[SampleFDO] Add an option to turn on/off warning about samples unused

[wmi]

Summary

If a function has sample to use, but cannot use them because of no debug information, currently a warning will be issued to inform the missing opportunity.

This warning assumes the binary generating the profile and the binary using the profile are similar enough. In some cases even if it is not similar enough, we may still get some benefit by using sampleFDO. In those cases, we may not want to see a lot of such warnings pop up.

The patch adds an option for the warning.

[tejohnson]

Does this happen when the code the profile is being applied to is not being built with some necessary debug options like -gmlt? Can/should it be fixed by adding that or another option?

[wmi]

In D48510#1141389, @tejohnson wrote:

Does this happen when the code the profile is being applied to is not being built with some necessary debug options like -gmlt? Can/should it be fixed by adding that or another option?

Yes, it is because -g0 is used in the code where the profile is applied. I am not sure whether the -g0 is required or not in that specific case. But I can imagine there is case where some module need to be built using -g0, and if we got sample for such module from elsewhere, the option will be needed there.

[tejohnson]

LGTM

llvm/trunk/lib/Transforms/IPO/SampleProfile.cpp

static cl::opt<unsigned> SampleProfileRecordCoverage(
     cl::desc("Emit a warning if less than N% of records in the input profile "
              "are matched to the IR."));
 
 static cl::opt<unsigned> SampleProfileSampleCoverage(
     "sample-profile-check-sample-coverage", cl::init(0), cl::value_desc("N"),
     cl::desc("Emit a warning if less than N% of samples in the input profile "
              "are matched to the IR."));
 
+static cl::opt<bool> NoWarnSampleUnused(
+    "no-warn-sample-unused", cl::init(false), cl::Hidden,
+    cl::desc("Use this option to turn off/on warnings about function with "
+             "samples but without debug information to use those samples. "));
+
 namespace {
 
 using BlockWeightMap = DenseMap<const BasicBlock *, uint64_t>;
 using EquivalenceClassMap = DenseMap<const BasicBlock *, const BasicBlock *>;
 using Edge = std::pair<const BasicBlock *, const BasicBlock *>;
 using EdgeWeightMap = DenseMap<Edge, uint64_t>;
 using BlockEdgeMap =
     DenseMap<const BasicBlock *, SmallVector<const BasicBlock *, 8>>;

 /// \param F  Function object to query.
 ///
 /// \returns the line number where \p F is defined. If it returns 0,
 ///          it means that there is no debug information available for \p F.
 unsigned SampleProfileLoader::getFunctionLoc(Function &F) {
   if (DISubprogram *S = F.getSubprogram())
     return S->getLine();
 
+  if (NoWarnSampleUnused)
+    return 0;
+
   // If the start of \p F is missing, emit a diagnostic to inform the user
   // about the missed opportunity.
   F.getContext().diagnose(DiagnosticInfoSampleProfile(
       "No debug information found in function " + F.getName() +
           ": Function profile not used",
       DS_Warning));
   return 0;
 }