3 files changed, 46 insertions, 5 deletions
diff --git a/clang/include/clang/Driver/Multilib.h b/clang/include/clang/Driver/Multilib.h
index 353aefde354..abf0d5fa6ea 100644
--- a/clang/include/clang/Driver/Multilib.h
+++ b/clang/include/clang/Driver/Multilib.h
@@ -34,10 +34,11 @@ private:
   std::string OSSuffix;
   std::string IncludeSuffix;
   flags_list Flags;
+  int Priority;
 
 public:
   Multilib(StringRef GCCSuffix = {}, StringRef OSSuffix = {},
-           StringRef IncludeSuffix = {});
+           StringRef IncludeSuffix = {}, int Priority = 0);
 
   /// Get the detected GCC installation path suffix for the multi-arch
   /// target variant. Always starts with a '/', unless empty
@@ -77,6 +78,10 @@ public:
   const flags_list &flags() const { return Flags; }
   flags_list &flags() { return Flags; }
 
+  /// Returns the multilib priority. When more than one multilib matches flags,
+  /// the one with the highest priority is selected, with 0 being the default.
+  int priority() const { return Priority; }
+
   /// Add a flag to the flags list
   /// \p Flag must be a flag accepted by the driver with its leading '-' removed,
   ///     and replaced with either:
diff --git a/clang/lib/Driver/Multilib.cpp b/clang/lib/Driver/Multilib.cpp
index 29c633e39b1..303047e05f7 100644
--- a/clang/lib/Driver/Multilib.cpp
+++ b/clang/lib/Driver/Multilib.cpp
@@ -51,8 +51,9 @@ static void normalizePathSegment(std::string &Segment) {
 }
 
 Multilib::Multilib(StringRef GCCSuffix, StringRef OSSuffix,
-                   StringRef IncludeSuffix)
-    : GCCSuffix(GCCSuffix), OSSuffix(OSSuffix), IncludeSuffix(IncludeSuffix) {
+                   StringRef IncludeSuffix, int Priority)
+    : GCCSuffix(GCCSuffix), OSSuffix(OSSuffix), IncludeSuffix(IncludeSuffix),
+      Priority(Priority) {
   normalizePathSegment(this->GCCSuffix);
   normalizePathSegment(this->OSSuffix);
   normalizePathSegment(this->IncludeSuffix);
@@ -265,8 +266,19 @@ bool MultilibSet::select(const Multilib::flags_list &Flags, Multilib &M) const {
     return true;
   }
 
-  // TODO: pick the "best" multlib when more than one is suitable
-  assert(false);
+  // Sort multilibs by priority and select the one with the highest priority.
+  llvm::sort(Filtered.begin(), Filtered.end(),
+             [](const Multilib &a, const Multilib &b) -> bool {
+               return a.priority() > b.priority();
+             });
+
+  if (Filtered[0].priority() > Filtered[1].priority()) {
+    M = Filtered[0];
+    return true;
+  }
+
+  // TODO: We should consider returning llvm::Error rather than aborting.
+  assert(false && "More than one multilib with the same priority");
   return false;
 }
 
diff --git a/clang/unittests/Driver/MultilibTest.cpp b/clang/unittests/Driver/MultilibTest.cpp
index 9dca139ab43..0731c81d9f5 100644
--- a/clang/unittests/Driver/MultilibTest.cpp
+++ b/clang/unittests/Driver/MultilibTest.cpp
@@ -349,3 +349,27 @@ TEST(MultilibTest, SetCombineWith) {
   Latte.combineWith(Milk);
   ASSERT_EQ(Latte.size(), (unsigned)2);
 }
+
+TEST(MultilibTest, SetPriority) {
+  MultilibSet MS;
+  MS.push_back(Multilib("foo", {}, {}, 1).flag("+foo"));
+  MS.push_back(Multilib("bar", {}, {}, 2).flag("+bar"));
+
+  Multilib::flags_list Flags1;
+  Flags1.push_back("+foo");
+  Flags1.push_back("-bar");
+  Multilib Selection1;
+  ASSERT_TRUE(MS.select(Flags1, Selection1))
+      << "Flag set was {\"+foo\"}, but selection not found";
+  ASSERT_TRUE(Selection1.gccSuffix() == "/foo")
+      << "Selection picked " << Selection1 << " which was not expected";
+
+  Multilib::flags_list Flags2;
+  Flags2.push_back("+foo");
+  Flags2.push_back("+bar");
+  Multilib Selection2;
+  ASSERT_TRUE(MS.select(Flags2, Selection2))
+      << "Flag set was {\"+bar\"}, but selection not found";
+  ASSERT_TRUE(Selection2.gccSuffix() == "/bar")
+      << "Selection picked " << Selection2 << " which was not expected";
+}