From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wm1-f51.google.com (mail-wm1-f51.google.com [209.85.128.51]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C2E0721766A for ; Tue, 29 Apr 2025 14:38:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.51 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745937536; cv=none; b=EWd68mVctmXPkmfFQXZ7XAtJkgJtWmgW+j085PaQ14zjFW1A1E47mqyPew++UKaCNSSeRY8Dyzs+Tk7ajWe22Up9dxuFpMxVSkFkgxiJv7GfdkZ3nsPAJ1hiRskwKRqYZjc/xMhwJWQsN/WbQS5Nz7G1qbHAvInJ23jkuBfs+Gc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745937536; c=relaxed/simple; bh=uOGR0qB95uIbNvLLFjMJe/rZXFYnfta9yQxHsTCxptU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=T9fL0ub0xTcT/HyuLmNvQwCjq0iDoSf0wPIeT9Kv8HX2GPYi7kw4HTZYGhOGEXZZ05qO9zCjUl6ovlZkGgYz/z9pTcfn0lTRTdidrhOdiMrEwJd4ouCz/vMnva/kYKNp81q0JI7XZxtLPgObe9bG9DyavDh1L45aJDMvKbsXX9s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=citrix.com; spf=pass smtp.mailfrom=cloud.com; dkim=pass (1024-bit key) header.d=citrix.com header.i=@citrix.com header.b=rl+tjRKL; arc=none smtp.client-ip=209.85.128.51 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=citrix.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=cloud.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=citrix.com header.i=@citrix.com header.b="rl+tjRKL" Received: by mail-wm1-f51.google.com with SMTP id 5b1f17b1804b1-43cf680d351so40595345e9.0 for ; Tue, 29 Apr 2025 07:38:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=citrix.com; s=google; t=1745937533; x=1746542333; darn=vger.kernel.org; h=content-transfer-encoding:in-reply-to:autocrypt:from :content-language:references:cc:to:subject:user-agent:mime-version :date:message-id:from:to:cc:subject:date:message-id:reply-to; bh=+bH2VtO965DApheflq6WJZVHbuWGpEGuuDOfcTC4myY=; b=rl+tjRKLR0w5iqS+ugNaJTm4zZeTc4CmIcZYMsuQSjptP3mcYv1rX+DTO584fBhX4V l0Fh3OTAlFgZpvuFzMuI1sq/yVR2Joux7KTKyNA3q7k89L6ndghogouhp8bcWW7uteHf yrwHEAwXwrR9AR8zsoJFP4m5DS+/XknDtAvY4= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1745937533; x=1746542333; h=content-transfer-encoding:in-reply-to:autocrypt:from :content-language:references:cc:to:subject:user-agent:mime-version :date:message-id:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=+bH2VtO965DApheflq6WJZVHbuWGpEGuuDOfcTC4myY=; b=bmLscokJ2tp7QIZfdrOW8mimwpMoogHMTVnJFDf+flPUjUcWTAa95x9NvkcpkQwz32 zfE8RfzHTEN3kmNECgovCJutem90N10sGbM6pCoTveWa2wRpRGeY7A/RmCJrwdyVtGzU caqdmoKPPRbtZBuCAsrvM19l/Fdn+x/UASMXYwjl8doUvQVqNXxKTjyoG67brQZQvp2Z N2DGFwYz69Zj7tCKfRxooUl4m1deaSbUMXeEqhr+iVP7UmfGNGuCHxSQeA78MrgYuLKw QmopAwCW0KzhcUE5wjg2CI7Q78OkPHhTb5VumsPrnRXU/0ryi2aKtmkoShtZzTfV7doi NSXA== X-Forwarded-Encrypted: i=1; AJvYcCWwZJNuvhV9s3ZMqFzPStNMshO7XXvdEZcS4wZhLyGxnT63igR4b1hUBkjwJ38glLafPJOp2BnCcImeE3Q=@vger.kernel.org X-Gm-Message-State: AOJu0YzuPZCZgTpGpXO11chjCzcgortICzuUaoVso/Zd3hGChQyiDIBg ClgfhhfRVBym0UjP9+0BhDKfvyRMEKzmcsCcbOmdOWFKIslZsNUdmPRrg5FAlxk= X-Gm-Gg: ASbGncsv43UC0cR8h6Nkl3B0NZI/N8mYtbeu7P2Rgs+hgGTzNqnewLOeWXF+P5A91dK 58iL/Tcx4AKmio5mr69coJh8EbgzJCckxT0I+o8/5hD4nwS95g8eWbIxNd0XjYdeLTpwL0y725e GpWKkO0O38JHnWnkx2BFzM2LTUlDZPCRR7H+P6Lx35K1VHVWCWTrb9DT/mVmWLU2Y8lI5IHeCaA VlQDfbm0KFvK0aQmVDO2ovLfXLr0KySjhJp9o3lE/Esehxyx4TwyVOLORWTTK2duxrnyIxZi2rI huZGfm8WKxBKfGIsq4bfxPCzbU6tZHqq1rX4+194EpCWAYae07CYT5qRvjolHJIFs7Stl4RrJSb A75Gy6A== X-Google-Smtp-Source: AGHT+IEz0M3YvmQrPHT1D/TQ+n1bkMRH0XKuCVufus5QpckGV8BdchYH4uo1PkS82FK1285Fo/Zw+Q== X-Received: by 2002:a05:600c:4f8b:b0:439:9434:4f3b with SMTP id 5b1f17b1804b1-441acb20523mr29568085e9.8.1745937532997; Tue, 29 Apr 2025 07:38:52 -0700 (PDT) Received: from [192.168.1.183] (host-92-26-98-202.as13285.net. [92.26.98.202]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-441b0232267sm4173865e9.39.2025.04.29.07.38.