Date: Tue, 11 Feb 2025 05:23:21 +0000 From: Jessica Clarke <jrtc27@freebsd.org> To: Zhenlei Huang <zlei@FreeBSD.org> Cc: Doug Moore <dougm@FreeBSD.org>, "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> Subject: Re: git: 7bcc7a0b88cc - stable/14 - libkern: avoid local var in order_base_2() Message-ID: <803D3DBC-A4F5-47FB-8034-5241461D4A71@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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On 11 Feb 2025, at 03:30, Zhenlei Huang <zlei@FreeBSD.org> 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 ? It’s not about pollution. GNU statement expressions are just not valid at file scope, they must be used within a function (since otherwise the implementation would have to have a full blown interpreter to constant evaluate the whole thing, just like C++ and now C to some extent have ended up with these days with constexpr). That is, you cannot write: int x = ({ ... }); at file scope. Jess
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