[ThinLTO] Populate declaration import status except for distributed ThinLTO under a default-off new option (#88024)

The goal is to populate `declaration` import status if a new flag`-import-declaration` is on.

* For in-process ThinLTO, the `declaration` status is visible to backend
`function-import` pass, so `FunctionImporter::importFunctions` should
read the import status and be no-op for declaration summaries.
Basically, the postlink pipeline is updated to keep its current behavior
(import definitions), but not updated to handle `declaration` summaries.
Two use cases (better call-graph sort and cross-module auto-init)
would use this bit differently.

* For distributed ThinLTO, the `declaration` status is not serialized to
bitcode. As discussed, https://github.com/llvm/llvm-project/pull/87600
will do this.

[1] https://discourse.llvm.org/t/rfc-for-better-call-graph-sort-build-a-more-complete-call-graph-by-adding-more-indirect-call-edges/74029#support-cross-module-function-declaration-import-5
[2] https://github.com/llvm/llvm-project/pull/87597#discussion_r1556067195

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
index d4b89ed..58434fe 100644
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/lib/LTO/LTOBackend.cpp
@@ -721,7 +721,14 @@ bool lto::initImportList(const Module &M,
       if (Summary->modulePath() == M.getModuleIdentifier())
         continue;
       // Add an entry to provoke importing by thinBackend.
-      ImportList[Summary->modulePath()].insert(GUID);
+      // Try emplace the entry first. If an entry with the same key already
+      // exists, set the value to 'std::min(existing-value, new-value)' to make
+      // sure a definition takes precedence over a declaration.
+      auto [Iter, Inserted] = ImportList[Summary->modulePath()].try_emplace(
+          GUID, Summary->importType());
+
+      if (!Inserted)
+        Iter->second = std::min(Iter->second, Summary->importType());
     }
   }
   return true;