Proposal: Decoupling Int Width from Pointer Size

Mojo Language Design
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Proposal: Decoupling Int Width from Pointer Size

Summary: Define Int as a platform-dependent type with a guaranteed minimum width of 64 bits, distinct from the system’s pointer width.

Motivation

  • Default Robustness: As the default type for literals and arguments, Int requires a generous minimum width to ensure portability and prevent overflow across platforms.
  • Divergent Requirements: The assumption that size_of(Pointer) == size_of(Integer) is a legacy constraint. Future architectural needs (e.g., 128-bit contexts) may not align linearly for both types.
  • The 32-bit Indexing Limit: Current 32-bit constraints prevent indexing collections larger than ~2GB, a critical limitation for Wasm32 targets. Adopting a 64-bit standard eliminates this bottleneck entirely.

Performance & Implementation

  • 64-bit Systems: Zero runtime overhead; maps directly to native registers.
  • 32-bit Systems (inc. Wasm): Minimal impact. Modern 32-bit architectures and Wasm32 utilize efficient hardware support for 64-bit arithmetic, adhering to Mojo’s strict performance mandates while significantly improving ergonomics and safety.