Reland "[ThinLTO][Bitcode] Generate import type in bitcode" (#97253)

https://github.com/llvm/llvm-project/pull/87600 was reverted in order to
revert
https://github.com/llvm/llvm-project/commit/6262763341fcd71a2b0708cf7485f9abd1d26ba8.
Now https://github.com/llvm/llvm-project/pull/95482 is fix forward for
https://github.com/llvm/llvm-project/commit/6262763341fcd71a2b0708cf7485f9abd1d26ba8.
This patch is a reland for
https://github.com/llvm/llvm-project/pull/87600

**Changes on top of original patch**
In `llvm/include/llvm/IR/ModuleSummaryIndex.h`, make the type of
`GVSummaryPtrSet` an `unordered_set` which is more memory efficient when
the number of elements is smaller than 128 [1]

**Original commit message**

For distributed ThinLTO, the LTO indexing step generates combined
summary for each module, and postlink pipeline reads the combined
summary which stores the information for link-time optimization.

This patch populates the 'import type' of a summary in bitcode, and
updates bitcode reader to parse the bit correctly.

[1]
https://github.com/llvm/llvm-project/blob/393eff4e02e7ab3d234d246a8d6912c8e745e6f9/llvm/lib/Support/SmallPtrSet.cpp#L43

1 files changed, 5 insertions, 3 deletions

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index 5382b11..4ffacbb 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -1400,18 +1400,20 @@ public:
                   llvm::StringRef ModulePath,
                   const std::string &NewModulePath) {
     std::map<std::string, GVSummaryMapTy> ModuleToSummariesForIndex;
+    GVSummaryPtrSet DeclarationSummaries;
 
     std::error_code EC;
     gatherImportedSummariesForModule(ModulePath, ModuleToDefinedGVSummaries,
-                                     ImportList, ModuleToSummariesForIndex);
+                                     ImportList, ModuleToSummariesForIndex,
+                                     DeclarationSummaries);
 
     raw_fd_ostream OS(NewModulePath + ".thinlto.bc", EC,
                       sys::fs::OpenFlags::OF_None);
     if (EC)
       return errorCodeToError(EC);
 
-    // TODO: Serialize declaration bits to bitcode.
-    writeIndexToFile(CombinedIndex, OS, &ModuleToSummariesForIndex);
+    writeIndexToFile(CombinedIndex, OS, &ModuleToSummariesForIndex,
+                     &DeclarationSummaries);
 
     if (ShouldEmitImportsFiles) {
       EC = EmitImportsFiles(ModulePath, NewModulePath + ".imports",