From: Kuan-Wei Chiu <visitorckw@gmail.com>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: dan.j.williams@intel.com, vishal.l.verma@intel.com,
dave.jiang@intel.com, ira.weiny@intel.com, lenb@kernel.org,
nvdimm@lists.linux.dev, linux-acpi@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] ACPI: NFIT: Optimize nfit_mem_cmp() for efficiency
Date: Thu, 19 Oct 2023 12:30:25 +0800 [thread overview]
Message-ID: <20231019043025.GA577714@sivslab-System-Product-Name> (raw)
In-Reply-To: <CAJZ5v0gSB_ACBpK1nKu3sbA0HQ1xsk2mn3oc9AjpoFtge9Opdw@mail.gmail.com>
On Wed, Oct 18, 2023 at 01:17:31PM +0200, Rafael J. Wysocki wrote:
> On Fri, Oct 13, 2023 at 2:22 PM Kuan-Wei Chiu <visitorckw@gmail.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@gmail.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?
I believe your concerns are valid. Perhaps this was a stupid mistake I
made. Initially, I proposed this patch because I noticed that other
parts of the Linux kernel, such as the sram_reserve_cmp() function in
drivers/misc/sram.c, directly used subtraction for comparisons
involving u32. However, this approach could potentially lead to issues
when the size of int is 2 bytes instead of 4 bytes. I think maybe we
should consider dropping this patch. I apologize for proposing an
incorrect patch.
Thanks,
Kuan-Wei Chiu
>
> > }
> >
> > static int nfit_mem_init(struct acpi_nfit_desc *acpi_desc)
> > --
prev parent reply other threads:[~2023-10-19 4:30 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-12 21:59 [PATCH] ACPI: NFIT: Optimize nfit_mem_cmp() for efficiency Kuan-Wei Chiu
2023-10-13 12:22 ` [PATCH v2] " Kuan-Wei Chiu
2023-10-18 11:17 ` Rafael J. Wysocki
2023-10-19 4:30 ` Kuan-Wei Chiu [this message]
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