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  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]
Date:   Sun, 7 Oct 2018 13:52:24 -0600
From:   Jeff Law <>
To:     Nadav Amit <>, Richard Biener <>,
        Borislav Petkov <>,
        "" <>, Michael Matz <>
Cc:     Ingo Molnar <>,
        "" <>,
        "" <>,
        Masahiro Yamada <>,
        Sam Ravnborg <>,
        Alok Kataria <>,
        Christopher Li <>,
        Greg Kroah-Hartman <>,
        "H. Peter Anvin" <>, Jan Beulich <>,
        Josh Poimboeuf <>,
        Juergen Gross <>,
        Kate Stewart <>,
        Kees Cook <>,
        "" <>,
        Peter Zijlstra <>,
        Philippe Ombredanne <>,
        Thomas Gleixner <>,
        Linus Torvalds <>,
        Chris Zankel <>,
        Max Filippov <>
Subject: Re: PROPOSAL: Extend inline asm syntax with size spec

On 10/7/18 1:06 PM, Nadav Amit wrote:
> at 9:46 AM, Richard Biener <> wrote:
>> On October 7, 2018 6:09:30 PM GMT+02:00, Nadav Amit <> wrote:
>>> at 2:18 AM, Borislav Petkov <> wrote:
>>>> Hi people,
>>>> this is an attempt to see whether gcc's inline asm heuristic when
>>>> estimating inline asm statements' cost for better inlining can be
>>>> improved.
>>>> AFAIU, the problematic arises when one ends up using a lot of inline
>>>> asm statements in the kernel but due to the inline asm cost
>>> estimation
>>>> heuristic which counts lines, I think, for example like in this here
>>>> macro:
>>>> the resulting code ends up not inlining the functions themselves
>>> which
>>>> use this macro. I.e., you see a CALL <function> instead of its body
>>>> getting inlined directly.
>>>> Even though it should be because the actual instructions are only a
>>>> couple in most cases and all those other directives end up in another
>>>> section anyway.
>>>> The issue is explained below in the forwarded mail in a larger detail
>>>> too.
>>>> Now, Richard suggested doing something like:
>>>> 1) inline asm ("...")
>>>> 2) asm ("..." : : : : <size-expr>)
>>>> 3) asm ("...") __attribute__((asm_size(<size-expr>)));
>>>> with which user can tell gcc what the size of that inline asm
>>> statement
>>>> is and thus allow for more precise cost estimation and in the end
>>> better
>>>> inlining.
>>>> And FWIW 3) looks pretty straight-forward to me because attributes
>>> are
>>>> pretty common anyways.
>>>> But I'm sure there are other options and I'm sure people will have
>>>> better/different ideas so feel free to chime in.
>>> Thanks for taking care of it. I would like to mention a second issue,
>>> since
>>> you may want to resolve both with a single solution: not inlining
>>> conditional __builtin_constant_p(), in which there are two code-paths -
>>> one
>>> for constants and one for variables.
>>> Consider for example the Linux kernel ilog2 macro, which has a
>>> condition
>>> based on __builtin_constant_p() (
>>> ). The compiler mistakenly considers the “heavy” code-path that is
>>> supposed
>>> to be evaluated only in compilation time to evaluate the code size.
>> But this is a misconception about __builtin_constant_p. It doesn't guard sth like 'constexpr' regions. If you try to use it with those semantics you'll fail (appearantly you do). 
>> Of course IPA CP code size estimates when seeing a constant fed to bcp might be not optimal, that's another issue of course. 
> I understand that this is might not be the right way to implement macros
> such as ilog2() and test_bit(), but this code is around for some time.
That doesn't make it right -- and there's been numerous bogus bugs
reported against ilog2 because the authors of ilog2 haven't had a clear
understanding of the semantics of builtin_constant_p.


Powered by blists - more mailing lists