Speed up and slightly simplify Mutex

Eliminate the separate seqentially consistent contention counter load
in Mutex::ExclusiveUnlock by putting the contenion counter in the
Mutex's state word.

Replace some CHECK_GE checks with CHECK_GT. We were checking quantities
intended to be non-negative against >= 0 just before decrementing them.

Remove a pointless volatile declaration.

Introduce constants for the first FUTEX_WAKE argument. Remove all uses
of -1 as that argument, everywhere in ART. It appears to work, but the
documentation says it's wrong.

This does not yet address the ReaderWriterMutex issue, which is handled
in a different way in a separate CL.

Benchmark runs with and without this CL weakly suggest a tiny, not
statistically significant, improvement in both time and space with this
CL.

Bug: 111835365
Test: Build and boot AOSP. TreeHugger.
Change-Id: Ie53c65f2ce774a8cb4d224e2c1b3a110eb880f0c

1 files changed, 1 insertions, 1 deletions

diff --git a/runtime/base/mutex-inl.h b/runtime/base/mutex-inl.h
index 98e23b4036..22b76d231b 100644
--- a/runtime/base/mutex-inl.h
+++ b/runtime/base/mutex-inl.h
@@ -209,7 +209,7 @@ inline void ReaderWriterMutex::SharedUnlock(Thread* self) {
       if (done && (cur_state - 1) == 0) {  // Weak CAS may fail spuriously.
         if (num_contenders_.load(std::memory_order_seq_cst) > 0) {
           // Wake any exclusive waiters as there are now no readers.
-          futex(state_.Address(), FUTEX_WAKE_PRIVATE, -1, nullptr, nullptr, 0);
+          futex(state_.Address(), FUTEX_WAKE_PRIVATE, kWakeAll, nullptr, nullptr, 0);
         }
       }
     } else {