Implement CPU feature detection for ARM Linux.

BUG=webrtc:5057
NOTRY=True

Review URL: https://codereview.webrtc.org/1820133002

Cr-Commit-Position: refs/heads/master@{#12270}

webrtc/system_wrappers/BUILD.gn

@@ -107,6 +107,10 @@ static_library("system_wrappers") {
   if (is_linux) {
     defines += [ "WEBRTC_THREAD_RR" ]
 
+    if (!build_with_chromium) {
+      deps += [ ":cpu_features_linux" ]
+    }
+
     libs += [ "rt" ]
   }
 
@@ -177,3 +181,14 @@ if (is_android) {
     ]
   }
 }
+
+if (is_linux) {
+  source_set("cpu_features_linux") {
+    sources = [
+      "source/cpu_features_linux.c",
+    ]
+
+    configs += [ "..:common_config" ]
+    public_configs = [ "..:common_inherited_config" ]
+  }
+}

webrtc/system_wrappers/cpu_features_webrtc.gyp

@@ -7,6 +7,7 @@
 # be found in the AUTHORS file in the root of the source tree.
 
 {
+  'includes': [ '../build/common.gypi', ],
   'conditions': [
     ['OS=="android"', {
       'targets': [
@@ -22,5 +23,19 @@
         },
       ],
     }],
+    ['OS=="linux"', {
+      'targets': [
+        {
+          'target_name': 'cpu_features_linux',
+          'type': 'static_library',
+          'sources': [
+            'source/cpu_features_linux.c',
+          ],
+          'dependencies': [
+            '<(webrtc_root)/common.gyp:webrtc_common',
+          ],
+        },
+      ],
+    }],
   ], # conditions
 }

webrtc/system_wrappers/source/cpu_features_linux.c

@@ -0,0 +1,83 @@
+/*
+ *  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) {
+      if (read(fd, &auxv, sizeof(auxv)) < (ssize_t)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;
+      }
+    }
+    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' &&
+        (platform[2] == 'l' || platform[2] == 'b')) {
+      architecture = platform[1] - '0';
+    }
+  }
+  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

webrtc/system_wrappers/system_wrappers.gyp

@@ -122,6 +122,13 @@
           'defines': [
             'WEBRTC_THREAD_RR',
           ],
+          'conditions': [
+            ['build_with_chromium==0', {
+              'dependencies': [
+                'cpu_features_webrtc.gyp:cpu_features_linux',
+              ],
+            }],
+          ],
           'link_settings': {
             'libraries': [ '-lrt', ],
           },