  Compilers face an intrinsic tradeoff between compilation speed and code
quality. The tradeoff is particularly stark in a dynamic setting where JIT
compilation time contributes to application runtime. Many systems now employ
multiple compilation tiers, where one tier offers fast compile speed while
another has much slower compile speed but produces higher quality code. With
proper heuristics on when to use each, the overall performance is better than
using either compiler in isolation. At the introduction of WebAssembly into the
Web platform in 2017, most engines employed optimizing compilers and
pre-compiled entire modules before execution. Yet since that time, all Web
engines have introduced new "baseline" compiler tiers for Wasm to improve
startup time. Further, many new non-web engines have appeared, some of which
also employ simple compilers. In this paper, we demystify single-pass compilers
for Wasm, explaining their internal algorithms and tradeoffs, as well as
providing a detailed empirical study of those employed in production. We show
the design of a new single-pass compiler for a research Wasm engine that
integrates with an in-place interpreter and host garbage collector using value
tags, while also supporting flexible instrumentation. In experiments, we
measure the effectiveness of optimizations targeting value tags and find,
somewhat surprisingly, that the runtime overhead can be reduced to near zero.
We also assess the relative compile speed and execution time of six baseline
compilers and place these baseline compilers in a two-dimensional tradeoff
space with other execution tiers for Wasm.
