Changes imported from Abseil "staging" branch:

- fbff677ef850865ea67ed6771a8ed348be181e8e Modify sysinfo.cc to support GetTID on Akaros. by Abseil Team <absl-team@google.com>
  - f1c2929e08a3d4181e08cb5014c4a569306fd922 Two functions that did not refer to their arguments unles... by Abseil Team <absl-team@google.com>
  - ee43cc3bfdb4d84d40eee31fb25ecdc1aa060f47 Support Akaros (https://akaros.org) in the ABSL spinlock_... by Abseil Team <absl-team@google.com>
  - 6869c8c5253126459d6c7f0aa708d8612c8e5963 Make sure vdso_base_ is constant-intialized. by Abseil Team <absl-team@google.com>
  - d54e0366efc8d44cd5da5fd157734da966dc45e8 Add missing include for assert used by ABSL_ASSERT. by Derek Mauro <dmauro@google.com>
  - a5139775f3917bb5201e7fc838135766daa05b8d When building against GLIBC-2.16 or newer, use getauxval(... by Abseil Team <absl-team@google.com>

GitOrigin-RevId: fbff677ef850865ea67ed6771a8ed348be181e8e
Change-Id: Ie3549f6ef054783dd104304d2faf8d9800c16b83

absl/base/internal/spinlock_akaros.inc

@@ -0,0 +1,35 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This file is an Akaros-specific part of spinlock_wait.cc
+
+#include <atomic>
+
+#include "absl/base/internal/scheduling_mode.h"
+
+extern "C" {
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
+    std::atomic<uint32_t>* /* lock_word */, uint32_t /* value */,
+    int /* loop */, absl::base_internal::SchedulingMode /* mode */) {
+  // In Akaros, one must take care not to call anything that could cause a
+  // malloc(), a blocking system call, or a uthread_yield() while holding a
+  // spinlock. Our callers assume will not call into libraries or other
+  // arbitrary code.
+}
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(
+    std::atomic<uint32_t>* /* lock_word */, bool /* all */) {}
+
+}  // extern "C"

absl/base/internal/spinlock_wait.cc

@@ -23,6 +23,8 @@
 
 #if defined(_WIN32)
 #include "absl/base/internal/spinlock_win32.inc"
+#elif defined(__akaros__)
+#include "absl/base/internal/spinlock_akaros.inc"
 #else
 #include "absl/base/internal/spinlock_posix.inc"
 #endif

absl/base/internal/sysinfo.cc

@@ -284,6 +284,30 @@ pid_t GetTID() {
   return syscall(SYS_gettid);
 }
 
+#elif defined(__akaros__)
+
+pid_t GetTID() {
+  // Akaros has a concept of "vcore context", which is the state the program
+  // is forced into when we need to make a user-level scheduling decision, or
+  // run a signal handler.  This is analogous to the interrupt context that a
+  // CPU might enter if it encounters some kind of exception.
+  //
+  // There is no current thread context in vcore context, but we need to give
+  // a reasonable answer if asked for a thread ID (e.g., in a signal handler).
+  // Thread 0 always exists, so if we are in vcore context, we return that.
+  //
+  // Otherwise, we know (since we are using pthreads) that the uthread struct
+  // current_uthread is pointing to is the first element of a
+  // struct pthread_tcb, so we extract and return the thread ID from that.
+  //
+  // TODO(dcross): Akaros anticipates moving the thread ID to the uthread
+  // structure at some point. We should modify this code to remove the cast
+  // when that happens.
+  if (in_vcore_context())
+    return 0;
+  return reinterpret_cast<struct pthread_tcb *>(current_uthread)->id;
+}
+
 #else
 
 // Fallback implementation of GetTID using pthread_getspecific.