From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id 72AA443DCA; Mon, 8 Apr 2024 17:53:28 +0200 (CEST) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 3C8CC402C9; Mon, 8 Apr 2024 17:53:28 +0200 (CEST) Received: from dkmailrelay1.smartsharesystems.com (smartserver.smartsharesystems.com [77.243.40.215]) by mails.dpdk.org (Postfix) with ESMTP id 4FB4B402C0; Mon, 8 Apr 2024 17:53:27 +0200 (CEST) Received: from smartserver.smartsharesystems.com (smartserver.smartsharesys.local [192.168.4.10]) by dkmailrelay1.smartsharesystems.com (Postfix) with ESMTP id 5176220C85; Mon, 8 Apr 2024 17:53:26 +0200 (CEST) Content-class: urn:content-classes:message MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Subject: RE: [PATCH 0/4] RFC samples converting VLA to alloca Date: Mon, 8 Apr 2024 17:53:23 +0200 X-MimeOLE: Produced By Microsoft Exchange V6.5 Message-ID: <98CBD80474FA8B44BF855DF32C47DC35E9F380@smartserver.smartshare.dk> In-Reply-To: <20240408152703.GA25804@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: [PATCH 0/4] RFC samples converting VLA to alloca Thread-Index: AdqJyTdTuQgOryYdRnqv9u0M0rpWawAANkaw References: <20231107193220.GA15232@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> <1712250913-1977-1-git-send-email-roretzla@linux.microsoft.com> <98CBD80474FA8B44BF855DF32C47DC35E9F379@smartserver.smartshare.dk> <20240407100306.36c9688f@hermes.local> <20240408152703.GA25804@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> From: =?iso-8859-1?Q?Morten_Br=F8rup?= To: "Tyler Retzlaff" , "Stephen Hemminger" , Cc: =?iso-8859-1?Q?Mattias_R=F6nnblom?= , , "Bruce Richardson" , "Thomas Monjalon" X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org > From: Tyler Retzlaff [mailto:roretzla@linux.microsoft.com] > Sent: Monday, 8 April 2024 17.27 >=20 > For next technboard meeting. >=20 > On Sun, Apr 07, 2024 at 10:03:06AM -0700, Stephen Hemminger wrote: > > On Sun, 7 Apr 2024 13:07:06 +0200 > > Morten Br=F8rup wrote: > > > > > > From: Mattias R=F6nnblom [mailto:hofors@lysator.liu.se] > > > > Sent: Sunday, 7 April 2024 11.32 > > > > > > > > On 2024-04-04 19:15, Tyler Retzlaff wrote: > > > > > This series is not intended for merge. It insteat provides = examples > > > > of > > > > > converting use of VLAs to alloca() would look like. > > > > > > > > > > what's the advantages of VLA over alloca()? > > > > > > > > > > * sizeof(array) works as expected. > > > > > > > > > > * multi-dimensional arrays are still arrays instead of = pointers to > > > > > dynamically allocated space. this means multiple subscript = syntax > > > > > works (unlike on a pointer) and calculation of addresses = into > > > > allocated > > > > > space in ascending order is performed by the compiler = instead of > > > > manually. > > > > > > > > > > > > > alloca() is a pretty obscure mechanism, and also not a part of = the C > > > > standard. VLAs are C99, and well-known and understood, and very > > > > efficient. > > > > > > The RFC fails to mention why we need to replace VLAs with = something else: > > > > > > VLAs are C99, but not C++; VLAs were made optional in C11. > > > > > > MSVC doesn't support VLAs, and is not going to: > > > = https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support- > arriving-in-msvc/#variable-length-arrays > > > > > > > > > I dislike alloca() too, and the notes section in the alloca(3) man = page > even discourages the use of alloca(): > > > https://man7.org/linux/man-pages/man3/alloca.3.html > > > > > > But I guess alloca() is the simplest replacement for VLAs. > > > This RFC patch series opens the discussion for alternatives in = different > use cases. > > > > > > > The other issue with VLA's is that if the number is something that = can be > externally > > input, then it can be a source of stack overflow bugs. That is why = the Linux > kernel > > has stopped using them; for security reasons. DPDK has much less of = a > security > > trust domain. Mostly need to make sure that no data from network is = being > > used to compute VLA size. > > >=20 > Looks like we need to discuss this at the next techboard meeting. >=20 > * MSVC doesn't support C11 optional VLAs (and never will). > * alloca() is an alternative that is available on all = platforms/toolchain > combinations. > * it's reasonable for some VLAs to be turned into regular arrays but = it > would be unsatisfactory to be stuck waiting discussions of defining = new > constant expression macros on a per-use basis. We must generally stop using VLAs, for many reasons. The only available 1:1 replacement is alloca(), so we have to accept = that. If anyone still cares about improvements, we can turn alloca()'d arrays = into regular arrays after this patch series. Alternatives to VLAs are very interesting discussions, but let's not = stall MSVC progress because of it! > * there is resistance to using alloca() vs VLA so my proposal is to > change only the code that is built to target windows. I would prefer to get rid of them all, so the CI can build with -Wvla to = prevent them from being introduced again. Not a strong preference. On the other hand, the CI's MSVC builds will catch them if used for a = Windows target. And limiting to Windows code reduces the amount of work, so that's = probably the most realistic solution.