From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ffbox0-bg.mplayerhq.hu (ffbox0-bg.ffmpeg.org [79.124.17.100]) by master.gitmailbox.com (Postfix) with ESMTP id 0E57448526 for ; Wed, 6 Dec 2023 13:25:15 +0000 (UTC) Received: from [127.0.1.1] (localhost [127.0.0.1]) by ffbox0-bg.mplayerhq.hu (Postfix) with ESMTP id 1336868CF8C; Wed, 6 Dec 2023 15:25:13 +0200 (EET) Received: from mail8.parnet.fi (mail8.parnet.fi [77.234.108.134]) by ffbox0-bg.mplayerhq.hu (Postfix) with ESMTPS id DF5C068C67E for ; Wed, 6 Dec 2023 15:25:06 +0200 (EET) Received: from mail9.parnet.fi (mail9.parnet.fi [77.234.108.21]) by mail8.parnet.fi with ESMTP id 3B6DP5Mi020178-3B6DP5Mj020178; Wed, 6 Dec 2023 15:25:05 +0200 Received: from foo.martin.st (host-97-144.parnet.fi [77.234.97.144]) by mail9.parnet.fi (Postfix) with ESMTPS id 4E25CA142A; Wed, 6 Dec 2023 15:25:05 +0200 (EET) Date: Wed, 6 Dec 2023 15:25:03 +0200 (EET) From: =?ISO-8859-15?Q?Martin_Storsj=F6?= To: FFmpeg development discussions and patches In-Reply-To: <20231203201027.2255-1-timo@rothenpieler.org> Message-ID: <6ebfe490-d27e-14c3-ac81-8c4a2446ba17@martin.st> References: <20231203201027.2255-1-timo@rothenpieler.org> MIME-Version: 1.0 X-FE-Policy-ID: 3:14:2:SYSTEM Subject: Re: [FFmpeg-devel] [PATCH] avutil/mem: always align by at least 32 bytes X-BeenThere: ffmpeg-devel@ffmpeg.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: FFmpeg development discussions and patches List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: FFmpeg development discussions and patches Cc: Timo Rothenpieler Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: ffmpeg-devel-bounces@ffmpeg.org Sender: "ffmpeg-devel" Archived-At: List-Archive: List-Post: On Sun, 3 Dec 2023, Timo Rothenpieler wrote: > FFmpeg has instances of DECLARE_ALIGNED(32, ...) in a lot of structs, > which then end up heap-allocated. > By declaring any variable in a struct, or tree of structs, to be 32 byte > aligned, it allows the compiler to safely assume the entire struct > itself is also 32 byte aligned. > > This might make the compiler emit code which straight up crashes or > misbehaves in other ways, and at least in one instances is now > documented to actually do (see ticket 10549 on trac). > The issue there is that an unrelated variable in SingleChannelElement is > declared to have an alignment of 32 bytes. So if the compiler does a copy > in decode_cpe() with avx instructions, but ffmpeg is built with > --disable-avx, this results in a crash, since the memory is only 16 byte > aligned. > Mind you, even if the compiler does not emit avx instructions, the code > is still invalid and could misbehave. It just happens not to. Declaring > any variable in a struct with a 32 byte alignment promises 32 byte > alignment of the whole struct to the compiler. > > Instead of now going through all instances of variables in structs > being declared as 32 byte aligned, this patch bumps the minimum alignment > to 32 bytes. > --- > libavutil/mem.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/libavutil/mem.c b/libavutil/mem.c > index 36b8940a0c..26a9b9753b 100644 > --- a/libavutil/mem.c > +++ b/libavutil/mem.c > @@ -62,7 +62,7 @@ void free(void *ptr); > > #endif /* MALLOC_PREFIX */ > > -#define ALIGN (HAVE_AVX512 ? 64 : (HAVE_AVX ? 32 : 16)) > +#define ALIGN (HAVE_AVX512 ? 64 : 32) LGTM It could be good to add a comment here, to indicate how this value relates to the alignemnts used in structs. For others who commented in this thread, it all boils down to something like this: struct MyData { uint8_t __attribute__((aligned(32))) aligned_data[1024]; }; void func(void) { struct MyData *obj = malloc(sizeof(*obj)); // Uusally only aligned to 8 bytes // operate on obj->aligned_data[] } Due to how aligned_data is declared, we promise to the compiler that it is aligned to 32 bytes, and that the compiler can assume this wherever. Depending on -march or whatever, this can be to access it with instructions that assume 32 byte alignment. // Martin _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".