From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/15031 Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: max_align_t mess on i386 Date: Mon, 16 Dec 2019 10:56:22 -0500 Message-ID: <20191216155622.GC1666@brightrain.aerifal.cx> References: <20191214151932.GW1666@brightrain.aerifal.cx> <20191215182242.GA1666@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="237941"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: Mutt/1.5.21 (2010-09-15) To: musl@lists.openwall.com Original-X-From: musl-return-15047-gllmg-musl=m.gmane.org@lists.openwall.com Mon Dec 16 16:56:37 2019 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by blaine.gmane.org with smtp (Exim 4.89) (envelope-from ) id 1igsjd-000zig-8G for gllmg-musl@m.gmane.org; Mon, 16 Dec 2019 16:56:37 +0100 Original-Received: (qmail 18091 invoked by uid 550); 16 Dec 2019 15:56:35 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Original-Received: (qmail 18073 invoked from network); 16 Dec 2019 15:56:34 -0000 Content-Disposition: inline In-Reply-To: Original-Sender: Rich Felker Xref: news.gmane.org gmane.linux.lib.musl.general:15031 Archived-At: On Mon, Dec 16, 2019 at 10:30:30AM -0500, Jeffrey Walton wrote: > On Sun, Dec 15, 2019 at 1:22 PM Rich Felker 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 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 - #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. > > 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. Rich