linux-kernel - Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <956DB0ED-F3DB-456D-8D06-6F40DBDB815A@zytor.com>
Date: Mon, 28 Apr 2025 20:13:52 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
CC: Arnd Bergmann <arnd@...db.de>, Arnd Bergmann <arnd@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Alexander Usyskin <alexander.usyskin@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mateusz Jończyk <mat.jonczyk@...pl>,
        Mike Rapoport <rppt@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C

On April 28, 2025 7:25:17 PM PDT, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>On 29/04/2025 3:00 am, H. Peter Anvin wrote:
>> On April 28, 2025 5:12:13 PM PDT, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>>> On 28/04/2025 10:38 pm, H. Peter Anvin wrote:
>>>> On April 28, 2025 9:14:45 AM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>>>>> On Mon, 28 Apr 2025 at 00:05, Ingo Molnar <mingo@...nel.org> 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?