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Fri, 31 May 2024 18:48:00 -0700 (PDT) Message-ID: Date: Sat, 1 Jun 2024 02:47:59 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 07/13] x86/bitops: Improve arch_ffs() in the general case To: Jan Beulich Cc: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Wei Liu , Stefano Stabellini , Julien Grall , Volodymyr Babchuk , Bertrand Marquis , Michal Orzel , Oleksii Kurochko , Shawn Anastasio , "consulting @ bugseng . com" , Simone Ballarin , Federico Serafini , Nicola Vetrini , Xen-devel References: <20240524200338.1232391-1-andrew.cooper3@citrix.com> <20240524200338.1232391-8-andrew.cooper3@citrix.com> <1660a2a7-cea4-4e6f-9286-0c134c34b6fb@suse.com> <57a47c76-c484-4309-8a87-a51f79dd48b6@suse.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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 28/05/2024 2:12 pm, Jan Beulich wrote: > On 28.05.2024 14:30, Andrew Cooper wrote: >> On 27/05/2024 2:37 pm, Jan Beulich wrote: >>> On 27.05.2024 15:27, Jan Beulich wrote: >>>> On 24.05.2024 22:03, Andrew Cooper wrote: >>>>> --- a/xen/arch/x86/include/asm/bitops.h >>>>> +++ b/xen/arch/x86/include/asm/bitops.h >>>>> @@ -432,12 +432,28 @@ static inline int ffsl(unsigned long x) >>>>> >>>>> static always_inline unsigned int arch_ffs(unsigned int x) >>>>> { >>>>> - int r; >>>>> + unsigned int r; >>>>> + >>>>> + if ( __builtin_constant_p(x > 0) && x > 0 ) >>>>> + { >>>>> + /* Safe, when the compiler knows that x is nonzero. */ >>>>> + asm ( "bsf %[val], %[res]" >>>>> + : [res] "=r" (r) >>>>> + : [val] "rm" (x) ); >>>>> + } >>>> In patch 11 relevant things are all in a single patch, making it easier >>>> to spot that this is dead code: The sole caller already has a >>>> __builtin_constant_p(), hence I don't see how the one here could ever >>>> return true. With that the respective part of the description is then >>>> questionable, too, I'm afraid: Where did you observe any actual effect >>>> from this? Or if you did - what am I missing? >>> Hmm, thinking about it: I suppose that's why you have >>> __builtin_constant_p(x > 0), not __builtin_constant_p(x). I have to admit >>> I'm (positively) surprised that the former may return true when the latter >>> doesn't. >> So was I, but this recommendation came straight from the GCC mailing >> list.  And it really does work, even back in obsolete versions of GCC. >> >> __builtin_constant_p() operates on an expression not a value, and is >> documented as such. > Of course. > >>> Nevertheless I'm inclined to think this deserves a brief comment. >> There is a comment, and it's even visible in the snippet. > The comment is about the asm(); it is neither placed to clearly relate > to __builtin_constant_p(), nor is it saying anything about this specific > property of it. You said you were equally surprised; don't you think > that when both of us are surprised, a specific (even if brief) comment > is warranted? Spell it out for me like I'm an idiot. Because I'm looking at the patch I submitted, and at your request for "a brief comment", and I still have no idea what you think is wrong at the moment. I'm also not included to write a comment saying "go and read the GCC manual more carefully". > >>> As an aside, to better match the comment inside the if()'s body, how about >>> >>> if ( __builtin_constant_p(!!x) && x ) >>> >>> ? That also may make a little more clear that this isn't just a style >>> choice, but actually needed for the intended purpose. >> I am not changing the logic. >> >> Apart from anything else, your suggestion is trivially buggy.  I care >> about whether the RHS collapses to a constant, and the only way of doing >> that correctly is asking the compiler about the *exact* expression.  >> Asking about some other expression which you hope - but do not know - >> that the compiler will treat equivalently is bogus.  It would be >> strictly better to only take the else clause, than to have both halves >> emitted. >> >> This is the form I've tested extensively.  It's also the clearest form >> IMO.  You can experiment with alternative forms when we're not staring >> down code freeze of 4.19. > "Clearest form" is almost always a matter of taste. To me, comparing > unsigned values with > or < against 0 is generally at least suspicious. > Using != is typically better (again: imo), and simply omitting the != 0 > then is shorter with no difference in effect. Except in peculiar cases > like this one, where indeed it took me some time to figure why the > comparison operator may not be omitted. > > All that said: I'm not going to insist on any change; the R-b previously > offered still stands. I would highly appreciate though if the (further) > comment asked for could be added. > > What I definitely dislike here is you - not for the first time - turning > down remarks because a change of yours is late. Actually it's not to do with the release.  I'd reject it at any point because it's an unreasonable request to make; to me, or to anyone else. It would be a matter of taste (which again you have a singular view on), if it wasn't for the fact that what you actually said was: "I don't like it, and you should discard all the careful analysis you did because here's a form I prefer, that I haven't tested concerning a behaviour I didn't even realise until this email." and even if it wasn't a buggy suggestion to begin with, it's still toxic maintainer feedback. Frankly, I'd have more time to review other peoples patches if I wasn't wasting all of my time on premium grade manure like this, while trying to help Oleksii who's had it far worse this release trying to clean up droppings of maintainers-past. ~Andrew