matt godboltDec 11

Day 11 of Advent of Compiler Optimisations!

A clever loop that counts set bits using the "clear bottom bit" trick: value &= value - 1. Works great, generates tight assembly. But change one compiler flag to target a slightly newer CPU and something extraordinary happens to your loop. The compiler spots a pattern you didn't even know was there. What replaces your careful bit manipulation?

Read more: https://xania.org/202512/11-pop-goes-the-weasel-er-count
Watch: https://youtu.be/Hu0vu1tpZnc

#AoCO2025

Show thread🌸 lily 🏳️‍⚧️   θΔ ⋐ & ∞Dec 11
@mattgodbolt i'm banned off your website for some reason
Show threadМя ��Dec 11

@tauon same.

@mattgodbolt are you blocking Hetzner IP ranges or something broke?

Show threadmatt godboltDec 11
@mo @tauon sorry I typod something in my deploy scripts...
Show threadМя ��Dec 11

@mattgodbolt last picture[1] still returns AccessDenied, but other parts of the page loads fine, tnx

[1] https://xania.org/202512/11-popcount-clang-opt.png

@tauon

Show threadmatt godbolt
@mo @tauon thanks! On it! I tested this stuff haha but until you go live it's always a crapshoot
Dec 11 at 2:01pmWeb
Show threadmatt godboltDec 11
@mo @tauon oops
Show threadmatt godboltDec 11
@mo @tauon fixed now. And my release scripts updated