Mittwoch, Februar 21, 2018

TableGen #2: Functional Programming

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

When the basic pattern of having classes with variables that are filled in via template arguments or let-statements reaches the limits of its expressiveness, it can become useful to calculate values on the fly. TableGen provides string concatenation out of the box with the paste operator ('#'), and there are built-in functions which can be easily recognized since they start with an exclamation mark, such as !add, !srl, !eq, and !listconcat. There is even an !if-builtin and a somewhat broken and limited !foreach.

There is no way of defining new functions, but there is a pattern that can be used to make up for it: classes with ret-values:
class extractBit<int val, int bitnum> {
  bit ret = !and(!srl(val, bitnum), 1);

class Foo<int val> {
  bit bitFour = extractBit<val, 4>.ret;

def Foo1 : Foo<5>;
def Foo2 : Foo<17>;
This doesn't actually work in LLVM trunk right now because of the deficiencies around anonymous record instantiations that I mentioned in the first part of the series, but after a lot of refactoring and cleanups, I got it to work reliably. It turns out to be an extremely useful tool.

In case you're wondering, this does not support recursion and it's probably better that way. It's possible that TableGen is already accidentally Turing complete, but giving it that power on purpose seems unnecessary and might lead to abuse.

Without recursion, a number of builtin functions are required. There has been a !foreach for a long time, and it is a very odd duck:
def Defs {
  int num;

class Example<list<int> nums> {
  list<int> doubled = !foreach(Defs.num, nums, !add(Defs.num, Defs.num));

def MyNums : Example<[4, 1, 9, -3]>;
In many ways it does what you'd expect, except that having to define a dummy record with a dummy variable in this way is clearly odd and fragile. Until very recently it did not actually support everything you'd think even then, and even with the recent fixes there are plenty of bugs. Clearly, this is how !foreach should look instead:
class Example<list<int> nums> {
  list<int> doubled =
      !foreach(x, nums, !add(x, x));

def MyNums : Example<[4, 1, 9, -3]>;
... and that's what I've implemented.

This ends up being a breaking change (the only one in the whole series, hopefully), but !foreach isn't actually used in upstream LLVM proper anyway, and external projects can easily adapt.

A new feature that I have found very helpful is a fold-left operation:
class Enumeration<list<string> items> {
  list<string> ret = !foldl([], items, lhs, item,
      !listconcat(lhs, [!size(lhs) # ": " # item]));

def MyList : Enumeration<["foo", "bar", "baz"]>;
This produces the following record:
def MyList {    // Enumeration
  list<string> ret = ["0: foo", "1: bar", "2: baz"];
  string NAME = ?;
Needless to say, it was necessary to refactor the TableGen tool very deeply to enable this kind of feature, but I am quite happy with how it ended up.

The title of this entry is "Functional Programming", and in a sense I lied. Functions are not first-class values in TableGen even with my changes, so one of the core features of functional programming is missing. But that's okay: most of what you'd expect to have and actually need is now available in a consistent manner, even if it's still clunkier than in a "real" programming language. And again: making functions first-class would immediately make TableGen Turing complete. Do we really want that?

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