linux-kernel - Re: [PATCH v2] x86/lib: Optimize num_digits() and fix INT

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Message-ID: <alpine.DEB.2.21.2601231041380.6421@angie.orcam.me.uk>
Date: Fri, 23 Jan 2026 10:55:20 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: "H. Peter Anvin" <hpa@...or.com>
cc: David Desobry <david.desobry@...malgen.com>, 
    David Laight <david.laight.linux@...il.com>, tglx@...nel.org, 
    Ingo Molnar <mingo@...hat.com>, bp@...en8.de, dave.hansen@...ux.intel.com, 
    x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/lib: Optimize num_digits() and fix INT_MIN
 overflow

On Thu, 22 Jan 2026, H. Peter Anvin wrote:

> > I guess you chose to use SBB rather than somewhat less mind-twisting ADC 
> >for the entertainment of the reader?
[...]
> 
> No, I use it because SBB subtracts CF, whereas ADC adds CF.

 Ah, right, on x86 an immediate needs to be the subtrahend, so you don't 
have the freedom to swap the operands of CMP, which would let you use CF 
either way (as say on VAX), which is what I had in mind.  I stand 
corrected then, it's the only way.

  Maciej