Messages in this thread | ![/](/images/icornerl.gif) | | From | Linus Torvalds <> | Date | Tue, 2 May 2023 09:00:06 -0700 | Subject | Re: [GIT PULL] x86/mm for 6.4 |
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On Tue, May 2, 2023 at 8:42 AM Dave Hansen <dave.hansen@intel.com> wrote: > > Have anyone seen any actual code generation difference between: > > return (long)ptr >= 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 >= 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 it.
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
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