Write summaries for merged modules when splitting modules for ThinLTO.

This is to prepare to allow for dead stripping of globals in the
merged modules.

Differential Revision: https://reviews.llvm.org/D33921

llvm-svn: 305027

3 files changed, 29 insertions, 3 deletions

diff --git a/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp b/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
index 3253f27c010..095647e1bd2 100644
--- a/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
+++ b/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp
@@ -447,6 +447,11 @@ ModuleSummaryIndex llvm::buildModuleSummaryIndex(
         });
   }
 
+  bool IsThinLTO = true;
+  if (auto *MD =
+          mdconst::extract_or_null<ConstantInt>(M.getModuleFlag("ThinLTO")))
+    IsThinLTO = MD->getZExtValue();
+
   for (auto &GlobalList : Index) {
     // Ignore entries for references that are undefined in the current module.
     if (GlobalList.second.SummaryList.empty())
@@ -455,6 +460,11 @@ ModuleSummaryIndex llvm::buildModuleSummaryIndex(
     assert(GlobalList.second.SummaryList.size() == 1 &&
            "Expected module's index to have one summary per GUID");
     auto &Summary = GlobalList.second.SummaryList[0];
+    if (!IsThinLTO) {
+      Summary->setNotEligibleToImport();
+      continue;
+    }
+
     bool AllRefsCanBeExternallyReferenced =
         llvm::all_of(Summary->refs(), [&](const ValueInfo &VI) {
           return !CantBePromoted.count(VI.getGUID());
diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index 9043b8c12d2..d5879fec95c 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -3305,7 +3305,15 @@ static const uint64_t INDEX_VERSION = 3;
 /// Emit the per-module summary section alongside the rest of
 /// the module's bitcode.
 void ModuleBitcodeWriter::writePerModuleGlobalValueSummary() {
-  Stream.EnterSubblock(bitc::GLOBALVAL_SUMMARY_BLOCK_ID, 4);
+  // By default we compile with ThinLTO if the module has a summary, but the
+  // client can request full LTO with a module flag.
+  bool IsThinLTO = true;
+  if (auto *MD =
+          mdconst::extract_or_null<ConstantInt>(M.getModuleFlag("ThinLTO")))
+    IsThinLTO = MD->getZExtValue();
+  Stream.EnterSubblock(IsThinLTO ? bitc::GLOBALVAL_SUMMARY_BLOCK_ID
+                                 : bitc::FULL_LTO_GLOBALVAL_SUMMARY_BLOCK_ID,
+                       4);
 
   Stream.EmitRecord(bitc::FS_VERSION, ArrayRef<uint64_t>{INDEX_VERSION});
 
diff --git a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
index 9dede4cedd1..a7bcc7cc553 100644
--- a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -318,6 +318,12 @@ void splitAndWriteThinLTOBitcode(
   ProfileSummaryInfo PSI(M);
   ModuleSummaryIndex Index = buildModuleSummaryIndex(M, nullptr, &PSI);
 
+  // Mark the merged module as requiring full LTO. We still want an index for
+  // it though, so that it can participate in summary-based dead stripping.
+  MergedM->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
+  ModuleSummaryIndex MergedMIndex =
+      buildModuleSummaryIndex(*MergedM, nullptr, &PSI);
+
   SmallVector<char, 0> Buffer;
 
   BitcodeWriter W(Buffer);
@@ -327,7 +333,8 @@ void splitAndWriteThinLTOBitcode(
   ModuleHash ModHash = {{0}};
   W.writeModule(&M, /*ShouldPreserveUseListOrder=*/false, &Index,
                 /*GenerateHash=*/true, &ModHash);
-  W.writeModule(MergedM.get());
+  W.writeModule(MergedM.get(), /*ShouldPreserveUseListOrder=*/false,
+                &MergedMIndex);
   W.writeStrtab();
   OS << Buffer;
 
@@ -340,7 +347,8 @@ void splitAndWriteThinLTOBitcode(
     StripDebugInfo(M);
     W2.writeModule(&M, /*ShouldPreserveUseListOrder=*/false, &Index,
                    /*GenerateHash=*/false, &ModHash);
-    W2.writeModule(MergedM.get());
+    W2.writeModule(MergedM.get(), /*ShouldPreserveUseListOrder=*/false,
+                   &MergedMIndex);
     W2.writeStrtab();
     *ThinLinkOS << Buffer;
   }