Calling all arguments Written by me, proof-read by an LLM. Details at end. Today we’re looking at calling conventions1 - which aren’t purely optimisation related but are important to understand. The calling convention is part of the ABI (Application Binary Interface), and varies from architecture to…

Your complimentary articles You’ve read one of your four complimentary articles for this month. You can read four articles free per month. To have complete access to the thousands of philosophy articles on this site, please SUBSCRIBE NOW Happiness The Good Life Paradox Matthew Hammerton points out…

Loop-Invariant Code Motion Written by me, proof-read by an LLM. Details at end. Look back at our simple loop example - there’s an optimisation I completely glossed over. Let me show you what I mean: On every loop iteration we are calling vec.size() to compare the index value, and to check if the…

When LICM fails us Written by me, proof-read by an LLM. Details at end. Yesterday’s LICM post ended with the compiler pulling invariants like size() and get_range out of our loop - clean assembly, great performance. Job done, right? Not quite. Let’s see how that optimisation can disappear. Let’s say…

tl;dr: Weak AVL trees are replacements for AVL trees and red-black trees. The 2014 paper Rank-Balanced Trees (Haeupler, Sen, Tarjan) presents a framework using ranks and rank differences to define binary search trees. Each node has a non-negative integer rank r(x). Null nodes have rank -1. The rank…

Unrolling loops Written by me, proof-read by an LLM. Details at end. A common theme for helping the compiler optimise is to give it as much information as possible. Using the right signedness of types, targeting the right CPU model, keeping loop iterations independent, and for today’s topic: telling…

Pop goes the...population count? Written by me, proof-read by an LLM. Details at end. Who among us hasn’t looked at a number and wondered, “How many one bits are in there?” No? Just me then? Actually, this “population count” operation can be pretty useful in some cases like data compression…

What makes a game tick? Part 8 - Data Driven Multi-Threading Implementation 11 Dec 2025 on C++, Game development Let’s talk about game simulations. Today we dive into the nitty-gritty bits of implementing data driven multi-threading. In our last episode in this series we presented the concept of…

You can't fool the optimiser Written by me, proof-read by an LLM. Details at end. Sometimes you’ll step through code in a debugger and find a complex-looking loop… that executes as a single instruction. The compiler saw through the obfuscation and generated the obvious code anyway. Consider this…

Addressing the adding situation Written by me, proof-read by an LLM. Details at end. Yesterday we saw how compilers zero registers efficiently. Today let’s look at something a tiny bit less trivial (though not by much): adding two integers. What do you think a simple x86 function to add two ints1…

Multiplying with a constant Written by me, proof-read by an LLM. Details at end. So far we’ve covered addition, and subtraction mostly follows suit. There’s an obvious next step: multiplication. Specifically, let’s try multiplying by constants on x861. We’ll try several constants: 2, 3, 4, 16, 25…

ARM's barrel shifter tricks Written by me, proof-read by an LLM. Details at end. Yesterday we talked about how the compiler handles multiplication with a constant on x86. x86 has some architectural quirks (like lea) that give the compiler quite some latitude for clever optimisations. But do other…

Division Written by me, proof-read by an LLM. Details at end. Broadly computers have the same kind of trouble with integer arithmetic that we do, addition and subtraction are quick, multiplication takes a bit longer, and then division is where things start to get really slow. On an average x86,…

Multiplying our way out of division Written by me, proof-read by an LLM. Details at end. I occasionally give presentations to undergraduates, and one of my favourites is taking the students on a journey of optimising a “binary to decimal” routine1. There are a number of tricks, which I won’t go in…

Going loopy Written by me, proof-read by an LLM. Details at end. Which loop style is “best”? This very question led to the creation of Compiler Explorer! In 2011 I was arguing with my team about whether we could switch all our loops from ordinal or iterator-style to the “new” range-for1. I wrote a…

Induction variables and loops Written by me, proof-read by an LLM. Details at end. Loop optimisations often surprise us. What looks expensive might be fast, and what looks clever might be slow. Yesterday we saw how the compiler canonicalises loops so it (usually) doesn’t matter how you write them,…