[mutter] region-utils: Reduce temporary allocations

[Marge Bot <marge-bot src gnome org>]

commit 91c9416259f744a9f1ae123a1f319943705ff23d
Author: Robert Mader <robert mader posteo de>
Date:   Mon Nov 16 13:43:52 2020 +0100

    region-utils: Reduce temporary allocations
    
    This applies the optimizations from 0c55e87d8fb70848e to serveral
    similar places in region-utils.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1565>

 src/compositor/region-utils.c | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)
---
diff --git a/src/compositor/region-utils.c b/src/compositor/region-utils.c
index 4f5a2346a4..aabe76b52d 100644
--- a/src/compositor/region-utils.c
+++ b/src/compositor/region-utils.c
@@ -26,6 +26,15 @@
 
 #include <math.h>
 
+#define META_REGION_MAX_STACK_RECTS 256
+
+#define META_REGION_CREATE_RECTANGLE_ARRAY_SCOPED(n_rects, rects) \
+  g_autofree cairo_rectangle_int_t *G_PASTE(__n, __LINE__) = NULL; \
+  if (n_rects < META_REGION_MAX_STACK_RECTS) \
+    rects = g_newa (cairo_rectangle_int_t, n_rects); \
+  else \
+    rects = G_PASTE(__n, __LINE__) = g_new (cairo_rectangle_int_t, n_rects);
+
 /* MetaRegionBuilder */
 
 /* Various algorithms in this file require unioning together a set of rectangles
@@ -191,8 +200,7 @@ meta_region_scale_double (cairo_region_t       *region,
     return cairo_region_copy (region);
 
   n_rects = cairo_region_num_rectangles (region);
-
-  rects = g_malloc (sizeof(cairo_rectangle_int_t) * n_rects);
+  META_REGION_CREATE_RECTANGLE_ARRAY_SCOPED (n_rects, rects);
   for (i = 0; i < n_rects; i++)
     {
       cairo_region_get_rectangle (region, i, &rects[i]);
@@ -203,8 +211,6 @@ meta_region_scale_double (cairo_region_t       *region,
 
   scaled_region = cairo_region_create_rectangles (rects, n_rects);
 
-  g_free (rects);
-
   return scaled_region;
 }
 
@@ -219,8 +225,7 @@ meta_region_scale (cairo_region_t *region, int scale)
     return cairo_region_copy (region);
 
   n_rects = cairo_region_num_rectangles (region);
-
-  rects = g_malloc (sizeof(cairo_rectangle_int_t) * n_rects);
+  META_REGION_CREATE_RECTANGLE_ARRAY_SCOPED (n_rects, rects);
   for (i = 0; i < n_rects; i++)
     {
       cairo_region_get_rectangle (region, i, &rects[i]);
@@ -232,8 +237,6 @@ meta_region_scale (cairo_region_t *region, int scale)
 
   scaled_region = cairo_region_create_rectangles (rects, n_rects);
 
-  g_free (rects);
-
   return scaled_region;
 }
 
@@ -389,8 +392,7 @@ meta_region_transform (const cairo_region_t *region,
     return cairo_region_copy (region);
 
   n_rects = cairo_region_num_rectangles (region);
-
-  rects = g_new0 (cairo_rectangle_int_t, n_rects);
+  META_REGION_CREATE_RECTANGLE_ARRAY_SCOPED (n_rects, rects);
   for (i = 0; i < n_rects; i++)
     {
       cairo_region_get_rectangle (region, i, &rects[i]);
@@ -404,8 +406,6 @@ meta_region_transform (const cairo_region_t *region,
 
   transformed_region = cairo_region_create_rectangles (rects, n_rects);
 
-  g_free (rects);
-
   return transformed_region;
 }
 
@@ -437,8 +437,7 @@ meta_region_crop_and_scale (cairo_region_t  *region,
     }
 
   n_rects = cairo_region_num_rectangles (region);
-
-  rects = g_new0 (cairo_rectangle_int_t, n_rects);
+  META_REGION_CREATE_RECTANGLE_ARRAY_SCOPED (n_rects, rects);
   for (i = 0; i < n_rects; i++)
     {
       cairo_region_get_rectangle (region, i, &rects[i]);
@@ -452,7 +451,5 @@ meta_region_crop_and_scale (cairo_region_t  *region,
 
   viewport_region = cairo_region_create_rectangles (rects, n_rects);
 
-  g_free (rects);
-
   return viewport_region;
 }