AoCO2025 Archive — Matt Godbolt’s blog

Archive — AoCO2025

December 2025

When compilers surprise you: Sometimes compilers can surprise and delight even a jaded old engineer like me
Switching it up a bit: Taking a look at the various ways the compiler can optimise switch statements
Clever memory tricks: We learn that compilers have tricks to access memory efficiently
When SIMD Fails: Floating Point Associativity: Why floating point maths doesn't vectorise like integers, and what to do about it
SIMD City: Auto-vectorisation: Doing more with less: vectorising can speed your code up 8x or more!
Chasing your tail: The art of not (directly) coming back: tail call optimisation
Partial inlining: Inlining doesn't have to be all-or-nothing
Inlining - the ultimate optimisation: Copy paste can sometimes be a good thing, at least if the compiler does it for you
Calling all arguments: Knowing how compilers call functions can help with design - and optimisation
Aliasing: Knowing when the compiler can't optimise is important too
When LICM fails us: When aliasing can prevent loop-invariant code motion
Loop-Invariant Code Motion: The compiler can move code outside of loops to speed things up
Unswitching loops for fun and profit: Duplicating loops around can yield some decent optimisations
Pop goes the...population count?: Compilers can take advantage of some very specific instructions
Unrolling loops: Learning when the compiler decides to unroll loops for performance
Induction variables and loops: Compilers can rewrite loops to avoid expensive calculations
Going loopy: Exploring the ways optimisers deal with loop constructs
Multiplying our way out of division: How compilers avoid expensive division with multiplication tricks
Division: Division doesn't have to be slow with some clever tricks
ARM's barrel shifter tricks: The ARM architecture has a cool feature, and compilers know how to use it
Multiplying with a constant: Compilers know all the tricks for avoiding doing actual multiplies.
You can't fool the optimiser: Pattern recognition can see through obfuscated code to find the right instruction
Addressing the adding situation: We learn why adding on x86 isn't as obvious as you might think
Why xor eax, eax?: Why do compilers love xor-ing registers so much?

November 2025

Introducing the Advent of Compiler Optimisations 2025: Announcing AoCO 2025 - daily December posts about compiler optimisations

About Matt Godbolt

Matt Godbolt is a C++ developer living in Chicago. He works for Hudson River Trading on super fun but secret things. He is one half of the Two's Complement podcast. Follow him on Mastodon or Bluesky.