for magical lookups in the implementation of !cast; this is a topic for another time, though. At the same time, variables referencing R are supposed to be resolved, but only if RV is null. If RV is non-null, then only references to that specific variable are supposed to be resolved. Additionally, some behaviors around unset depend on this.

This is replaced in my changes with
virtual Init *resolveReferences(Resolver &R) const;
where Resolver is an abstract base class / interface which can lookup values based on their variable names:
class Resolver {
  Record *CurRec;

  explicit Resolver(Record *CurRec) : CurRec(CurRec) {}
  virtual ~Resolver() {}

  Record *getCurrentRecord() const { return CurRec; }
  virtual Init *resolve(Init *VarName) = 0;
  virtual bool keepUnsetBits() const { return false; }
The "current record" is used as a reference for the aforementioned magical !casts, and keepUnsetBits instructs the implementation of bit sequences in BitsInit not to resolve to ? (as was explained in the third part of the series). resolve itself is implemented by one of the subclasses, most notably:
  1. MapResolver: Resolve based on a dictionary of name-value pairs.
  2. RecordResolver: Resolve variable names that appear in the current record.
  3. ShadowResolver: Delegate requests to an underlying resolver, but filter out some names.
 This last type of resolver is used by the implementations of !foreach and !foldl to avoid mistakes with nesting. Consider, for example:
class Exclamation<list<string> messages> {
  list Messages = !foreach(s, messages, s # "!");

class Greetings<list<string> names>
    : Exclamation&lt!foreach(s, names, "Hello, " # s)>;

def : Greetings<["Alice", "Bob"]>;
This effectively becomes a nested !foreach. The iteration variable is named s in both, so when substituting s for the outer !foreach, we must ensure that we don't also accidentally substitute s in the inner !foreach. We achieve this by having !foreach wrap the given resolver with a ShadowResolver. The same principle applies to !foldl as well, of course.

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