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/BUILD.bazel

@@ -28,6 +28,7 @@ licenses(["notice"])  # Apache 2.0
 cc_library(
     name = "spinlock_wait",
     srcs = [
+        "internal/spinlock_akaros.inc",
         "internal/spinlock_posix.inc",
         "internal/spinlock_wait.cc",
         "internal/spinlock_win32.inc",

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.

absl/base/macros.h

@@ -29,6 +29,7 @@
 #ifndef ABSL_BASE_MACROS_H_
 #define ABSL_BASE_MACROS_H_
 
+#include <cassert>
 #include <cstddef>
 
 #include "absl/base/port.h"

absl/debugging/internal/elf_mem_image.cc

@@ -75,8 +75,9 @@ const T *GetTableElement(const ElfW(Ehdr) * ehdr, ElfW(Off) table_offset,
 
 }  // namespace
 
-const void *const ElfMemImage::kInvalidBase =
-    reinterpret_cast<const void *>(~0L);
+// The value of this variable doesn't matter; it's used only for its
+// unique address.
+const int ElfMemImage::kInvalidBaseSentinel = 0;
 
 ElfMemImage::ElfMemImage(const void *base) {
   ABSL_RAW_CHECK(base != kInvalidBase, "bad pointer");

absl/debugging/internal/elf_mem_image.h

@@ -43,9 +43,14 @@ namespace debug_internal {
 
 // An in-memory ELF image (may not exist on disk).
 class ElfMemImage {
+ private:
+  // Sentinel: there could never be an elf image at &kInvalidBaseSentinel.
+  static const int kInvalidBaseSentinel;
+
  public:
   // Sentinel: there could never be an elf image at this address.
-  static const void *const kInvalidBase;
+  static constexpr const void *const kInvalidBase =
+    static_cast<const void*>(&kInvalidBaseSentinel);
 
   // Information about a single vdso symbol.
   // All pointers are into .dynsym, .dynstr, or .text of the VDSO.

absl/debugging/internal/stacktrace_x86-inl.inc

@@ -114,7 +114,9 @@ static const int kMaxFrameBytes = 100000;
 // vuc is a ucontext_t *.  We use void* to avoid the use
 // of ucontext_t on non-POSIX systems.
 static uintptr_t GetFP(const void *vuc) {
-#if defined(__linux__)
+#if !defined(__linux__)
+  static_cast<void>(vuc);  // Avoid an unused argument compiler warning.
+#else
   if (vuc != nullptr) {
     auto *uc = reinterpret_cast<const ucontext_t *>(vuc);
 #if defined(__i386__)

absl/debugging/internal/vdso_support.cc

@@ -20,10 +20,15 @@
 
 #ifdef ABSL_HAVE_VDSO_SUPPORT     // defined in vdso_support.h
 
+#include <errno.h>
 #include <fcntl.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
+#if __GLIBC_PREREQ(2, 16)  // GLIBC-2.16 implements getauxval.
+#include <sys/auxv.h>
+#endif
+
 #include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/port.h"
@@ -35,8 +40,10 @@
 namespace absl {
 namespace debug_internal {
 
+ABSL_CONST_INIT
 std::atomic<const void *> VDSOSupport::vdso_base_(
     debug_internal::ElfMemImage::kInvalidBase);
+
 std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(&InitAndGetCPU);
 VDSOSupport::VDSOSupport()
     // If vdso_base_ is still set to kInvalidBase, we got here
@@ -56,9 +63,18 @@ VDSOSupport::VDSOSupport()
 // Finally, even if there is a race here, it is harmless, because
 // the operation should be idempotent.
 const void *VDSOSupport::Init() {
-  if (vdso_base_.load(std::memory_order_relaxed) ==
-      debug_internal::ElfMemImage::kInvalidBase) {
-    {
+  const auto kInvalidBase = debug_internal::ElfMemImage::kInvalidBase;
+#if __GLIBC_PREREQ(2, 16)
+  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
+    errno = 0;
+    const void *const sysinfo_ehdr =
+        reinterpret_cast<const void *>(getauxval(AT_SYSINFO_EHDR));
+    if (errno == 0) {
+      vdso_base_.store(sysinfo_ehdr, std::memory_order_relaxed);
+    }
+  }
+#endif  // __GLIBC_PREREQ(2, 16)
+  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
     // Valgrind zaps AT_SYSINFO_EHDR and friends from the auxv[]
     // on stack, and so glibc works as if VDSO was not present.
     // But going directly to kernel via /proc/self/auxv below bypasses
@@ -84,9 +100,7 @@ const void *VDSOSupport::Init() {
       }
     }
     close(fd);
-    }
-    if (vdso_base_.load(std::memory_order_relaxed) ==
-        debug_internal::ElfMemImage::kInvalidBase) {
+    if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
       // Didn't find AT_SYSINFO_EHDR in auxv[].
       vdso_base_.store(nullptr, std::memory_order_relaxed);
     }
@@ -135,6 +149,7 @@ long VDSOSupport::GetCPUViaSyscall(unsigned *cpu,  // NOLINT(runtime/int)
   return syscall(SYS_getcpu, cpu, nullptr, nullptr);
 #else
   // x86_64 never implemented sys_getcpu(), except as a VDSO call.
+  static_cast<void>(cpu);  // Avoid an unused argument compiler warning.
   errno = ENOSYS;
   return -1;
 #endif