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Message-ID: <CAKwvOd=H5kB=nsParYME2KoQxj-eDC_DAN09y1T2E7yqS43H4Q@mail.gmail.com> Date: Wed, 5 Oct 2022 11:57:55 -0700 From: Nick Desaulniers <ndesaulniers@...gle.com> To: YingChi Long <me@...lyc.cn> Cc: bp@...en8.de, chang.seok.bae@...el.com, dave.hansen@...ux.intel.com, hpa@...or.com, linux-kernel@...r.kernel.org, mingo@...hat.com, pbonzini@...hat.com, tglx@...utronix.de, x86@...nel.org, peterz@...radead.org, david.laight@...lab.com Subject: Re: [PATCH v2] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN On Wed, Oct 5, 2022 at 11:38 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote: > > On Wed, Oct 5, 2022 at 11:30 AM Nick Desaulniers > <ndesaulniers@...gle.com> wrote: > > > > On Wed, Oct 5, 2022 at 12:29 AM YingChi Long <me@...lyc.cn> wrote: > > > > > > Kindly ping :) > > > > Hi YingChi, > > Sorry for the delay in review. > > > > I think https://godbolt.org/z/sPs1GEhbT has convinced me that > > TYPE_ALIGN is analogous to _Alignof and not __alignof__; so your patch > > is correct to use _Alignof rather than __alignof__. I think that test > > case demonstrates this clearer than the other links in the commit > > message. Please consider replacing the existing godbolt links with > > that one if you agree. > > > > Please reword the paragraphs in the commit message from: > > ``` > > In PATCH v1 "TYPE_ALIGN" was substituted with "__alignof__" which is a > > GCC extension, which returns the *preferred alignment*, that is > > different from C11 "_Alignof" returning *ABI alignment*. For example, on > > i386 __alignof__(long long) evaluates to 8 but _Alignof(long long) > > evaluates to 4. See godbolt links below. > > > > In this patch, I'd like to use "__alignof__" to "_Alignof" to preserve > > the behavior here. > > ``` > > to: > > ``` > > ISO C11 _Alignof is subtly different from the GNU C extension > > __alignof__. _Alignof expressions evaluate to a multiple of the object > > size, while __alignof__ expressions evaluate to the alignment dictated > > by the target machine's ABI. In the case of long long on i386, > > _Alignof (long long) is 8 while __alignof__ (long long) is 4. > > Oops, and I had that backwards. > > In the case of long long on i386, _Alignof (long long) is 4 while > __alignof__ (long long) is 8. > > So I guess my commentary on "multiple of the object size" is > wrong...hmm...this wording can probably be improved further still... https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3054.pdf Section 6.2.8 "Alignment of objects" refers to "fundamental alignment" and "extended alignment." I wonder if it would be precise to say that "_Alignof evaluates to the fundamental alignment while __alignof__ evaluates to the extended alignment (which is implementation defined, typically by the machine specific ABI)." Though even that seems imprecise since it sounds like a fundamental alignment could be less than or equal to what alignof evaluates to. Grepping for `alignment requirement` turns up perhaps relevant portions of the spec. > > > > > The macro TYPE_ALIGN we're replacing has behavior that matches > > _Alignof rather than __alignof__. > > ``` > > In particular, I think it's best to avoid language like "returns" in > > favor of "evaluates to" since these are expressions, not function > > calls. I think it's also good to avoid the term "preferred alignment" > > since that isn't meaningful; it looks like it was pulled from one of > > the GCC bug reports rather than the GCC docs or latest ISO C standard > > (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3054.pdf). I'm not > > sure that the links to the GCC bug tracker add anything meaningful > > here; I think those can get dropped, too. It's also perhaps confusing > > to refer to earlier versions of the patch. One thing you can do is > > include comments like that "below the fold" in a commit message as a > > meta comment to reviewers. See > > https://lore.kernel.org/llvm/20220512205545.992288-1-twd2.me@gmail.com/ > > as an example of commentary "below the fold" on differences between > > patch versions. Text in that area is discarded by git when a patch is > > applied. > > > > With those changes to the commit message in a v3, I'd be happy to sign > > off on the change. Thanks for your work on this! > > -- > > Thanks, > > ~Nick Desaulniers > > > > -- > Thanks, > ~Nick Desaulniers -- Thanks, ~Nick Desaulniers
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