* This file was part of the Independent JPEG Group's software:
* Copyright (C) 1995-1997, Thomas G. Lane.
* libjpeg-turbo Modifications:
- * Copyright (C) 2015, D. R. Commander.
+ * Copyright (C) 2011, 2015, 2018, D. R. Commander.
* For conditions of distribution and use, see the accompanying README.ijg
* file.
*
#include "jinclude.h"
#include "jpeglib.h"
#include "jchuff.h" /* Declarations shared with jchuff.c */
+#include <limits.h>
#ifdef C_PROGRESSIVE_SUPPORTED
+/*
+ * NOTE: If USE_CLZ_INTRINSIC is defined, then clz/bsr instructions will be
+ * used for bit counting rather than the lookup table. This will reduce the
+ * memory footprint by 64k, which is important for some mobile applications
+ * that create many isolated instances of libjpeg-turbo (web browsers, for
+ * instance.) This may improve performance on some mobile platforms as well.
+ * This feature is enabled by default only on ARM processors, because some x86
+ * chips have a slow implementation of bsr, and the use of clz/bsr cannot be
+ * shown to have a significant performance impact even on the x86 chips that
+ * have a fast implementation of it. When building for ARMv6, you can
+ * explicitly disable the use of clz/bsr by adding -mthumb to the compiler
+ * flags (this defines __thumb__).
+ */
+
+/* NOTE: Both GCC and Clang define __GNUC__ */
+#if defined __GNUC__ && (defined __arm__ || defined __aarch64__)
+#if !defined __thumb__ || defined __thumb2__
+#define USE_CLZ_INTRINSIC
+#endif
+#endif
+
+#ifdef USE_CLZ_INTRINSIC
+#define JPEG_NBITS_NONZERO(x) (32 - __builtin_clz(x))
+#define JPEG_NBITS(x) (x ? JPEG_NBITS_NONZERO(x) : 0)
+#else
+#include "jpeg_nbits_table.h"
+#define JPEG_NBITS(x) (jpeg_nbits_table[x])
+#define JPEG_NBITS_NONZERO(x) JPEG_NBITS(x)
+#endif
+
+
/* Expanded entropy encoder object for progressive Huffman encoding. */
typedef struct {
if (entropy->EOBRUN > 0) { /* if there is any pending EOBRUN */
temp = entropy->EOBRUN;
- nbits = 0;
- while ((temp >>= 1))
- nbits++;
+ nbits = JPEG_NBITS_NONZERO(temp) - 1;
/* safety check: shouldn't happen given limited correction-bit buffer */
if (nbits > 14)
ERREXIT(entropy->cinfo, JERR_HUFF_MISSING_CODE);
encode_mcu_DC_first (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
{
phuff_entropy_ptr entropy = (phuff_entropy_ptr) cinfo->entropy;
- register int temp, temp2;
+ register int temp, temp2, temp3;
register int nbits;
int blkn, ci;
int Al = cinfo->Al;
entropy->last_dc_val[ci] = temp2;
/* Encode the DC coefficient difference per section G.1.2.1 */
- temp2 = temp;
- if (temp < 0) {
- temp = -temp; /* temp is abs value of input */
- /* For a negative input, want temp2 = bitwise complement of abs(input) */
- /* This code assumes we are on a two's complement machine */
- temp2--;
- }
+
+ /* This is a well-known technique for obtaining the absolute value without
+ * a branch. It is derived from an assembly language technique presented
+ * in "How to Optimize for the Pentium Processors", Copyright (c) 1996,
+ * 1997 by Agner Fog.
+ */
+ temp3 = temp >> (CHAR_BIT * sizeof(int) - 1);
+ temp ^= temp3;
+ temp -= temp3; /* temp is abs value of input */
+ /* For a negative input, want temp2 = bitwise complement of abs(input) */
+ temp2 = temp ^ temp3;
/* Find the number of bits needed for the magnitude of the coefficient */
- nbits = 0;
- while (temp) {
- nbits++;
- temp >>= 1;
- }
+ nbits = JPEG_NBITS(temp);
/* Check for out-of-range coefficient values.
* Since we're encoding a difference, the range limit is twice as much.
*/
encode_mcu_AC_first (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
{
phuff_entropy_ptr entropy = (phuff_entropy_ptr) cinfo->entropy;
- register int temp, temp2;
+ register int temp, temp2, temp3;
register int nbits;
register int r, k;
int Se = cinfo->Se;
* in C, we shift after obtaining the absolute value; so the code is
* interwoven with finding the abs value (temp) and output bits (temp2).
*/
- if (temp < 0) {
- temp = -temp; /* temp is abs value of input */
- temp >>= Al; /* apply the point transform */
- /* For a negative coef, want temp2 = bitwise complement of abs(coef) */
- temp2 = ~temp;
- } else {
- temp >>= Al; /* apply the point transform */
- temp2 = temp;
- }
+ temp3 = temp >> (CHAR_BIT * sizeof(int) - 1);
+ temp ^= temp3;
+ temp -= temp3; /* temp is abs value of input */
+ temp >>= Al; /* apply the point transform */
+ /* For a negative coef, want temp2 = bitwise complement of abs(coef) */
+ temp2 = temp ^ temp3;
/* Watch out for case that nonzero coef is zero after point transform */
if (temp == 0) {
r++;
}
/* Find the number of bits needed for the magnitude of the coefficient */
- nbits = 1; /* there must be at least one 1 bit */
- while ((temp >>= 1))
- nbits++;
+ nbits = JPEG_NBITS_NONZERO(temp); /* there must be at least one 1 bit */
/* Check for out-of-range coefficient values */
if (nbits > MAX_COEF_BITS)
ERREXIT(cinfo, JERR_BAD_DCT_COEF);
encode_mcu_AC_refine (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
{
phuff_entropy_ptr entropy = (phuff_entropy_ptr) cinfo->entropy;
- register int temp;
+ register int temp, temp3;
register int r, k;
int EOB;
char *BR_buffer;
* is an integer division with rounding towards 0. To do this portably
* in C, we shift after obtaining the absolute value.
*/
- if (temp < 0)
- temp = -temp; /* temp is abs value of input */
+ temp3 = temp >> (CHAR_BIT * sizeof(int) - 1);
+ temp ^= temp3;
+ temp -= temp3; /* temp is abs value of input */
temp >>= Al; /* apply the point transform */
absvalues[k] = temp; /* save abs value for main pass */
if (temp == 1)
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