Great write-up. I really feel like any kind of performance optimization, compromise, or other detail should be accompanied by tests or assertions that capture all of the inputs that supported the decision. In this example, ideally the compromise necessary to support Windows XP would have come with an assertion that the minimum supported version of Windows was still XP or earlier. This way, the decision is remembered and revisited if XP stops being supported, because the build would break. I don't know what the chrome code looks like but I imagine something like<p><pre><code> // TODO: Remove this hack if we drop Windows XP
assert(min_win < win7)
</code></pre>
... simple. I recall finding a function deep in Google search that had been "optimized" in x86 assembly, but way back when the cache lines were 32 bytes. On Opteron and later the "optimized" code was slower than idiomatic C++. That's when I decided any kind of performance decision needs to be recorded, somehow. Either something like `assert cache_bytes==32` or just a FIXME($date) that forces someone to revisit the decision every year.