[Coverage] Fix branch coverage merging in FunctionCoverageSummary::get() for instantiation

Fix branch coverage merging in FunctionCoverageSummary::get() for instantiation
groups.

This change corrects the implementation for the branch coverage summary to do
the same thing for branches that is done for lines and regions.  That is,
across function instantiations in an instantiation group, the maximum branch
coverage found in any of those instantiations is returned, with the total
number of branches being the same across instantiations.

Differential Revision: https://reviews.llvm.org/D102193

(cherry picked from commit eccb925)

Author: evodius96

llvm/test/tools/llvm-cov/branch-templates.cpp

@@ -1,9 +1,9 @@
 // RUN: llvm-profdata merge %S/Inputs/branch-templates.proftext -o %t.profdata
 // RUN: llvm-cov show --show-expansions --show-branches=count %S/Inputs/branch-templates.o32l -instr-profile %t.profdata -path-equivalence=/tmp,%S %s | FileCheck %s
 // RUN: llvm-cov report --show-branch-summary %S/Inputs/branch-templates.o32l -instr-profile %t.profdata -show-functions -path-equivalence=/tmp,%S %s | FileCheck %s -check-prefix=REPORT
+// RUN: llvm-cov report --show-branch-summary %S/Inputs/branch-templates.o32l -instr-profile %t.profdata -path-equivalence=/tmp,%S %s | FileCheck %s -check-prefix=REPORTFILE
 
 #include <stdio.h>
-
 template<typename T>
 void unused(T x) {
   return;
@@ -45,3 +45,17 @@ int main() {
 // REPORT-NEXT: _Z4funcIfEiT_                     5       2  60.00%         7       3  57.14%         2       1  50.00%
 // REPORT-NEXT: ---
 // REPORT-NEXT: TOTAL                            22       7  68.18%        31      11  64.52%        12       6  50.00%
+
+// Make sure the covered branch tally for the function instantiation group is
+// merged to reflect maximum branch coverage of a single instantiation, just
+// like what is done for lines and regions. Also, the total branch tally
+// summary for an instantiation group should agree with the total number of
+// branches in the definition (In this case, 2 and 6 for func<>() and main(),
+// respectively).  This is returned by: FunctionCoverageSummary::get(const
+// InstantiationGroup &Group, ...)
+
+// REPORTFILE: Filename                      Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
+// REPORTFILE-NEXT: ---
+// REPORTFILE-NEXT: branch-templates.cpp          12                 3    75.00%           2                 0   100.00%          17                 4    76.47%           8                 4    50.00%
+// REPORTFILE-NEXT: ---
+// REPORTFILE-NEXT: TOTAL                              12                 3    75.00%           2                 0   100.00%          17                 4    76.47%           8                 4    50.00%

llvm/tools/llvm-cov/CoverageSummaryInfo.cpp

@@ -100,11 +100,7 @@ FunctionCoverageSummary::get(const InstantiationGroup &Group,
   for (const auto &FCS : Summaries.drop_front()) {
     Summary.RegionCoverage.merge(FCS.RegionCoverage);
     Summary.LineCoverage.merge(FCS.LineCoverage);
-
-    // Sum branch coverage across instantiation groups for the summary rather
-    // than "merge" the maximum count. This is a clearer view into whether all
-    // created branches are covered.
-    Summary.BranchCoverage += FCS.BranchCoverage;
+    Summary.BranchCoverage.merge(FCS.BranchCoverage);
   }
   return Summary;
 }

llvm/tools/llvm-cov/CoverageSummaryInfo.h

@@ -123,6 +123,11 @@ class BranchCoverageInfo {
     return *this;
   }
 
+  void merge(const BranchCoverageInfo &RHS) {
+    Covered = std::max(Covered, RHS.Covered);
+    NumBranches = std::max(NumBranches, RHS.NumBranches);
+  }
+
   size_t getCovered() const { return Covered; }
 
   size_t getNumBranches() const { return NumBranches; }