Export of internal Abseil changes

--
3fad46c668edd864a62511f2a6875b8b79e38f34 by Evan Brown <ezb@google.com>:

Use switches instead of lookup tables for zap_desig_waker and ignore_waiting_writers so that we can avoid dTLB misses.

See, e.g., https://godbolt.org/z/a7Gb9vzzj.

Also, now that these are functions, follow function style in naming and comments.

PiperOrigin-RevId: 418654693

--
ba5107744023a4e9163a44d706fbe8e4a1bc0fd9 by Abseil Team <absl-team@google.com>:

Check for Clang before attempting to expand __clang_major__.

This avoids a warning about an undefined macro on Windows when compiling with
MinGW-GCC.

PiperOrigin-RevId: 418287329
GitOrigin-RevId: 3fad46c668edd864a62511f2a6875b8b79e38f34
Change-Id: I28104980c4d3b204537b248447a6bd1022c9ef5d

1 files changed, 45 insertions, 29 deletions

diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index 376ea794..8f25e3dd 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc
@@ -1744,23 +1744,33 @@ ABSL_XRAY_LOG_ARGS(1) void Mutex::ReaderUnlock() {
   ABSL_TSAN_MUTEX_POST_UNLOCK(this, __tsan_mutex_read_lock);
 }
 
-// The zap_desig_waker bitmask is used to clear the designated waker flag in
-// the mutex if this thread has blocked, and therefore may be the designated
-// waker.
-static const intptr_t zap_desig_waker[] = {
-    ~static_cast<intptr_t>(0),  // not blocked
-    ~static_cast<intptr_t>(
-        kMuDesig)  // blocked; turn off the designated waker bit
-};
-
-// The ignore_waiting_writers bitmask is used to ignore the existence
-// of waiting writers if a reader that has already blocked once
-// wakes up.
-static const intptr_t ignore_waiting_writers[] = {
-    ~static_cast<intptr_t>(0),  // not blocked
-    ~static_cast<intptr_t>(
-        kMuWrWait)  // blocked; pretend there are no waiting writers
-};
+// Clears the designated waker flag in the mutex if this thread has blocked, and
+// therefore may be the designated waker.
+static intptr_t ClearDesignatedWakerMask(int flag) {
+  assert(flag >= 0);
+  assert(flag <= 1);
+  switch (flag) {
+    case 0:  // not blocked
+      return ~static_cast<intptr_t>(0);
+    case 1:  // blocked; turn off the designated waker bit
+      return ~static_cast<intptr_t>(kMuDesig);
+  }
+  ABSL_INTERNAL_UNREACHABLE;
+}
+
+// Conditionally ignores the existence of waiting writers if a reader that has
+// already blocked once wakes up.
+static intptr_t IgnoreWaitingWritersMask(int flag) {
+  assert(flag >= 0);
+  assert(flag <= 1);
+  switch (flag) {
+    case 0:  // not blocked
+      return ~static_cast<intptr_t>(0);
+    case 1:  // blocked; pretend there are no waiting writers
+      return ~static_cast<intptr_t>(kMuWrWait);
+  }
+  ABSL_INTERNAL_UNREACHABLE;
+}
 
 // Internal version of LockWhen().  See LockSlowWithDeadline()
 ABSL_ATTRIBUTE_NOINLINE void Mutex::LockSlow(MuHow how, const Condition *cond,
@@ -1852,8 +1862,10 @@ bool Mutex::LockSlowWithDeadline(MuHow how, const Condition *cond,
   bool unlock = false;
   if ((v & how->fast_need_zero) == 0 &&  // try fast acquire
       mu_.compare_exchange_strong(
-          v, (how->fast_or | (v & zap_desig_waker[flags & kMuHasBlocked])) +
-                 how->fast_add,
+          v,
+          (how->fast_or |
+           (v & ClearDesignatedWakerMask(flags & kMuHasBlocked))) +
+              how->fast_add,
           std::memory_order_acquire, std::memory_order_relaxed)) {
     if (cond == nullptr ||
         EvalConditionAnnotated(cond, this, true, false, how == kShared)) {
@@ -1927,9 +1939,10 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
     CheckForMutexCorruption(v, "Lock");
     if ((v & waitp->how->slow_need_zero) == 0) {
       if (mu_.compare_exchange_strong(
-              v, (waitp->how->fast_or |
-                  (v & zap_desig_waker[flags & kMuHasBlocked])) +
-                     waitp->how->fast_add,
+              v,
+              (waitp->how->fast_or |
+               (v & ClearDesignatedWakerMask(flags & kMuHasBlocked))) +
+                  waitp->how->fast_add,
               std::memory_order_acquire, std::memory_order_relaxed)) {
         if (waitp->cond == nullptr ||
             EvalConditionAnnotated(waitp->cond, this, true, false,
@@ -1946,8 +1959,9 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
       if ((v & (kMuSpin|kMuWait)) == 0) {   // no waiters
         // This thread tries to become the one and only waiter.
         PerThreadSynch *new_h = Enqueue(nullptr, waitp, v, flags);
-        intptr_t nv = (v & zap_desig_waker[flags & kMuHasBlocked] & kMuLow) |
-                      kMuWait;
+        intptr_t nv =
+            (v & ClearDesignatedWakerMask(flags & kMuHasBlocked) & kMuLow) |
+            kMuWait;
         ABSL_RAW_CHECK(new_h != nullptr, "Enqueue to empty list failed");
         if (waitp->how == kExclusive && (v & kMuReader) != 0) {
           nv |= kMuWrWait;
@@ -1961,12 +1975,13 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
           waitp->thread->waitp = nullptr;
         }
       } else if ((v & waitp->how->slow_inc_need_zero &
-                  ignore_waiting_writers[flags & kMuHasBlocked]) == 0) {
+                  IgnoreWaitingWritersMask(flags & kMuHasBlocked)) == 0) {
         // This is a reader that needs to increment the reader count,
         // but the count is currently held in the last waiter.
         if (mu_.compare_exchange_strong(
-                v, (v & zap_desig_waker[flags & kMuHasBlocked]) | kMuSpin |
-                       kMuReader,
+                v,
+                (v & ClearDesignatedWakerMask(flags & kMuHasBlocked)) |
+                    kMuSpin | kMuReader,
                 std::memory_order_acquire, std::memory_order_relaxed)) {
           PerThreadSynch *h = GetPerThreadSynch(v);
           h->readers += kMuOne;       // inc reader count in waiter
@@ -1987,8 +2002,9 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
         }
       } else if ((v & kMuSpin) == 0 &&  // attempt to queue ourselves
                  mu_.compare_exchange_strong(
-                     v, (v & zap_desig_waker[flags & kMuHasBlocked]) | kMuSpin |
-                            kMuWait,
+                     v,
+                     (v & ClearDesignatedWakerMask(flags & kMuHasBlocked)) |
+                         kMuSpin | kMuWait,
                      std::memory_order_acquire, std::memory_order_relaxed)) {
         PerThreadSynch *h = GetPerThreadSynch(v);
         PerThreadSynch *new_h = Enqueue(h, waitp, v, flags);