Segment Arena Builtin

The Segment Arena extends Cairo VM's memory handling by tracking segment endpoints. This approach simplifies memory operations where segments need to be allocated and finalized.

Cells Organization

Each Segment Arena builtin instance works with blocks of 3 cells that maintain the state of dictionaries:

• First cell: Contains the base address of the info pointer
• Second cell: Contains the current number of allocated segments
• Third cell: Contains the current number of squashed/finalized segments

This structure works in close conjunction with an Info segment, which is also organized in blocks of 3 cells:

• First cell: Base address of the segment
• Second cell: End address of the segment (when squashed)
• Third cell: Current number of squashed segments (squashing index)

Let's take a look at two snapshots of a Segment Arena segment, during the execution of a dummy program by the Cairo VM.

In the first snapshot, Let's look at first case when a dictionary is allocated:

• info_ptr points to new info segment
• n_dicts increments to 1
• Info segment created with three cells
• Dictionary gets new segment <3:0>

Now, In the second case one more dictionary is allocated:

• Info segment grows by three cells per dictionary
• Squashed dictionaries have end addresses set
• Squashing indices assigned sequentially
• Unfinished dictionaries have 0 end address

The second snapshot shows two error conditions. In the first case, an invalid state occurs when info_ptr contains the non-relocatable value ABC. The error is triggered when accessing the info segment. In the second case, the error occurs when there's an inconsistent state as shown in the snapshot, n_squashed is greater than n_segments.

Key Validation Rules

The builtin enforces several rules:

• Each segment must be allocated and finalized exactly once
• All cell values must be valid field elements
• Segment sizes must be non-negative
• Squashing operations must maintain sequential order
• Info segment entries must correspond to segment allocations

Implementation References

These implementation references of the Segment Arena builtin might not be exhaustive.

• TypeScript Segment Arena Builtin
• Rust Segment Arena Builtin
• Zig Segment Arena Builtin