linux-kernel - Re: [PATCH v2] ACPI: NFIT: Optimize nfit_mem

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Message-ID: <CAJZ5v0gSB_ACBpK1nKu3sbA0HQ1xsk2mn3oc9AjpoFtge9Opdw@mail.gmail.com>
Date:   Wed, 18 Oct 2023 13:17:31 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Kuan-Wei Chiu <visitorckw@...il.com>
Cc:     dan.j.williams@...el.com, vishal.l.verma@...el.com,
        dave.jiang@...el.com, ira.weiny@...el.com, rafael@...nel.org,
        lenb@...nel.org, nvdimm@...ts.linux.dev,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ACPI: NFIT: Optimize nfit_mem_cmp() for efficiency

On Fri, Oct 13, 2023 at 2:22 PM Kuan-Wei Chiu <visitorckw@...il.com> wrote:
>
> The original code used conditional branching in the nfit_mem_cmp
> function to compare two values and return -1, 1, or 0 based on the
> result. However, the list_sort comparison function only needs results
> <0, >0, or =0. This patch optimizes the code to make the comparison
> branchless, improving efficiency and reducing code size. This change
> reduces the number of comparison operations from 1-2 to a single
> subtraction operation, thereby saving the number of instructions.
>
> Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> ---
> v1 -> v2:
> - Add explicit type cast in case the sizes of u32 and int differ.
>
>  drivers/acpi/nfit/core.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/drivers/acpi/nfit/core.c b/drivers/acpi/nfit/core.c
> index f96bf32cd368..563a32eba888 100644
> --- a/drivers/acpi/nfit/core.c
> +++ b/drivers/acpi/nfit/core.c
> @@ -1138,11 +1138,7 @@ static int nfit_mem_cmp(void *priv, const struct list_head *_a,
>
>         handleA = __to_nfit_memdev(a)->device_handle;
>         handleB = __to_nfit_memdev(b)->device_handle;
> -       if (handleA < handleB)
> -               return -1;
> -       else if (handleA > handleB)
> -               return 1;
> -       return 0;
> +       return (int)handleA - (int)handleB;

Are you sure that you are not losing bits in these conversions?

>  }
>
>  static int nfit_mem_init(struct acpi_nfit_desc *acpi_desc)
> --