Date: Mon, 16 Dec 2019 11:36:42 -0500 From: Jeffrey Walton <noloader@...il.com> To: musl@...ts.openwall.com Subject: Re: max_align_t mess on i386 On Mon, Dec 16, 2019 at 10:56 AM Rich Felker <dalias@...c.org> wrote: > > On Mon, Dec 16, 2019 at 10:30:30AM -0500, Jeffrey Walton wrote: > > On Sun, Dec 15, 2019 at 1:22 PM Rich Felker <dalias@...c.org> wrote: > > > > > > 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: > > > > > > Generally, you should look to the C11 or POSIX (man 3p) specifications > > > for the functions rather than the "man 3" ones, but here it's pretty > > > close to the same, just imprecisely worded: > > > > > > > The malloc() and calloc() functions return a pointer to the > > > > allocated memory that is suitably aligned for any kind of variable. > > > > > > > > I expect to be able to use a pointer returned by malloc (and friends) > > > > in MMX, SSE and AVX functions. > > > > > > "Any kind of variable" isn't "any kind of load/store instruction". For > > > example you most certainly will not get 32- or 64-byte alignment that > > > you may want for AVX-256 or AVX-512 without memalign. > > > > GCC tells us the largest alignment that we can expect: > > > > $ gcc -dM -E - </dev/null | grep -i align > > #define __BIGGEST_ALIGNMENT__ 16 > > > > Because __BIGGEST_ALIGNMENT__ is 16, I don't expect to get 32-byte or > > 64-byte aligned buffers. > > I wasn't aware of this gcc feature. Do you know if it's documented and > what it's derived from? It seems to match what max_align_t is expected > to be, including on i386 (16) and powerpc (16) and indeed it's only 4 > on a few 32-bit archs and even 2 on m68k. I believe it is documented at https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html. The linker problem discussed in the same area has bitten me several times in the past. It usually arises on 32-bit systems. But PowerPC also got me when using AIX. > > > A max_align_t > > > (and corresponding malloc alignment constraint) that heavily aligned > > > would be awful to use, with memory waste possibly exceeding 1000% and > > > over 500% likely for real-world data structures. Over-alignment also > > > weakens hardening properties by making pointers more predictable. > > > > It sounds like you are moving the fragmentation problem from the > > runtime library to the application. (When fragmentation is a problem). > > I don't understand what you mean. When we can't get properly aligned buffers in userland, then we (userland) have to over-commit in our allocators and play the pointer games. For example, if I can only get 8-byte aligned pointers, then I always have to allocate n+16 bytes, move the pointer 'p' to the right for a 16 byte alignment, and store the offset at p-1 so I can delete the base pointer on delete/free. Those kind of pointer games are usually played out in the runtime library. I can only says "usually" and not always because we have to do them on AIX and GNU Hurd (among others). Jeff
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