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 29 Apr 2025 07:38:52 -0700 (PDT) Message-ID: <06d04c13-76e8-45fb-a563-e365e1f19362@citrix.com> Date: Tue, 29 Apr 2025 15:38:51 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C To: "H. Peter Anvin" , Linus Torvalds , Ingo Molnar Cc: Arnd Bergmann , Arnd Bergmann , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, Juergen Gross , Boris Ostrovsky , Alexander Usyskin , Greg Kroah-Hartman , =?UTF-8?Q?Mateusz_Jo=C5=84czyk?= , Mike Rapoport , Ard Biesheuvel , Peter Zijlstra , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org References: <20250425141740.734030-1-arnd@kernel.org> <67be5eee-b67b-409a-8309-829f891b9944@citrix.com> <916BD58C-E6A7-495E-9A60-722E130AC7A7@zytor.com> <956DB0ED-F3DB-456D-8D06-6F40DBDB815A@zytor.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: <956DB0ED-F3DB-456D-8D06-6F40DBDB815A@zytor.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 29/04/2025 4:13 am, H. Peter Anvin wrote: > On April 28, 2025 7:25:17 PM PDT, Andrew Cooper wrote: >> On 29/04/2025 3:00 am, H. Peter Anvin wrote: >>> On April 28, 2025 5:12:13 PM PDT, Andrew Cooper wrote: >>>> On 28/04/2025 10:38 pm, H. Peter Anvin wrote: >>>>> On April 28, 2025 9:14:45 AM PDT, Linus Torvalds wrote: >>>>>> On Mon, 28 Apr 2025 at 00:05, Ingo Molnar wrote: >>>>>>> And once we remove 486, I think we can do the optimization below to >>>>>>> just assume the output doesn't get clobbered by BS*L in the zero-case, >>>>>>> right? >>>>>> We probably can't, because who knows what "Pentium" CPU's are out there. >>>>>> >>>>>> Or even if Pentium really does get it right. I doubt we have any >>>>>> developers with an original Pentium around. >>>>>> >>>>>> So just leave the "we don't know what the CPU result is for zero" >>>>>> unless we get some kind of official confirmation. >>>>>> >>>>>> Linus >>>>> If anyone knows for sure, it is probably Christian Ludloff. However, there was a *huge* tightening of the formal ISA when the i686 was introduced (family=6) and I really believe this was part of it. >>>>> >>>>> I also really don't trust that family=5 really means conforms to undocumented P5 behavior, e.g. for Quark. >>>> https://www.sandpile.org/x86/flags.htm >>>> >>>> That's a lot of "can't even characterise the result" in the P5. >>>> >>>> Looking at P4 column, that is clearly what the latest SDM has >>>> retroactively declared to be architectural. >>>> >>>> ~Andrew >>> Yes, but it wasn't about flags here. >>> >>> Now, question: can we just use __builtin_*() for these? I think gcc should always generate inline code for these on x86. >> Yes it does generate inline code.  https://godbolt.org/z/M45oo5rqT >> >> GCC does it branchlessly, but cannot optimise based on context. >> >> Clang can optimise based on context, except the 0 case it seems. >> >> Moving to -march=i686 causes both GCC and Clang to switch to CMOV and >> create branchless code, but is still GCC still can't optimise out the >> CMOV based on context. >> >> ~Andrew > Maybe a gcc bug report would be better than trying to hack around this in the kernel? I tried that.  (The thread started as a question around __builtin_constant_p() but did grow to cover __builtin_ffs().) https://gcc.gnu.org/pipermail/gcc/2024-March/243465.html ~Andrew