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Thu, 14 Mar 2024 09:23:56 -0700 (PDT) Message-ID: <6a3c09a1-f35f-4d1e-bbba-342388da71ef@citrix.com> Date: Thu, 14 Mar 2024 16:23:52 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/7] xen/bitops: Implement ffs() in common logic Content-Language: en-GB 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: <20240313172716.2325427-1-andrew.cooper3@citrix.com> <20240313172716.2325427-3-andrew.cooper3@citrix.com> <76a1e81d-86d1-438c-975e-c4db46af93ee@suse.com> 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: <76a1e81d-86d1-438c-975e-c4db46af93ee@suse.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 14/03/2024 2:16 pm, Jan Beulich wrote: > On 13.03.2024 18:27, Andrew Cooper wrote: >> --- a/xen/arch/arm/include/asm/bitops.h >> +++ b/xen/arch/arm/include/asm/bitops.h >> @@ -157,7 +157,7 @@ static inline int fls(unsigned int x) >> } >> >> >> -#define ffs(x) ({ unsigned int __t = (x); fls(ISOLATE_LSB(__t)); }) >> +#define arch_ffs(x) ({ unsigned int __t = (x); fls(ISOLATE_LSB(__t)); }) > The way the macro is invoked, I don't think the helper local variable > is then needed anymore? I strongly suspect It is still needed.  ISOLATE_LSB() double-expands its parameter. Either way, I'm not reopening that can of worms that lead to this form. >> --- a/xen/arch/x86/include/asm/bitops.h >> +++ b/xen/arch/x86/include/asm/bitops.h >> @@ -430,16 +430,23 @@ static inline int ffsl(unsigned long x) >> return (int)r+1; >> } >> >> -static inline int ffs(unsigned int x) >> +static inline unsigned int arch_ffs(unsigned int x) >> { >> - int r; >> + int r = -1; >> + >> + /* >> + * The AMD manual states that BSF won't modify the destination register if >> + * x=0. The Intel manual states that the result is undefined, but the >> + * architects have said that the register is written back with it's old >> + * value, possibly zero extended above 32 bits. >> + */ >> + asm ( "bsf %[val], %[res]" >> + : [res] "+r" (r) >> + : [val] "rm" (x) ); > And this isn't what the compiler would be doing anyway? No.  The builtin avoids all undefined behaviour, and is quite a lot of asm as a result. With some help from the gcc mailing list https://gcc.gnu.org/pipermail/gcc/2024-March/243465.html I've found a solution which improves things in the common case. > Also, just to mention it: I take it that you/we are sure that disallowing > both operands to be the same register is still better than ... > >> - asm ( "bsf %1,%0\n\t" >> - "jnz 1f\n\t" >> - "mov $-1,%0\n" >> - "1:" : "=r" (r) : "rm" (x)); > ... the original form? Yes.  Without any doubt, on a 64bit CPU. This transformation isn't safe on a 486, but I expect even the later 32bit CPUs lacking register renaming would still be better with the non-branch form. >> --- a/xen/include/xen/bitops.h >> +++ b/xen/include/xen/bitops.h >> @@ -110,6 +110,21 @@ static inline int generic_flsl(unsigned long x) >> >> #include >> >> +/* >> + * Find First Set bit. Bits are labelled from 1. >> + */ >> +static always_inline __pure unsigned int ffs(unsigned int x) > Why always_inline? For all the normal reasons to counter Clang and GCC doing stupid things with inlines that contain assembly. > >> +{ >> + if ( __builtin_constant_p(x) ) >> + return __builtin_ffs(x); >> + >> +#ifndef arch_ffs >> +#define arch_ffs __builtin_ffs >> +#endif >> + >> + return arch_ffs(x); >> +} > Just to mention it: __builtin_ffs() takes and returns plain int. I'm > happy about our own helper being unsigned-correct, but anything like > this has a Misra angle too. I did note that, and decided it could wait until some other point. ~Andrew