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Fri, 26 Oct 2018 12:32:47 -0700 (PDT) Date: Fri, 26 Oct 2018 12:32:47 -0700 (PDT) X-Google-Original-Date: Fri, 26 Oct 2018 12:32:44 PDT (-0700) Subject: Re: [GIT PULL] RISC-V Patches for the 4.20 Merge Window, Part 1 In-Reply-To: From: Palmer Dabbelt To: Linus Torvalds Message-ID: Mime-Version: 1.0 (MHng) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20181026_123300_472699_92206EA6 X-CRM114-Status: GOOD ( 18.83 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Greg KH , linux-riscv@lists.infradead.org Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-riscv" Errors-To: linux-riscv-bounces+infradead-linux-riscv=archiver.kernel.org@lists.infradead.org Message-ID: <20181026193247.q0Vi8ik1fC_ePKa1H3TupuG4Ti57u8h9ts6Sft4YiH0@z> On Thu, 25 Oct 2018 18:12:46 PDT (-0700), Linus Torvalds wrote: > On Wed, Oct 24, 2018 at 1:42 PM Palmer Dabbelt wrote: >> >> I'm pretty sure this is our largest patch set since the original kernel >> contribution, and it's certainly the one with the most contributors. >> While I don't have anything else I know I'm going to submit for the >> merge window, I would be somewhat surprised if I didn't screw anything >> up. > > So I'm not entirely sure this is a screw-up, but it does have some > signs of being one: > >> lib: Add umoddi3 and udivmoddi4 of GCC library routines > > We have *actively* tried to avoid needing umoddi3 and friends, and > have so far succceeded in doing that across many different > architectures, and many many years. > > And when we _have_ added it, we've added it as architecture-specific > optimized routines when people really need it. > > The reason to avoid a full 64-by-64 divide is that > > (a) it is really expensive > > (b) code that needs it is almost always actually wrong > > Almost always, the 64-bit divide turns out to be entirely pointless, > and the code can be written as a (much cheaper) 64-by-32 divide > instead using the functions in instead. > > Does RISC-V _really_ need that disgusting full 64-bit divide, or was > the addition of that simply a "we didn't know better, and didn't > realize just how expensive and wrong it is"? > > I've pulled this, but I really _really_ hope that I will get a second > pull request that gets rid of this again, and fixes whatever RISC-V > code that triggered this to just do not do an insane full 64-bit > divide on a 32-bit architecture. Thanks. We'll go figure out how to remove it. _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv