linux-kernel - Re: [PATCH] drm/i915/gvt: Optimize mmio_offset

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Date:   Fri, 13 Oct 2023 13:36:17 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Kuan-Wei Chiu <visitorckw@...il.com>
Cc:     zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        tvrtko.ursulin@...ux.intel.com,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, rodrigo.vivi@...el.com
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 <visitorckw@...il.com>
> ---
>  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 Syrjälä
Intel