## 10 Sizes and offsets as runtime invariants

GCC allows the size of a hardware register to be a runtime invariant rather than a compile-time constant. This in turn means that various sizes and offsets must also be runtime invariants rather than compile-time constants, such as:

- the size of a general `machine_mode` (see Machine Modes);
- the size of a spill slot;
- the offset of something within a stack frame;
- the number of elements in a vector;
- the size and offset of a `mem` rtx (see Regs and Memory); and
- the byte offset in a `subreg` rtx (see Regs and Memory).

The motivating example is the Arm SVE ISA, whose vector registers can be any multiple of 128 bits between 128 and 2048 inclusive. The compiler normally produces code that works for all SVE register sizes, with the actual size only being known at runtime.

GCC’s main representation of such runtime invariants is the `poly_int` class. This chapter describes what `poly_int` does, lists the available operations, and gives some general usage guidelines.