png: Optimize by avoiding divisions.

diff --git a/grub-core/video/readers/png.c b/grub-core/video/readers/png.c
index 1f03f88d3965bf11f796b58e523f93d1d11488de..e1a01e99fd09190040d8a3f3ee61a633f11eb796 100644 (file)
--- a/grub-core/video/readers/png.c
+++ b/grub-core/video/readers/png.c
@@ -227,7 +227,7 @@ grub_png_decode_image_palette (struct grub_png_data *data,
 {
   unsigned i = 0, j;
 
-  if (len == 0 || len % 3 != 0)
+  if (len == 0)
     return GRUB_ERR_NONE;
 
   for (i = 0; 3 * i < len && i < 256; i++)
@@ -851,15 +851,26 @@ grub_png_convert_image (struct grub_png_data *data)
       int mask = (1 << data->color_bits) - 1;
       unsigned j;
       if (data->is_gray)
-       for (i = 0; i < (1U << data->color_bits); i++)
-         {
-           grub_uint8_t col = (0xff * i) / ((1U << data->color_bits) - 1);
-           palette[i][0] = col;
-           palette[i][1] = col;
-           palette[i][2] = col;
-         }
+       {
+         /* Generic formula is
+            (0xff * i) / ((1U << data->color_bits) - 1)
+            but for allowed bit depth of 1, 2 and for it's
+            equivalent to
+            (0xff / ((1U << data->color_bits) - 1)) * i
+            Precompute the multipliers to avoid division.
+         */
+
+         const grub_uint8_t multipliers[5] = { 0xff, 0xff, 0x55, 0x24, 0x11 };
+         for (i = 0; i < (1U << data->color_bits); i++)
+           {
+             grub_uint8_t col = multipliers[data->color_bits] * i;
+             palette[i][0] = col;
+             palette[i][1] = col;
+             palette[i][2] = col;
+           }
+       }
       else
-       grub_memcpy (palette, data->palette, 16 * 3);
+       grub_memcpy (palette, data->palette, 3 << data->color_bits);
       d1c = d1;
       d2c = d2;
       for (j = 0; j < data->image_height; j++, d1c += data->image_width * 3,