| Message ID | 20231013122236.2127269-1-visitorckw@gmail.com (mailing list archive) |
|---|---|
| State | Handled Elsewhere, archived |
| Headers | show |
| Series | [v2] ACPI: NFIT: Optimize nfit_mem_cmp() for efficiency | expand |
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? > } > > static int nfit_mem_init(struct acpi_nfit_desc *acpi_desc) > --
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) > > --
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; } static int nfit_mem_init(struct acpi_nfit_desc *acpi_desc)
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(-)