linux-kernel - Re: [RFC PATCH 0/2] Helper to isolate least-significant bit

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Message-ID: <b47b02cc-642b-4a21-a932-25bca8352ae3@citrix.com>
Date: Fri, 9 Jan 2026 19:26:13 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: ptesarik@...e.com
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Subject: Re: [RFC PATCH 0/2] Helper to isolate least-significant bit

> diff --git a/arch/alpha/kernel/core_cia.c b/arch/alpha/kernel/core_cia.c
> index 6e577228e175d..b3c273c13c104 100644
> --- a/arch/alpha/kernel/core_cia.c
> +++ b/arch/alpha/kernel/core_cia.c
> @@ -1035,7 +1035,7 @@ cia_decode_ecc_error(struct el_CIA_sysdata_mcheck *cia, const char *msg)
>      cia_decode_mem_error(cia, msg);
>  
>      syn = cia->cia_syn & 0xff;
> -    if (syn == (syn & -syn)) {
> +    if (syn == ffs_val(syn)) {
>          fmt = KERN_CRIT "  ECC syndrome %#x -- check bit %d\n";
>          i = ffs(syn) - 1;
>      } else {

This is a check for multiple bits set, which has a simpler expression to
calculate.  (the ffs() in context shows that syn isn't expected to be 0.)

In Xen I added this helper while tiding things up, and it's applicable
to the more common pattern of checking hweight() of a bitmap against 1.

/*
 * Calculate if a value has two or more bits set.  Always use this in
 * preference to an expression of the form 'hweight(x) > 1'.
 */
#define multiple_bits_set(x)                    \
    ({                                          \
        typeof(x) _v = (x);                     \
        (_v & (_v - 1)) != 0;                   \
    })


~Andrew