linux-kernel - [PATCH v2] drm/i915/gvt: Optimize mmio_offset

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Message-Id: <20231013121355.2125824-1-visitorckw@gmail.com>
Date:   Fri, 13 Oct 2023 20:13:55 +0800
From:   Kuan-Wei Chiu <visitorckw@...il.com>
To:     ville.syrjala@...ux.intel.com, zhenyuw@...ux.intel.com,
        zhi.a.wang@...el.com
Cc:     jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, tvrtko.ursulin@...ux.intel.com,
        airlied@...il.com, daniel@...ll.ch,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, Kuan-Wei Chiu <visitorckw@...il.com>
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 <visitorckw@...il.com>
---
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