Date: Mon, 10 Feb 2025 23:15:52 -0600 From: Doug Moore <unkadoug@gmail.com> To: Zhenlei Huang <zlei@FreeBSD.org> Cc: "src-committers@freebsd.org" <src-committers@FreeBSD.org>, "dev-commits-src-all@freebsd.org" <dev-commits-src-all@FreeBSD.org>, "dev-commits-src-branches@freebsd.org" <dev-commits-src-branches@FreeBSD.org>, "Bjoern A. Zeeb" <bz@FreeBSD.org> Subject: Re: git: 7bcc7a0b88cc - stable/14 - libkern: avoid local var in order_base_2() Message-ID: <d0333a09-0191-de98-20cc-e3d031cca784@freebsd.org> In-Reply-To: <858BD10B-A2C9-4D3A-9FF5-C5573597DF8B@FreeBSD.org> References: <202502101115.51ABFISj016298@gitrepo.freebsd.org> <858BD10B-A2C9-4D3A-9FF5-C5573597DF8B@FreeBSD.org>
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bz@freebsd.org presented the problem that this change fixed. I think it was in a driver. It may have been a case in which the macro argument had side effects, and having it appear twice in the macro made those side effects happen twice. But I have not preserved the original report from bz. I regret that I did not adequately describe the problem being fixed in the commit message. Perhaps bz can shed some light on the matter. Doug On 2/10/25 21:30, Zhenlei Huang wrote: > >> On Feb 10, 2025, at 7:15 PM, Doug Moore <dougm@FreeBSD.org> wrote: >> >> The branch stable/14 has been updated by dougm: >> >> URL: https://cgit.FreeBSD.org/src/commit/?id=7bcc7a0b88ccb5e1fe31de88ab9a17e46859318b >> >> commit 7bcc7a0b88ccb5e1fe31de88ab9a17e46859318b >> Author: Doug Moore <dougm@FreeBSD.org> >> AuthorDate: 2024-09-27 23:43:07 +0000 >> Commit: Doug Moore <dougm@FreeBSD.org> >> CommitDate: 2025-02-10 10:30:05 +0000 >> >> libkern: avoid local var in order_base_2() >> >> order_base_2(n) is implemented with a variable, which keeps it from >> being used at file scope. Implement it instead as ilog2(2*n-1), which >> produces a different result when 2*n overflows, which appears unlikely >> in practice. >> >> Reviewed by: bz >> Differential Revision: https://reviews.freebsd.org/D46826 >> >> (cherry picked from commit b7cbf741d55468ba34305a14ac3acc1c286af034) >> --- >> sys/sys/libkern.h | 5 +---- >> 1 file changed, 1 insertion(+), 4 deletions(-) >> >> diff --git a/sys/sys/libkern.h b/sys/sys/libkern.h >> index a10289d72dc7..835e5ffaf0b7 100644 >> --- a/sys/sys/libkern.h >> +++ b/sys/sys/libkern.h >> @@ -229,10 +229,7 @@ ilog2_long_long(long long n) >> >> #define ilog2(n) (__builtin_constant_p(n) ? ilog2_const(n) : ilog2_var(n)) >> #define rounddown_pow_of_two(n) ((__typeof(n))1 << ilog2(n)) >> -#define order_base_2(n) ({ \ >> - __typeof(n) _n = (n); \ > This local var `_n` is within the scope of the macro `order_base_2`, it is surrounded with > {} and is harmless, so it will not pollute the file scoped variables. > > Am I reading the commit message wrong ? > > Best regards, > Zhenlei > >> - _n == 1 ? 0 : 1 + ilog2(_n - 1); \ >> -}) >> +#define order_base_2(n) ilog2(2*(n)-1) >> #define roundup_pow_of_two(n) ((__typeof(n))1 << order_base_2(n)) >> >> #define bitcount64(x) __bitcount64((uint64_t)(x)) > >
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