tsan: use smaller trace parts for Go

[dvyukov]

Summary

In the old runtime we used to use different number of trace parts
for C++ and Go to reduce trace memory consumption for Go.
But now it's easier and better to use smaller parts because
we already use minimal possible number of parts for C++ (3).

[melver]

compiler-rt/lib/tsan/rtl/tsan_trace.h

195

Could do this in less code:

static constexpr uptr kByteSize = (SANITIZER_GO ? 128 : 256) << 10;

[dvyukov]

addressed the comment

compiler-rt/lib/tsan/rtl/tsan_trace.h

 struct Trace;
 
 struct TraceHeader {
   Trace* trace = nullptr;  // back-pointer to Trace containing this part
   INode trace_parts;       // in Trace::parts
 };
 
 struct TracePart : TraceHeader {
-  static constexpr uptr kByteSize = 256 << 10;
+  // There are a lot of goroutines in Go, so we use smaller parts.
+  static constexpr uptr kByteSize = (SANITIZER_GO ? 128 : 256) << 10;
   static constexpr uptr kSize =
       (kByteSize - sizeof(TraceHeader)) / sizeof(Event);
   // TraceAcquire does a fast event pointer overflow check by comparing
   // pointer into TracePart::events with kAlignment mask. Since TracePart's
   // are allocated page-aligned, this check detects end of the array
   // (it also have false positives in the middle that are filtered separately).
   // This also requires events to be the last field.
   static constexpr uptr kAlignment = 0xff0;