[TSan] Use max reduction to aggreagate StatThreadMaxAlive

[protze.joachim]

Summary

As long as only the initial thread creates all other threads, there is no difference. If different threads spawn threads, they all get a global max count. Sum reduction doesn't make sense for this counter. With fibers around, the value can become #threads * #fibers instead of #threads + #fibers

[vitalybuka]

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

22

what about StatThreadMaxTid?

24–25

[protze.joachim]

Abandon, since stats were removed with D106276 anyways.

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

 #include "tsan_rtl.h"
 
 namespace __tsan {
 
 #if TSAN_COLLECT_STATS
 
 void StatAggregate(u64 *dst, u64 *src) {
   for (int i = 0; i < StatCnt; i++)
+    if (i != StatThreadMaxAlive)
       dst[i] += src[i];
+    // max(StatThreadMaxAlive)
+    else if (dst[i] < src[i])
+      dst[i] = src[i];
 }
 
 void StatOutput(u64 *stat) {
   stat[StatShadowNonZero] = stat[StatShadowProcessed] - stat[StatShadowZero];
 
   static const char *name[StatCnt] = {};
   name[StatMop]                          = "Memory accesses                   ";
   name[StatMopRead]                      = "  Including reads                 ";