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Further optimisations to jbig2_decode_generic_template0_TPGDON.

Add a function to get a 'run' of pixels, and call that.

This takes the time spent in this function from 29% to 22% for
Jbig2_042_08.pdf.

--- a/jbig2_generic.c

+++ b/jbig2_generic.c

@@ -863,6 +863,10 @@

         left = -gmin;

     if (right < gmax)

         right = gmax;

+    /* We need to guarantee 9 pixels in the right margin to be able

+     * to use jbig2_image_get_pixels_fast. */

+    if (right < 9)

+        right = 9;

     right = GBW - right;

     /* So 0 <= x < left or right <= x < GBW needs bounds checking. */

@@ -892,21 +896,12 @@

                 }

                 if (y >= top && x >= left && x < right)

                 {

-                    CONTEXT = jbig2_image_get_pixel_fast(image, x - 1, y);

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x - 2, y) << 1;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x - 3, y) << 2;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x - 4, y) << 3;

+                    CONTEXT = jbig2_image_get_pixels_fast(image, x - 4, y, 4);

                     CONTEXT |= jbig2_image_get_pixel_fast(image, x + params->gbat[0], y + params->gbat[1]) << 4;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x + 2, y - 1) << 5;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x + 1, y - 1) << 6;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x, y - 1) << 7;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x - 1, y - 1) << 8;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x - 2, y - 1) << 9;

+                    CONTEXT |= jbig2_image_get_pixels_fast(image, x - 2, y - 1, 5) << 5;

                     CONTEXT |= jbig2_image_get_pixel_fast(image, x + params->gbat[2], y + params->gbat[3]) << 10;

                     CONTEXT |= jbig2_image_get_pixel_fast(image, x + params->gbat[4], y + params->gbat[5]) << 11;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x + 1, y - 2) << 12;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x, y - 2) << 13;

-                    CONTEXT |= jbig2_image_get_pixel_fast(image, x - 1, y - 2) << 14;

+                    CONTEXT |= jbig2_image_get_pixels_fast(image, x - 1, y - 2, 3) << 12;

                     CONTEXT |= jbig2_image_get_pixel_fast(image, x + params->gbat[6], y + params->gbat[7]) << 15;

                 }

                 else

--- a/jbig2_image.h

+++ b/jbig2_image.h

@@ -39,6 +39,7 @@

 int jbig2_image_get_pixel(Jbig2Image *image, int x, int y);

 void jbig2_image_set_pixel(Jbig2Image *image, int x, int y, bool value);

+/* Get a bit. No bounds checking. */

 static inline int

 jbig2_image_get_pixel_fast(Jbig2Image *image, int x, int y)

 {

@@ -48,7 +49,20 @@

     return ((image->data[byte] >> bit) & 1);

 }

-/* set an individual pixel value in an image */

+/* Get a run of up to 9 bits. This reads into the next byte, so the caller

+ * must ensure that we are always safe to read at least 9 pixels, even if

+ * it only wants less than that. */

+static inline int

+jbig2_image_get_pixels_fast(Jbig2Image *image, int x, int y, int bits)

+{

+    const int byte = (x >> 3) + y * image->stride;

+    const int bit = 7 - (x & 7);

+    int v = ((image->data[byte]<<8) | (image->data[byte+1]))>>(bit+9-bits);

+

+    return v & ((1<<bits)-1);

+}

+

+/* set an individual pixel value in an image - no bounds checking */

 static inline void

 jbig2_image_set_pixel_fast(Jbig2Image *image, int x, int y, bool value)

 {