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>
+#include <asm/hwcap.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)
+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)) {
+      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;
+        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);
+    }
+  }
+  if ((hwcap & HWCAP_VFPv3) != 0) result |= kCPUFeatureVFPv3;
+  if ((hwcap & HWCAP_NEON) != 0) result |= kCPUFeatureNEON;
+#endif
+  if (architecture >= 7) result |= kCPUFeatureARMv7;
+  if (architecture >= 6) result |= kCPUFeatureLDREXSTREX;
+  return result;
+}
+#endif  // WEBRTC_ARCH_ARM_FAMILY