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

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Message-ID: <8923623d-1a23-483d-add7-2fd03a3a2c3d@zytor.com>
Date: Tue, 20 Jan 2026 15:49:01 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: David Desobry <david.desobry@...malgen.com>,
        David Laight <david.laight.linux@...il.com>
Cc: tglx@...nel.org, 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 2026-01-20 15:32, David Desobry wrote:
> 
> Good catch. I have replaced the switch statement with a linear if-chain in v3
> to ensure better code generation for both GCC and Clang.
> 

I think a bigger deal is just to change it to unsigned.

Now, for really silly optimization:

int num_digits(unsigned int x)
{
    int n = 0;
    asm("cmp %2,%1; sbb $-2,%0" : "+r" (n) : "r" (x), "g" (10));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (100));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (1000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (10000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (100000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (1000000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (10000000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (100000000));
    asm("cmp %2,%1; sbb $-1,%0" : "+r" (n) : "r" (x), "g" (1000000000));

    return n;
}

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	-hpa