2023-10-12 23:05:47

by Kuan-Wei Chiu

Subject: [PATCH] drm/i915/gvt: Optimize mmio_offset_compare() for efficiency

The original code used conditional branching in the mmio_offset_compare
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 <[email protected]>
---
drivers/gpu/drm/i915/gvt/debugfs.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/debugfs.c b/drivers/gpu/drm/i915/gvt/debugfs.c
index baccbf1761b7..998d82a259c8 100644
--- a/drivers/gpu/drm/i915/gvt/debugfs.c
+++ b/drivers/gpu/drm/i915/gvt/debugfs.c
@@ -48,11 +48,7 @@ static int mmio_offset_compare(void *priv,

ma = container_of(a, struct diff_mmio, node);
mb = container_of(b, struct diff_mmio, node);
- if (ma->offset < mb->offset)
- return -1;
- else if (ma->offset > mb->offset)
- return 1;
- return 0;
+ return ma->offset - mb->offset;
}

static inline int mmio_diff_handler(struct intel_gvt *gvt,
--
2.25.1

2023-10-13 10:36:47

by Ville Syrjälä

Subject: Re: [PATCH] drm/i915/gvt: Optimize mmio_offset_compare() for efficiency

On Fri, Oct 13, 2023 at 07:04:49AM +0800, Kuan-Wei Chiu wrote:
> The original code used conditional branching in the mmio_offset_compare
> 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 <[email protected]>
> ---
> drivers/gpu/drm/i915/gvt/debugfs.c | 6 +-----
> 1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gvt/debugfs.c b/drivers/gpu/drm/i915/gvt/debugfs.c
> index baccbf1761b7..998d82a259c8 100644
> --- a/drivers/gpu/drm/i915/gvt/debugfs.c
> +++ b/drivers/gpu/drm/i915/gvt/debugfs.c
> @@ -48,11 +48,7 @@ static int mmio_offset_compare(void *priv,
>
> ma = container_of(a, struct diff_mmio, node);
> mb = container_of(b, struct diff_mmio, node);
> - if (ma->offset < mb->offset)
> - return -1;
> - else if (ma->offset > mb->offset)
> - return 1;
> - return 0;
> + return ma->offset - mb->offset;

Those are unsigned ints effectively, so this only works due
to the return value being the same size signed int. Might be
better to add some explicit casts.

> }
>
> static inline int mmio_diff_handler(struct intel_gvt *gvt,
> --
> 2.25.1

--
Ville Syrj?l?
Intel

2023-10-13 12:14:32

by Kuan-Wei Chiu

Subject: [PATCH v2] drm/i915/gvt: Optimize mmio_offset_compare() for efficiency

The original code used conditional branching in the mmio_offset_compare
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 <[email protected]>
---
v1 -> v2:
- Add explicit type cast in case the sizes of u32 and int differ.

drivers/gpu/drm/i915/gvt/debugfs.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/debugfs.c b/drivers/gpu/drm/i915/gvt/debugfs.c
index baccbf1761b7..d85d8a3b5ae5 100644
--- a/drivers/gpu/drm/i915/gvt/debugfs.c
+++ b/drivers/gpu/drm/i915/gvt/debugfs.c
@@ -48,11 +48,7 @@ static int mmio_offset_compare(void *priv,

ma = container_of(a, struct diff_mmio, node);
mb = container_of(b, struct diff_mmio, node);
- if (ma->offset < mb->offset)
- return -1;
- else if (ma->offset > mb->offset)
- return 1;
- return 0;
+ return (int)ma->offset - (int)mb->offset;
}

static inline int mmio_diff_handler(struct intel_gvt *gvt,
--
2.25.1