Actually, runtimes often allocate 8GB of address space because WASM has a [base32 + index32] address mode where the effective address could overflow into the 33rd bit.

On x86-64, the start of the linear memory is typically put into one of the two remaining segment registers: GS or FS. Then the code can simply use an address mode such as "GS:[RAX + RCX]" without any additional instructions for addition or bounds-checking.