diff --git a/llvm/lib/Transforms/IPO/SampleProfile.cpp b/llvm/lib/Transforms/IPO/SampleProfile.cpp
--- a/llvm/lib/Transforms/IPO/SampleProfile.cpp
+++ b/llvm/lib/Transforms/IPO/SampleProfile.cpp
@@ -540,21 +540,36 @@
   return getInstWeightImpl(Inst);
 }
 
+// Here use error_code to represent: 1) The dangling probe. 2) Ignore the weight
+// of non-probe instruction. So if all instructions of the BB give error_code,
+// tell the inference algorithm to infer the BB weight.
 ErrorOr<uint64_t> SampleProfileLoader::getProbeWeight(const Instruction &Inst) {
   assert(FunctionSamples::ProfileIsProbeBased &&
          "Profile is not pseudo probe based");
   Optional<PseudoProbe> Probe = extractProbe(Inst);
+  // Ignore the non-probe instruction. If none of the instruction in the BB is
+  // probe, we choose to infer the BB's weight.
   if (!Probe)
     return std::error_code();
 
-  // Ignore danling probes since they are logically deleted and should not
-  // consume any profile samples.
+  // This is not the dangling probe from the training pass but generated by the
+  // current compilation. Ignore this since they are logically deleted and
+  // should not consume any profile samples.
   if (Probe->isDangling())
     return std::error_code();
 
   const FunctionSamples *FS = findFunctionSamples(Inst);
+  // If none of the instruction has FunctionSample, we choose to return zero
+  // value sample to indicate the BB is cold. This could happen when the
+  // instruction is from inlinee and no profile data is found.
+  // FIXME: This should not be affected by the source drift issue as 1) if the
+  // newly added function is top-level inliner, it won't match the CFG checksum
+  // in the function profile or 2) if it's the inlinee, the inlinee should have
+  // a profile, otherwise it wouldn't be inlined. For non-probe based profile,
+  // we can improve it by adding a switch for profile-sample-block-accurate for
+  // block level counts in the future.
   if (!FS)
-    return std::error_code();
+    return 0;
 
   // For non-CS profile, If a direct call/invoke instruction is inlined in
   // profile (findCalleeFunctionSamples returns non-empty result), but not