WebAssembly: The Definitive Guide

Chapter 4. WebAssembly Memory

Perhaps one day this too will be pleasant to remember.

Virgil

If WebAssembly is going to behave like a regular runtime environment, it needs a way to allocate and free memory for its data-handling activities. In this chapter, we will introduce you to how it emulates this behavior for efficiency but without the risk of typical memory manipulation problems seen with languages like C and C++ (even if that is what we are running). As we are potentially downloading arbitrary code over the internet, this is an important safety consideration.

The entire concept of computation usually involves some form of data processing. Whether we are spell-checking a document, manipulating an image, doing machine learning, sequencing proteins, playing video games, watching movies, or simply crunching numbers in a spreadsheet, we are generally interacting with arbitrary blocks of data. One of the most crucial performance considerations in these systems is how to get the data where it needs to be in order to interrogate or transform it somehow.

Central Processing Units (CPUs) work the fastest when data is available in a register or an on-chip cache.¹ Obviously these are very small containers, so large data sets are never going to be loaded onto the CPU in their entirety. We have to spend some effort moving data into and out of memory. The cost of waiting for the data to be loaded to one of these locations is an eternity in CPU clock time. This is one of the reasons they ...