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 394B243DCA; Mon, 8 Apr 2024 17:27:07 +0200 (CEST) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id B6CE7402C9; Mon, 8 Apr 2024 17:27:06 +0200 (CEST) Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by mails.dpdk.org (Postfix) with ESMTP id C0552402C0; Mon, 8 Apr 2024 17:27:04 +0200 (CEST) Received: by linux.microsoft.com (Postfix, from userid 1086) id E77FE20EA43F; Mon, 8 Apr 2024 08:27:03 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com E77FE20EA43F DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1712590023; bh=xFXBTDVek4XD/fL4jw9oQByAt47UZf69ldBGc3ffNtc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ltZxA18sck+/Pj8zx8GtclZlbIx9Prue3NAwQ1Up/ysGLZznrC9QxmAxLOb5+/FvO gMxLYkC9RylVorw8ijkDwEn1goStjYC1RA4vTVe2xbHY2RYpLmkBRdHuhIEw6+Iho2 mtV0ngICkYuGkCBVRa/rKh7t1HhSEadi5vFUBN0I= Date: Mon, 8 Apr 2024 08:27:03 -0700 From: Tyler Retzlaff To: Stephen Hemminger , techboard@dpdk.org Cc: Morten =?iso-8859-1?Q?Br=F8rup?= , Mattias =?iso-8859-1?Q?R=F6nnblom?= , dev@dpdk.org, Bruce Richardson , Thomas Monjalon Subject: Re: [PATCH 0/4] RFC samples converting VLA to alloca Message-ID: <20240408152703.GA25804@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240407100306.36c9688f@hermes.local> User-Agent: Mutt/1.5.21 (2010-09-15) 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 For next technboard meeting. On Sun, Apr 07, 2024 at 10:03:06AM -0700, Stephen Hemminger wrote: > On Sun, 7 Apr 2024 13:07:06 +0200 > Morten Brørup wrote: > > > > From: Mattias Rönnblom [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. > Looks like we need to discuss this at the next techboard meeting. * 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. * there is resistance to using alloca() vs VLA so my proposal is to change only the code that is built to target windows.