Implement CPU feature detection for ARM Linux.

webrtc/system_wrappers/source/cpu_features_linux.c

+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include <stdlib.h>
+#include <string.h>
+#include <features.h>
+#if __GLIBC_PREREQ(2, 16)
+#include <sys/auxv.h>
+#else
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+#include <link.h>
+#endif
+#include "webrtc/system_wrappers/include/cpu_features_wrapper.h"
+
+#if defined(WEBRTC_ARCH_ARM_FAMILY)
+#include <asm/hwcap.h>
+
+uint64_t WebRtc_GetCPUFeaturesARM(void) {
+  uint64_t result = 0;
+  int architecture = 0;
+  unsigned long hwcap = 0;
+  const char* platform = NULL;
+#if __GLIBC_PREREQ(2, 16)
+  hwcap = getauxval(AT_HWCAP);
+  platform = (const char*)getauxval(AT_PLATFORM);
+#else
+  ElfW(auxv_t) auxv;
+  int fd = open("/proc/self/auxv", O_RDONLY);
+  if (fd >= 0) {
+    while ((hwcap == 0) || (platform == NULL)) {

[torbjorng (webrtc), 2016/04/04 08:35:42]

We don't usually use redundant parentheses.

[Simon Hosie, 2016/04/05 01:15:48]

Done.

+      if (read(fd, &auxv, sizeof(auxv)) < sizeof(auxv)) {
+        if (errno == EINTR) continue;
+        break;
+      }
+      switch (auxv.a_type) {
+      case AT_HWCAP:
+        hwcap = auxv.a_un.a_val;
+        break;
+      case AT_PLATFORM:
+        platform = (const char*)auxv.a_un.a_val;
+        break;
+      default:
+        (void)auxv.a_un.a_val;

[torbjorng (webrtc), 2016/04/04 08:35:42]

Please remove this.

[Simon Hosie, 2016/04/05 01:15:48]

Done.

+        break;
+      }
+    }
+    close(fd);
+  }
+#endif  // __GLIBC_PREREQ(2,16)
+#if defined(__aarch64__)
+  architecture = 8;
+  if ((hwcap & HWCAP_FP) != 0) result |= kCPUFeatureVFPv3;
+  if ((hwcap & HWCAP_ASIMD) != 0) result |= kCPUFeatureNEON;
+#else
+  if (platform != NULL) {
+    /* expect a string in the form "v6l" or "v7l", etc.
+     */
+    if ((platform[0] == 'v') &&
+        ('0' <= platform[1]) && (platform[1] <= '9')) {
+      architecture = atoi(platform + 1);

[torbjorng (webrtc), 2016/04/04 08:35:42]

If the |platform| string end up not being 0-terminated, this atoi might run off
in memory.

[Simon Hosie, 2016/04/05 01:15:48]

getauxval() defines `platform` as a pointer to a string.  Since there's no other
way to determine its length it really must be nul-terminated.

+    }
+  }
+  if ((hwcap & HWCAP_VFPv3) != 0) result |= kCPUFeatureVFPv3;
+  if ((hwcap & HWCAP_NEON) != 0) result |= kCPUFeatureNEON;
+#endif
+  if (architecture >= 7) result |= kCPUFeatureARMv7;

[torbjorng (webrtc), 2016/04/04 08:35:42]

This code will not detect CPU archs properly if the atoi above ever read more
than just one digit. E.g., no subarchs every append another digit, the result
will be greater than 7. To fix, please read just one digit above without using
atoi.

[Simon Hosie, 2016/04/05 01:15:48]

Reading just one digit makes "v10" less than "v7".  To avoid guessing which is
more likely I'll make it fail when there's more than one digit (so both "v10l"
and "v82l" will fail to parse).  Somebody can fix it either when ARMv10 ships or
when sub-architectures are inserted into this string.  (but it won't matter
because no caller it WebRTC actually uses this flag)

+  if (architecture >= 6) result |= kCPUFeatureLDREXSTREX;
+  return result;
+}
+#endif  // WEBRTC_ARCH_ARM_FAMILY