Test case: virtual regions

A virtual region holds a computed value instead of stored bytes. This test computes two values from a uint256 in storage: x, the value plus one, and tag, the first 4 bytes of the value's hash.

Test source

/**
 * the computed values example: `x` (`a + 1`) and `tag` (the first 4 bytes
 * of the hash of `a`) exist only as virtual regions computed from `a`
 */
const virtualRegionsTest: ObserveTraceTest<{ x: number; tag: string }> = {
  pointer: findExamplePointer("virtual-operand"),
  compileOptions: singleSourceCompilation({
    path: "VirtualRegions.sol",
    contractName: "VirtualRegions",
    content: `contract VirtualRegions {
      uint256 a;

      constructor() {
        a = 2;
      }
    }
    `,
  }),

  expectedValues: [
    { x: 1, tag: "0x290decd9" },
    { x: 3, tag: "0x405787fa" },
  ],

  async observe({ regions, read }) {
    const x = Number((await read(regions.lookup["x"])).asUint());

    const tag = (await read(regions.lookup["tag"])).toHex();

    return { x, tag };
  },

  equals(a, b) {
    return a.x === b.x && a.tag === b.tag;
  },
};

Tested pointer

# example computed values: `x = a + 1` after the optimizer removed `x`,
# and `tag`, the first 4 bytes of the hash of `a`, where `a` is a
# `uint256` in storage slot 0
group:
  - name: "virtual-operand"
    location: storage
    slot: 0

  - name: "x"
    location: virtual
    expression:
      ~wordsized:
        ~sum:
          - ~read: "virtual-operand"
          - 1

  - name: "tag"
    location: virtual
    expression:
      ~keccak256:
        - ~read: "virtual-operand"
    length: 4