https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125334
--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-16 branch has been updated by Patrick Palka <[email protected]>: https://gcc.gnu.org/g:d0c90c0034fcb526411a841b269a647c2690ba04 commit r16-9063-gd0c90c0034fcb526411a841b269a647c2690ba04 Author: Patrick Palka <[email protected]> Date: Fri Jun 5 17:05:33 2026 -0400 c++/modules: disable dependent ADL inside header unit [PR125334] This PR reports that building <bits/stdc++.h> as a header unit later slows down initial std module stream-in by around 2x compared to if the std module is built without the header unit, i.e. g++ -fmodules -fsearch-include-path bits/std.cc g++ -fmodules testcase.C # 15% slower vs before r16-6311 vs g++ -fmodules -fsearch-include-path bits/stdc++.h bits/std.cc g++ -fmodules testcase.C # 100% slower vs before r16-6311 This happens after the dependent ADL patch r16-6311. My initial theory for the slowdown was that the ADL step adds in many more dependency edges between entities and results in larger SCC clusters, making lazy loading less lazy, but the number of overall clusters doesn't seem to significantly before/after the patch. So I have no idea why there's such a big slowdown and why header units are affected more. But my basic understanding of the dependent ADL part of reachability analysis is that it's not really needed when building a header unit because header units don't discard or hide any entities. So this patch disables it for header units which restores std module stream-in back to pre r16-6311 levels when <bits/stdc++.h> is built as a header unit. When built without the header unit then std module stream-in is still about 15% slower. Maybe there's ways to safely speed up that case too (e.g. only add dependency edges to GMF entities?) but I'll leave that as future work. PR c++/125334 gcc/cp/ChangeLog: * module.cc (depset::hash::add_dependent_adl_entities): Disable when inside a header unit. Reviewed-by: Jason Merrill <[email protected]> (cherry picked from commit 8a3aa549299337ffee8d808bf7d997b7cb1f99fe)
