[Coverage] Ignore 'unused' functions with non-zero execution counts

Frontends emit 'unused' coverage mapping records for functions which are
provably unused in a TU. These unused records contain a single counter
with CounterKind::Zero. However, a function may be unused in one TU and
used in another. When this happens, prefer the records with a full set
of counters instead of arbitrarily picking the first loaded record.

There is no impact on the single-TU case. In the multiple-TU case, this
resolves issues causing a function to appear unused when it's not.

Testing: check-{llvm,clang,compiler-rt}

rdar://42981322

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339194 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 71a5ad4)

Author: vedantk

lib/ProfileData/Coverage/CoverageMapping.cpp

@@ -207,12 +207,6 @@ Error CoverageMapping::loadFunctionRecord(
   else
     OrigFuncName = getFuncNameWithoutPrefix(OrigFuncName, Record.Filenames[0]);
 
-  // Don't load records for (filenames, function) pairs we've already seen.
-  auto FilenamesHash = hash_combine_range(Record.Filenames.begin(),
-                                          Record.Filenames.end());
-  if (!RecordProvenance[FilenamesHash].insert(hash_value(OrigFuncName)).second)
-    return Error::success();
-
   CounterMappingContext Ctx(Record.Expressions);
 
   std::vector<uint64_t> Counts;
@@ -230,6 +224,15 @@ Error CoverageMapping::loadFunctionRecord(
 
   assert(!Record.MappingRegions.empty() && "Function has no regions");
 
+  // This coverage record is a zero region for a function that's unused in
+  // some TU, but used in a different TU. Ignore it. The coverage maps from the
+  // the other TU will either be loaded (providing full region counts) or they
+  // won't (in which case we don't unintuitively report functions as uncovered
+  // when they have non-zero counts in the profile).
+  if (Record.MappingRegions.size() == 1 &&
+      Record.MappingRegions[0].Count.isZero() && Counts[0] > 0)
+    return Error::success();
+
   FunctionRecord Function(OrigFuncName, Record.Filenames);
   for (const auto &Region : Record.MappingRegions) {
     Expected<int64_t> ExecutionCount = Ctx.evaluate(Region.Count);
@@ -240,6 +243,12 @@ Error CoverageMapping::loadFunctionRecord(
     Function.pushRegion(Region, *ExecutionCount);
   }
 
+  // Don't create records for (filenames, function) pairs we've already seen.
+  auto FilenamesHash = hash_combine_range(Record.Filenames.begin(),
+                                          Record.Filenames.end());
+  if (!RecordProvenance[FilenamesHash].insert(hash_value(OrigFuncName)).second)
+    return Error::success();
+
   Functions.push_back(std::move(Function));
   return Error::success();
 }