Summary:
This allows you to make some of the defs in a multiclass or `foreach`
conditional on an expression computed from the parameters or iteration
variables.
It was already possible to simulate an if statement using a `foreach`
with a dummy iteration variable and a list constructed using `!if` so
that it had length 0 or 1 depending on the condition, e.g.
foreach unusedIterationVar = !if(condition, [1], []<int>) in { ... }
But this syntax is nicer to read, and also more convenient because it
allows an else clause.
To avoid upheaval in the implementation, I've implemented `if` as pure
syntactic sugar on the `foreach` implementation: internally, `ParseIf`
actually does construct exactly the kind of foreach shown above (and
another reversed one for the else clause if present).
Reviewers: nhaehnle, hfinkel
Reviewed By: hfinkel
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D71474
Summary:
This allows you to define a global or local variable to an arbitrary
value, and refer to it in subsequent definitions.
The main use I anticipate for this is if you have to compute some
difficult function of the parameters of a multiclass, and then use it
many times. For example:
multiclass Foo<int i, string s> {
defvar op = !cast<BaseClass>("whatnot_" # s # "_" # i);
def myRecord {
dag a = (op this, (op that, the other), (op x, y, z));
int b = op.subfield;
}
def myOtherRecord<"template params including", op>;
}
There are a couple of ways to do this already, but they're not really
satisfactory. You can replace `defvar x = y` with a loop over a
singleton list, `foreach x = [y] in { ... }` - but that's unintuitive
to someone who hasn't seen that workaround idiom before, and requires
an extra pair of braces that you often didn't really want. Or you can
define a nested pair of multiclasses, with the inner one taking `x` as
a template parameter, and the outer one instantiating it just once
with the desired value of `x` computed from its other parameters - but
that makes it awkward to sequentially compute each value based on the
previous ones. I think `defvar` makes things considerably easier.
You can also use `defvar` at the top level, where it inserts globals
into the same map used by `defset`. That allows you to define global
constants without having to make a dummy record for them to live in:
defvar MAX_BUFSIZE = 512;
// previously:
// def Dummy { int MAX_BUFSIZE = 512; }
// and then refer to Dummy.MAX_BUFSIZE everywhere
Reviewers: nhaehnle, hfinkel
Reviewed By: hfinkel
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D71407
