[Clang] Simplify unifying target features

[jhuber6]

Summary

This patch simplifies how we unify target features. Now we simply
iterate the input in reverse and only insert the feature if it hasn't
been seen yet. The only reason we need to reverse this at the end is to
keep the features in order for the existing tests.

[tra]

clang/lib/Driver/ToolChains/CommonArgs.cpp

153

Nit: You could use UnifiedFeatures.insert(UnifiedFeatures.begin(), Feature) and that would make the following std::reverse unnecessary. Up to you.
The code is not performance critical, so we don't care that we have to copy the elements.

clang/lib/Driver/ToolChains/CommonArgs.h

 /// Note: Since \p Features may contain default values before calling
 /// this function, or may be appended with entries to override arguments,
 /// entries in \p Features are not unique.
 void handleTargetFeaturesGroup(const llvm::opt::ArgList &Args,
                                std::vector<StringRef> &Features,
                                llvm::opt::OptSpecifier Group);
 
 /// If there are multiple +xxx or -xxx features, keep the last one.
-std::vector<StringRef>
-unifyTargetFeatures(const std::vector<StringRef> &Features);
+SmallVector<StringRef> unifyTargetFeatures(ArrayRef<StringRef> Features);
 
 /// Handles the -save-stats option and returns the filename to save statistics
 /// to.
 SmallString<128> getStatsFileName(const llvm::opt::ArgList &Args,
                                   const InputInfo &Output,
                                   const InputInfo &Input, const Driver &D);
 
 /// \p Flag must be a flag accepted by the driver with its leading '-' removed,

clang/lib/Driver/ToolChains/CommonArgs.cpp

for (const Arg *A : Args.filtered(Group)) {
 
     bool IsNegative = Name.startswith("no-");
     if (IsNegative)
       Name = Name.substr(3);
     Features.push_back(Args.MakeArgString((IsNegative ? "-" : "+") + Name));
   }
 }
 
-std::vector<StringRef>
-tools::unifyTargetFeatures(const std::vector<StringRef> &Features) {
-  std::vector<StringRef> UnifiedFeatures;
-  // Find the last of each feature.
-  llvm::StringMap<unsigned> LastOpt;
-  for (unsigned I = 0, N = Features.size(); I < N; ++I) {
-    StringRef Name = Features[I];
-    assert(Name[0] == '-' || Name[0] == '+');
-    LastOpt[Name.drop_front(1)] = I;
-  }
-
-  for (unsigned I = 0, N = Features.size(); I < N; ++I) {
-    // If this feature was overridden, ignore it.
-    StringRef Name = Features[I];
-    llvm::StringMap<unsigned>::iterator LastI = LastOpt.find(Name.drop_front(1));
-    assert(LastI != LastOpt.end());
-    unsigned Last = LastI->second;
-    if (Last != I)
-      continue;
-
-    UnifiedFeatures.push_back(Name);
+SmallVector<StringRef>
+tools::unifyTargetFeatures(ArrayRef<StringRef> Features) {
+  // Only add a feature if it hasn't been seen before starting from the end.
+  SmallVector<StringRef> UnifiedFeatures;
+  llvm::DenseSet<StringRef> UsedFeatures;
+  for (StringRef Feature : llvm::reverse(Features)) {
+    if (UsedFeatures.insert(Feature.drop_front()).second)
+      UnifiedFeatures.push_back(Feature);
   }
+
+  std::reverse(UnifiedFeatures.begin(), UnifiedFeatures.end());
   return UnifiedFeatures;
 }
 
 void tools::addDirectoryList(const ArgList &Args, ArgStringList &CmdArgs,
                              const char *ArgName, const char *EnvVar) {
   const char *DirList = ::getenv(EnvVar);
   bool CombinedArg = false;