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Bitfield constants #1511

Merged
merged 4 commits into from
Sep 7, 2023
Merged

Bitfield constants #1511

merged 4 commits into from
Sep 7, 2023
+513 −52

Commits on Sep 7, 2023

  1. Change internal tuple representation of bitfield to store optional va… 

    …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).
    @rsmmr
    rsmmr committed Sep 7, 2023
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  2. Add bitfield constants. 

    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];
    @rsmmr
    rsmmr committed Sep 7, 2023
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  3. Extend bitfield type with per-item storage for constant values. 

    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.
    @rsmmr
    rsmmr committed Sep 7, 2023
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  4. Support bitfield constants in Spicy for parsing. 

    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.
    @rsmmr
    rsmmr committed Sep 7, 2023
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