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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a35ca4d1dfsm5743204f8f.18.2025.05.17.02.17.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 17 May 2025 02:17:50 -0700 (PDT) Date: Sat, 17 May 2025 10:17:42 +0100 From: David Laight To: Linus Torvalds Cc: Al Viro , Peter Zijlstra , dan.j.williams@intel.com, linux-cxl@vger.kernel.org, linux-kernel@vger.kernel.org, David Lechner , Ingo Molnar , "Fabio M. De Francesco" , Davidlohr Bueso , Jonathan Cameron , Dave Jiang , Alison Schofield , Vishal Verma , Ira Weiny Subject: Re: [PATCH 1/7] cleanup: Introduce DEFINE_ACQUIRE() a CLASS() for conditional locking Message-ID: <20250517101742.4c38830b@pumpkin> In-Reply-To: References: <20250509104028.GL4439@noisy.programming.kicks-ass.net> <681ea7d5ea04b_2a2bb100cf@dwillia2-mobl4.notmuch> <20250512105026.GP4439@noisy.programming.kicks-ass.net> <20250512182559.GB25891@noisy.programming.kicks-ass.net> <20250512185817.GA1808@noisy.programming.kicks-ass.net> <20250513070918.GB25763@noisy.programming.kicks-ass.net> <20250513085001.GC25891@noisy.programming.kicks-ass.net> <20250513200619.GJ2023217@ZenIV> <20250513203114.GK2023217@ZenIV> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 13 May 2025 14:28:37 -0700 Linus Torvalds wrote: > On Tue, 13 May 2025 at 13:31, Al Viro wrote: > > > > Nevermind - should've read back through the thread for context. > > Well, your comment did make me test what I can make gcc generate.. > > I still can't get gcc to do > > cmpq $-4095,%rdi > jns .L189 > > for IS_ERR_OR_NULL() however hard I try. > > The best I *can* get both gcc and clang to at least do > > movq %rdi, %rcx > addq $4095, %rcx > jns .L189 > > which I suspect it much better than the "lea+cmpq", because a pure > register move is handled by the renaming and has no cost aside from > the front end (ie decoding). > > So > > #define IS_ERR_OR_NULL(ptr) (MAX_ERRNO + (long)(ptr) >= 0) > > does seem to be potentially something we could use, and maybe we could > push the compiler people to realize that their current code generation > is bad. > > Of course, it doesn't actually *really* work for IS_ERR_OR_NULL(), > because it gets the wrong results for user pointers, and while the > *common* case for the kernel is to test various kernel pointers, the > user pointer case does happen (ie mmap() and friends). > > IOW, it's not actually correct in that form, I just wanted to see what > we could do for some limited form of this common pattern. > > Anyway, I am surprised that neither gcc nor clang seem to have > realized that you can turn an "add" that just checks the condition > codes for sign or equality into a "cmp" of the negative value. > > It seems such a trivial and obvious thing to do. But maybe I'm > confused and am missing something. Doing the signed compare (long)(ptr) >= -MAX_ERRNO generates cmp + jl (sign != overflow) which is a better test. To let user pointers through it might be possible to generate: leaq -1(%reg), %reg cmpq $-4097, %reg leaq 1(%reg), %reg ja label which trades a register for an instruction. It wouldn't be too bad if the second 'leaq' is moved to the branch target - especially for any cpu that don't have inc/dec that doesn't affect the flags. David