Date: Sat, 21 May 2022 10:22:46 +0200 From: Markus Wichmann <nullplan@....net> To: musl@...ts.openwall.com Subject: Re: add loongarch64 port v3 On Sat, May 21, 2022 at 02:38:20PM +0800, 王洪亮 wrote: > > 在 2022/5/16 下午10:27, Rich Felker 写道: > > Again,  is not valid C. If the extension field is going to be > > declared at all it needs to be declared in a way it can be accessed > > without invoking UB, e.g. as a FAM. > >  is allowed in GNU C, we can not use it in musl ? > > I don't understand UB ? e.g. as a FAM ? > musl tries to stay within ISO C as much as possible, and not use GNU C extensions if it possibly can. In this case, zero-sized arrays do the same thing as flexible array members, so there is absolutely no reason to use the extension over the standard way. UB = undefined behavior FAM = flexible array member. If the last member of a structure is an array, it can have indeterminate size. It will then not count towards the size of the structure (only towards its alignment, possibly), and can be indexed arbitrarily. This is a C99 feature. So basically, just nix the 0 in that line. > > I'm also not clear on how > > specifying the alignment here helps since any object created in a way > > that the alignment would affect cannot have the FAM present. > > > the __aligned__(16) here used to save 128bit vector later. But it has no effect, right? The array member is offset an integer multiple of sixteen bytes from the start of the structure, so it is already aligned with respect to that, and the declaration adds no further padding (and if it did, common style in both Linux and musl is to explicate the padding). And the pointer to the structure comes from the kernel. The only thing this alignment directive does is raise the alignment requirement of the structure up to sixteen, but that only possibly matters for variables of the structure type, be they automatic or program life time. But from here it is hard to see why the alignment should matter in either of those cases, since those would only possibly be local buffers of the kernel-provided mcontext. And the kernel provides the mcontext already with all the alignment it needs. I am not aware of any kernel API that receives a user provided mcontext. And even if there was one, the kernel would have to copy it into kernel space before doing anything with it, anyway. And, BTW, if the alignment really is needed on the mcontext_t itself, it might be better to put the alignment tag on the type, not one of the members. Ciao, Markus
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