tsan: fixes to ThreadClock::releaseStoreAcquire

[dfava]

Summary

Fixes:

1. Setting the number of entries in a thread's clock to max between the thread and the SyncClock the thread is acquiring from
2. Setting last_acquire_

Unit- and stress-test for releaseStoreAcquire added to tests/unit/tsan_clock_test.cpp

[dvyukov]

Please add a unit test for these bugs to tsan/tests/unit/tsan_clock_test.cpp and releaseStoreAcquire to the stress test in that file.

[dvyukov]

You can now land this yourself, right?

[dfava]

Committed in https://github.com/llvm/llvm-project/commit/3b222ef246ef9298e57d00b7c8c1ffd92d0fb44d

compiler-rt/lib/tsan/rtl/tsan_clock.cpp

void ThreadClock::releaseStoreAcquire(ClockCache *c, SyncClock *sc) {
 
   if (sc->size_ == 0) {
     // ReleaseStore will correctly set release_store_tid_,
     // which can be important for future operations.
     ReleaseStore(c, sc);
     return;
   }
 
-  // Check if we need to resize dst.
+  nclk_ = max(nclk_, (uptr) sc->size_);
+
+  // Check if we need to resize sc.
   if (sc->size_ < nclk_)
     sc->Resize(c, nclk_);
 
+  bool acquired = false;
+
   sc->Unshare(c);
   // Update sc->clk_.
   sc->FlushDirty();
   uptr i = 0;
   for (ClockElem &ce : *sc) {
     u64 tmp = clk_[i];
-    clk_[i] = max(ce.epoch, clk_[i]);
+    if (clk_[i] < ce.epoch) {
+      clk_[i] = ce.epoch;
+      acquired = true;
+    }
     ce.epoch = tmp;
     ce.reused = 0;
     i++;
   }
   sc->release_store_tid_ = kInvalidTid;
   sc->release_store_reused_ = 0;
+
+  if (acquired) {
+    CPP_STAT_INC(StatClockAcquiredSomething);
+    last_acquire_ = clk_[tid_];
+    ResetCached(c);
+  }
 }
 
 void ThreadClock::release(ClockCache *c, SyncClock *dst) {
   DCHECK_LE(nclk_, kMaxTid);
   DCHECK_LE(dst->size_, kMaxTid);
 
   if (dst->size_ == 0) {
     // ReleaseStore will correctly set release_store_tid_,