[AutoFDO] Fix handling of empty profiles

[david2050]

Summary

If a profile has no samples for a function, then the function "entry count" is set to the value 0. Several places in the code test that if the Function::getEntryCount is defined at all. Here we change to treat a 0 entry count the same as undefined.

In particular, this fixes a problem in getLayoutSuccessorProbThreshold in MachineBlockPlacement.cpp where we use a different and inferior heuristic for laying out basic blocks.

[david2050]

fix formatting

[dnovillo]

Adding Dehao. ISTR something special about 0 counts in profiles, but I don't recall details. Dehao, this looks fine to me, but will it break anything for samplepgo?

[danielcdh]

LGTM

I will have another patch to set entry count to be at least 1 if there is sample for the function.

[david2050]

Thanks, would one of you please accept?

[danielcdh]

In D23082#507015, @dnovillo wrote:

Adding Dehao. ISTR something special about 0 counts in profiles, but I don't recall details. Dehao, this looks fine to me, but will it break anything for samplepgo?

llvm/trunk/lib/IR/Function.cpp

 }
 
 Optional<uint64_t> Function::getEntryCount() const {
   MDNode *MD = getMetadata(LLVMContext::MD_prof);
   if (MD && MD->getOperand(0))
     if (MDString *MDS = dyn_cast<MDString>(MD->getOperand(0)))
       if (MDS->getString().equals("function_entry_count")) {
         ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(1));
-        return CI->getValue().getZExtValue();
+        uint64_t Count = CI->getValue().getZExtValue();
+        if (Count == 0)
+          return None;
+        return Count;
       }
   return None;
 }