| 10 Feb 2026 |
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 ris_ | ive just spotted something surprising | 22:42:40 |
| ris_ | when i merged https://github.com/NixOS/nixpkgs/pull/442945, i enabled them (both) by default | 22:43:20 |
| ris_ | this was not intended | 22:43:24 |
| ris_ | i should probably undo at least the extensive one | 22:44:08 |
| ris_ | though it's weird 26.xx shipping without flags 25.11 had enabled | 22:44:37 |
| ris_ | i'm sure some packages must be seeing a performance impact | 22:44:54 |
|  Adam joined the room. | 23:15:38 |
| 13 Feb 2026 |
 emily | are there numbers for the expected impact of the two? | 03:27:02 |
| emily |
Extensive mode, which contains all the checks from fast mode and some additional checks for undefined behavior that incur relatively little overhead but aren’t security-critical. Production builds requiring a broader set of checks than fast mode should consider enabling extensive mode. The additional rigour impacts performance more than fast mode: we recommend benchmarking to determine if that is acceptable for your program.
| 03:27:29 |
| emily | doesn't sound like it should be too bad | 03:27:33 |
| emily | specifically marked as suitable for production at least | 03:27:56 |
| emily | fwiw the main user of libc++ in Nixpkgs is macOS and _LIBCPP_HARDENING_MODE_DEFAULT is already 2 (fast) on that platform | 03:29:42 |
| emily | so libcxxhardeningfast is just a nop | 03:30:04 |
|  hoplopf joined the room. | 10:25:27 |
| ris_ | hmmmmmmmmmmmm | 21:51:10 |
| ris_ | well.. open to opinions. i should at least update the documentation to correspond to reality | 21:51:54 |
| emily | I like hardening, but I'm also okay with us matching the platform default here | 22:20:08 |
| emily | not sure if nobody noticing means that the impact is minimal or that nobody is monitoring Nixpkgs package perf 🫣 | 22:20:30 |
| ris_ | i fear the latter | 22:33:54 |
 Sergei Zimmerman (xokdvium) | In reply to @emilazy:matrix.org
not sure if nobody noticing means that the impact is minimal or that nobody is monitoring Nixpkgs package perf 🫣 I sure am monitoring nix perf :) glibcxx assertions absolutely tank inlining in the parser | 23:54:31 |
| emily | I don't think that cleanly maps to libc++ hardening though | 23:55:00 |
| emily | (and probably ~nobody is using Nix with libc++ on Linux) | 23:55:14 |
