Date: Sun, 15 Dec 2019 19:23:14 +0100 From: Joakim Sindholt <opensource@...sha.com> To: musl@...ts.openwall.com Subject: Re: max_align_t mess on i386 On Sun, Dec 15, 2019 at 01:06:29PM -0500, Jeffrey Walton wrote: > On Sat, Dec 14, 2019 at 10:19 AM Rich Felker <dalias@...c.org> wrote: > > > > In reserching how much memory could be saved, and how practical it > > would be, for the new malloc to align only to 8-byte boundaries > > instead of 16-byte on archs where alignof(max_align_t) is 8 (pretty > > much all 32-bit archs), I discovered that GCC quietly changed its > > idead of i386 max_align_t to 16-byte alignment in GCC 7, to better > > accommodate the new _Float128 access via SSE. Presumably (I haven't > > checked) the change is reflected with changes in the psABI document to > > make it "official". > > Be careful with policy changes like this. The malloc (3) man page says: > > The malloc() and calloc() functions return a pointer to the > allocated memory that is suitably aligned for any kind of variable. Your man pages are not the standard, but the standard does have this to say: > The pointer returned if the allocation succeeds shall be suitably > aligned so that it may be assigned to a pointer to any type of object > and then used to access such an object in the space allocated (until the > space is explicitly freed or reallocated). To me this sounds like my next suggestion is technically disallowed. > I expect to be able to use a pointer returned by malloc (and friends) > in MMX, SSE and AVX functions. I might agree, but would it not be feasible to have the alignment of the returned pointer be dependent on the size of the allocation? That way, if you allocate <16 bytes you can get 8 byte alignment. You might even be able to go all the way down to 4 byte alignment for <8 byte allocations. It might violate the standard technically speaking, but I don't know of any examples of types smaller than 16 bytes that require 16 byte alignment.
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