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Differential D54301

sys/bitcount.h: add __const_bitcount<n>
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Authored by bz on Dec 18 2025, 10:51 PM.
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emaste

Details

Reviewers
dumbbell
obiwac
imp
brooks
emaste
Group Reviewers
linuxkpi
Commits
rG3fc5eb05910f: sys/bitcount.h: add __const_bitcount<n>
rG605389e40b93: sys/bitcount.h: add __const_bitcount<n>
rG27aa23cee810: sys/bitcount.h: add __const_bitcount<n>
Summary

Add a version of const_bitcount<n> which can be used to get the
numbers at compile-time when builtin_popcountg() is not available
(see sys/compat/linuxkpi/common/include/linux/bitops.h for LLVM before
19 and gcc before 14).

Obtained from: https://reviews.freebsd.org/D50995#1174884 by obiwac

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Event Timeline

bz requested review of this revision.Dec 18 2025, 10:51 PM
bz created this revision.
Harbormaster completed remote builds in B69389: Diff 168375.Dec 18 2025, 10:51 PM
bz added a parent revision: D54297: LinuxKPI: bitcount fix gcc and older llvm builds.Dec 18 2025, 10:52 PM
bz mentioned this in D54297: LinuxKPI: bitcount fix gcc and older llvm builds.
bz added a reviewer: emaste.Dec 18 2025, 10:57 PM
bz added a subscriber: emaste.

Adding @emaste as I found @dumbbell 's version through the stack of local changes you carry.

brooks accepted this revision.Dec 19 2025, 9:55 AM

LGTM. Out of paranoia, I verified using compiler explorer that it's reliably const enough to use in a macro even at -O0 with an old clang.

This revision is now accepted and ready to land.Dec 19 2025, 9:55 AM
brooks mentioned this in D54266: iwlwifi: Don't compile for clang before 19.Dec 19 2025, 9:56 AM
emaste accepted this revision as: emaste.Dec 19 2025, 8:03 PM
bz added a comment.Dec 19 2025, 9:14 PM
In D54301#1241028, @brooks wrote:

LGTM. Out of paranoia, I verified using compiler explorer that it's reliably const enough to use in a macro even at -O0 with an old clang.

Neat tool. Didn't know about that. Thank you!

Closed by commit rG27aa23cee810: sys/bitcount.h: add __const_bitcount<n> (authored by bz). · Explain WhyDec 19 2025, 9:17 PM
This revision was automatically updated to reflect the committed changes.
bz added a commit: rG27aa23cee810: sys/bitcount.h: add __const_bitcount<n>.
bz added a commit: rG605389e40b93: sys/bitcount.h: add __const_bitcount<n>.Fri, Jan 16, 8:50 AM
bz added a commit: rG3fc5eb05910f: sys/bitcount.h: add __const_bitcount<n>.Fri, Jan 16, 7:44 PM

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sys/
sys/
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sys/sys/bitcount.h

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