Bitfield constants #1511
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Bitfield constants #1511
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bbannier
previously approved these changes
Sep 1, 2023
…lues. This will allow us to create bitfield constants where not at all elements are set. We also add an operator to test if an element is set (test forthcoming in a subsequent commit).
It's now possible to initialize a bitfield value through an assignment
from a struct constructor expression:
type BF = bitfield(8) {
a: 0..3;
b: 4..7;
c: 4..5;
};
global BF bf = [$a = 1, $c = 2];
This allows to associate an expression with each bit value. We don't use this from HILTI because there's isn't a good syntax to do so (and/or: it's not worth adding), but we'll use (and test) this from Spicy in a subsequent commit.
One can now define bitfield "constants" for parsing by providing
integer expressions with fields:
type Foo = unit {
x: bitfield(8) {
a: 0..3 = 2;
b: 4..7;
c: 7 = 1;
};
This will first parse the bitfield as usual and then enforce that the
two bit ranges that are coming with expressions (i.e., `a` and `c`)
indeed containing the expected values. If they don't, that's a parse
error.
We also support using such bitfield constants for look-ahead parsing:
type Foo = unit {
x: uint8[];
y: bitfield(8) {
a: 0..3 = 4;
b: 4..7;
};
};
This will parse uint8s until a value is discovered that has its bits
set as defined by the bitfield constant.
(We use the term "constant" loosely here: only the bits with values
are actually enforced to be constant, all others are parsed as usual.)
Closes #1467.
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