Re: [PATCH v2] ACPI: NFIT: Optimize nfit_mem_cmp() for efficiency

[Kuan-Wei Chiu]

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@xxxxxxxxx> 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@xxxxxxxxx>
> > ---
> > 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)
> > --