[clang] [Driver] Add ExclusiveGroup feature to multilib.yaml. (PR #69447)

[Fangrui Song via cfe-commits]

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@@ -96,13 +97,39 @@ bool MultilibSet::select(const Multilib::flags_list &Flags,
                          llvm::SmallVector<Multilib> &Selected) const {
   llvm::StringSet<> FlagSet(expandFlags(Flags));
   Selected.clear();
-  llvm::copy_if(Multilibs, std::back_inserter(Selected),
-                [&FlagSet](const Multilib &M) {
-                  for (const std::string &F : M.flags())
-                    if (!FlagSet.contains(F))
-                      return false;
-                  return true;
-                });
+
+  // Decide which multilibs we're going to select at all
+  std::vector<bool> IsSelected(Multilibs.size(), false);
+  std::map<std::string, size_t> ExclusiveGroupMembers;
+  for (size_t i = 0, e = Multilibs.size(); i < e; ++i) {
+    const Multilib &M = Multilibs[i];
+
+    // If this multilib doesn't match all our flags, don't select it
+    if (!llvm::all_of(M.flags(), [&FlagSet](const std::string &F) {
+          return FlagSet.contains(F);
+        }))
+      continue;
+
+    // If this multilib has the same ExclusiveGroup as one we've already
+    // selected, de-select the previous one
+    const std::string &group = M.exclusiveGroup();
+    if (!group.empty()) {
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MaskRay wrote:

Thanks for the update, but the new code still has the duplicate hash table lookup/insert problem, which can be fixed by using try_emplace.

https://github.com/llvm/llvm-project/pull/69447