ARM64 NEON SIMD implementation of Huffman encoding

Full-color compression speedups relative to previous commits:
Cortex-A53 (Nexus 5X), Android, 64-bit: 1.1-13% (avg. 6.0%)
Cortex-A57 (Nexus 5X), Android, 64-bit: 0.0-22% (avg. 6.3%)

Refer to libjpeg-turbo#47 and libjpeg-turbo#50 for discussion

Closes libjpeg-turbo#50

Note that this commit introduces a similar /proc/cpuinfo parser to that
of the ARM32 implementation.  It is used to specifically check whether
the code is running on Cavium ThunderX and, if so, disable the ARM64
SIMD Huffman routines (which slow performance by an average of 8% on
that CPU.)

Based on:
mayeut@a8c282e

simd/jsimd_arm64.c

@@ -27,6 +27,70 @@
 #include <ctype.h>
 
 static unsigned int simd_support = ~0;
+static unsigned int simd_huffman = 1;
+
+#if defined(__linux__) || defined(ANDROID) || defined(__ANDROID__)
+
+#define SOMEWHAT_SANE_PROC_CPUINFO_SIZE_LIMIT (1024 * 1024)
+
+LOCAL(int)
+check_cpuinfo (char *buffer, const char *field, char *value)
+{
+  char *p;
+  if (*value == 0)
+    return 0;
+  if (strncmp(buffer, field, strlen(field)) != 0)
+    return 0;
+  buffer += strlen(field);
+  while (isspace(*buffer))
+    buffer++;
+
+  /* Check if 'value' is present in the buffer as a separate word */
+  while ((p = strstr(buffer, value))) {
+    if (p > buffer && !isspace(*(p - 1))) {
+      buffer++;
+      continue;
+    }
+    p += strlen(value);
+    if (*p != 0 && !isspace(*p)) {
+      buffer++;
+      continue;
+    }
+    return 1;
+  }
+  return 0;
+}
+
+LOCAL(int)
+parse_proc_cpuinfo (int bufsize)
+{
+  char *buffer = (char *)malloc(bufsize);
+  FILE *fd;
+
+  if (!buffer)
+    return 0;
+
+  fd = fopen("/proc/cpuinfo", "r");
+  if (fd) {
+    while (fgets(buffer, bufsize, fd)) {
+      if (!strchr(buffer, '\n') && !feof(fd)) {
+        /* "impossible" happened - insufficient size of the buffer! */
+        fclose(fd);
+        free(buffer);
+        return 0;
+      }
+      if (check_cpuinfo(buffer, "CPU part", "0x0a1"))
+        /* The SIMD version of Huffman encoding is slower than the C version on
+           Cavium ThunderX. */
+        simd_huffman = 0;
+    }
+    fclose(fd);
+  }
+  free(buffer);
+  return 1;
+}
+
+#endif
 
 /*
  * Check what SIMD accelerations are supported.
@@ -44,13 +108,23 @@ LOCAL(void)
 init_simd (void)
 {
   char *env = NULL;
+#if defined(__linux__) || defined(ANDROID) || defined(__ANDROID__)
+  int bufsize = 1024; /* an initial guess for the line buffer size limit */
+#endif
 
   if (simd_support != ~0U)
     return;
 
   simd_support = 0;
 
   simd_support |= JSIMD_ARM_NEON;
+#if defined(__linux__) || defined(ANDROID) || defined(__ANDROID__)
+  while (!parse_proc_cpuinfo(bufsize)) {
+    bufsize *= 2;
+    if (bufsize > SOMEWHAT_SANE_PROC_CPUINFO_SIZE_LIMIT)
+      break;
+  }
+#endif
 
   /* Force different settings through environment variables */
   env = getenv("JSIMD_FORCENEON");
@@ -59,6 +133,9 @@ init_simd (void)
   env = getenv("JSIMD_FORCENONE");
   if ((env != NULL) && (strcmp(env, "1") == 0))
     simd_support = 0;
+  env = getenv("JSIMD_NOHUFFENC");
+  if ((env != NULL) && (strcmp(env, "1") == 0))
+    simd_huffman = 0;
 }
 
 GLOBAL(int)
@@ -657,6 +734,16 @@ jsimd_idct_float (j_decompress_ptr cinfo, jpeg_component_info * compptr,
 GLOBAL(int)
 jsimd_can_huff_encode_one_block (void)
 {
+  init_simd();
+
+  if (DCTSIZE != 8)
+    return 0;
+  if (sizeof(JCOEF) != 2)
+    return 0;
+
+  if (simd_support & JSIMD_ARM_NEON && simd_huffman)
+    return 1;
+
   return 0;
 }
 
@@ -665,5 +752,6 @@ jsimd_huff_encode_one_block (void * state, JOCTET *buffer, JCOEFPTR block,
                              int last_dc_val, c_derived_tbl *dctbl,
                              c_derived_tbl *actbl)
 {
-  return NULL;
+  return jsimd_huff_encode_one_block_neon(state, buffer, block, last_dc_val,
+                                          dctbl, actbl);
 }