PGO][InstrProf] Do not promote count if the exit blocks contains ret instruction

[xur]

Summary

Skip profile count promotion if any of the ExitBlocks contains a ret
instruction. This is to prevent dumping of incomplete profile -- if the
the loop is a long running loop and dump is called in the middle
of the loop, the result profile is incomplete.

ExitBlocks containing a ret instruction is an indication of a long running
loop -- early exit to error handling code.

[davidxl]

lgtm

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

292 ↗

(On Diff #280002)

Add a //FIXME here to add other heuristics to detect long running loop (runtime infinite), e.g. exit condition etc.

llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp

bool PGOUseFunc::setInstrumentedCounts(
   FuncInfo.getInstrumentBBs(InstrumentBBs);
   unsigned NumCounters =
       InstrumentBBs.size() + FuncInfo.SIVisitor.getNumOfSelectInsts();
   // The number of counters here should match the number of counters
   // in profile. Return if they mismatch.
   if (NumCounters != CountFromProfile.size()) {
     return false;
   }
+  auto *FuncEntry = &*F.begin();
+
   // Set the profile count to the Instrumented BBs.
   uint32_t I = 0;
   for (BasicBlock *InstrBB : InstrumentBBs) {
     uint64_t CountValue = CountFromProfile[I++];
     UseBBInfo &Info = getBBInfo(InstrBB);
+    // If we reach here, we know that we have some nonzero count
+    // values in this function. The entry count should not be 0.
+    // Fix it if necessary.
+    if (InstrBB == FuncEntry && CountValue == 0)
+      CountValue = 1;
     Info.setBBInfoCount(CountValue);
   }
   ProfileCountSize = CountFromProfile.size();
   CountPosition = I;
 
   // Set the edge count and update the count of unknown edges for BBs.
   auto setEdgeCount = [this](PGOUseEdge *E, uint64_t Value) -> void {
     E->setEdgeCount(Value);
#ifndef NDEBUG
   for (auto &BB : F) {
     auto BI = findBBInfo(&BB);
     if (BI == nullptr)
       continue;
     assert(BI->CountValid && "BB count is not valid");
   }
 #endif
   uint64_t FuncEntryCount = getBBInfo(&*F.begin()).CountValue;
-  F.setEntryCount(ProfileCount(FuncEntryCount, Function::PCT_Real));
   uint64_t FuncMaxCount = FuncEntryCount;
   for (auto &BB : F) {
     auto BI = findBBInfo(&BB);
     if (BI == nullptr)
       continue;
     FuncMaxCount = std::max(FuncMaxCount, BI->CountValue);
   }
+
+  // Fix the obviously inconsistent entry count.
+  if (FuncMaxCount > 0 && FuncEntryCount == 0)
+    FuncEntryCount = 1;
+  F.setEntryCount(ProfileCount(FuncEntryCount, Function::PCT_Real));
   markFunctionAttributes(FuncEntryCount, FuncMaxCount);
 
   // Now annotate select instructions
   FuncInfo.SIVisitor.annotateSelects(F, this, &CountPosition);
   assert(CountPosition == ProfileCountSize);
 
   LLVM_DEBUG(FuncInfo.dumpInfo("after reading profile."));
 }