Maximum size of an array in 32 bits?

Question

According to the Rust Reference:

The isize type is a signed integer type with the same number of bits as the platform\'s pointer type. Th

Answer 1

The internals of Vec do cap the value to 4GB, both in with_capacity and grow_capacity, using

let size = capacity.checked_mul(mem::size_of::<T>())
                   .expect("capacity overflow");

which will panic if the pointer overflows.

As such, Vec-allocated slices are also capped in this way in Rust. Given that this is because of an underlying restriction in the allocation API, I would be surprised if any typical type could circumvent this. And if they did, Index on slices would be unsafe due to pointer overflow. So I hope not.

It might still not be possible to allocate all 4GB for other reasons, though. In particular, allocate won't let you allocate more than 2GB (isize::MAX bytes), so Vec is restricted to that.

Answer 2

Rust uses LLVM as compiler backend. The LLVM instruction for pointer arithmetic (GetElementPtr) takes signed integer offsets and has undefined behavior on overflow, so it is impossible to index into arrays larger than 2GB when targeting a 32-bit platform.

To avoid undefined behavior, Rust will refuse to allocate more than 2 GB in a single allocation. See Rust issue #18726 for details.