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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 2 May 2023 09:00:06 -0700
Message-ID: <CAHk-=wgnGoj8qZg3SRh1Z1-mU=b3ryydXZykUuarnrAn6LuHBw@mail.gmail.com>
Subject: Re: [GIT PULL] x86/mm for 6.4
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com,
        Peter Zijlstra <peterz@infradead.org>
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On Tue, May 2, 2023 at 8:42=E2=80=AFAM Dave Hansen <dave.hansen@intel.com> =
wrote:
>
> Have anyone seen any actual code generation difference between:
>
>         return (long)ptr >=3D 0;
>
> and
>
>         return !((unsigned long)ptr & (1UL<<(BITS_PER_LONG-1)));

No, as far as I know, they both generate the same code.

> It's longer, but I'd rather read the explicit "check bit 63" than the
> positive/negative address space thing.  I certainly grok both, but have
> to think through the "(long)ptr >=3D 0" check every time.

I'm very much the other way. I think it's much clearer to say "check
the sign bit".

Doing the explicit bit check means that I have to look at what the bit
number is, and that is a much more complex expression.

In fact, I'd find it easier to read

       return !((unsigned long)ptr & (1UL<< 63));

just because then you go "Oh, checking bit 63" without having to parse
the expression.

But even then I find the '!' is easy to miss, so you really have to parse i=
t.

But:

> I guess it also wouldn't matter as much either if we hid it in a helper
> like the attached patch and I didn't have to read it twice. ;)

Yeah, I think that's a good solution.

                 Linus