[platform] Switch to std::atomic

We want to get rid of the atomicops library, hence switch all uses to
std::atomic.

R=mlippautz@chromium.org

Bug: v8:8926, v8:8834

Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Change-Id: I9b7cca83703775b1ddee4f16f51b7ad6535bb67c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1520717Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60233}

src/base/platform/platform-posix.cc

@@ -43,7 +43,7 @@
 #include "src/base/utils/random-number-generator.h"
 
 #ifdef V8_FAST_TLS_SUPPORTED
-#include "src/base/atomicops.h"
+#include <atomic>
 #endif
 
 #if V8_OS_MACOSX
@@ -815,7 +815,7 @@ static pthread_key_t LocalKeyToPthreadKey(Thread::LocalStorageKey local_key) {
 
 #ifdef V8_FAST_TLS_SUPPORTED
 
-static Atomic32 tls_base_offset_initialized = 0;
+static std::atomic<bool> tls_base_offset_initialized{false};
 intptr_t kMacTlsBaseOffset = 0;
 
 // It's safe to do the initialization more that once, but it has to be
@@ -851,7 +851,7 @@ static void InitializeTlsBaseOffset() {
     kMacTlsBaseOffset = 0;
   }
 
-  Release_Store(&tls_base_offset_initialized, 1);
+  tls_base_offset_initialized.store(true, std::memory_order_release);
 }
 
 
@@ -871,7 +871,7 @@ static void CheckFastTls(Thread::LocalStorageKey key) {
 Thread::LocalStorageKey Thread::CreateThreadLocalKey() {
 #ifdef V8_FAST_TLS_SUPPORTED
   bool check_fast_tls = false;
-  if (tls_base_offset_initialized == 0) {
+  if (!tls_base_offset_initialized.load(std::memory_order_acquire)) {
     check_fast_tls = true;
     InitializeTlsBaseOffset();
   }

src/base/platform/time.cc

@@ -19,7 +19,6 @@
 #include <ostream>
 
 #if V8_OS_WIN
-#include "src/base/atomicops.h"
 #include "src/base/lazy-instance.h"
 #include "src/base/win32-headers.h"
 #endif
@@ -493,7 +492,7 @@ DWORD (*g_tick_function)(void) = &timeGetTimeWrapper;
 // "rollover" counter.
 union LastTimeAndRolloversState {
   // The state as a single 32-bit opaque value.
-  base::Atomic32 as_opaque_32;
+  int32_t as_opaque_32;
 
   // The state as usable values.
   struct {
@@ -509,7 +508,7 @@ union LastTimeAndRolloversState {
     uint16_t rollovers;
   } as_values;
 };
-base::Atomic32 g_last_time_and_rollovers = 0;
+std::atomic<int32_t> g_last_time_and_rollovers{0};
 static_assert(sizeof(LastTimeAndRolloversState) <=
                   sizeof(g_last_time_and_rollovers),
               "LastTimeAndRolloversState does not fit in a single atomic word");
@@ -523,12 +522,12 @@ TimeTicks RolloverProtectedNow() {
   LastTimeAndRolloversState state;
   DWORD now;  // DWORD is always unsigned 32 bits.
 
+  // Fetch the "now" and "last" tick values, updating "last" with "now" and
+  // incrementing the "rollovers" counter if the tick-value has wrapped back
+  // around. Atomic operations ensure that both "last" and "rollovers" are
+  // always updated together.
+  int32_t original = g_last_time_and_rollovers.load(std::memory_order_acquire);
   while (true) {
-    // Fetch the "now" and "last" tick values, updating "last" with "now" and
-    // incrementing the "rollovers" counter if the tick-value has wrapped back
-    // around. Atomic operations ensure that both "last" and "rollovers" are
-    // always updated together.
-    int32_t original = base::Acquire_Load(&g_last_time_and_rollovers);
     state.as_opaque_32 = original;
     now = g_tick_function();
     uint8_t now_8 = static_cast<uint8_t>(now >> 24);
@@ -540,11 +539,13 @@ TimeTicks RolloverProtectedNow() {
 
     // Save the changed state. If the existing value is unchanged from the
     // original, exit the loop.
-    int32_t check = base::Release_CompareAndSwap(&g_last_time_and_rollovers,
-                                                 original, state.as_opaque_32);
-    if (check == original) break;
+    if (g_last_time_and_rollovers.compare_exchange_weak(
+            original, state.as_opaque_32, std::memory_order_acq_rel)) {
+      break;
+    }
 
     // Another thread has done something in between so retry from the top.
+    // {original} has been updated by the {compare_exchange_weak}.
   }
 
   return TimeTicks() +