[LTO] Record whether LTOUnit splitting is enabled in index

Summary:
Records in the module summary index whether the bitcode was compiled
with the option necessary to enable splitting the LTO unit
(e.g. -fsanitize=cfi, -fwhole-program-vtables, or -fsplit-lto-unit).

The information is passed down to the ModuleSummaryIndex builder via a
new module flag "EnableSplitLTOUnit", which is propagated onto a flag
on the summary index.

This is then used during the LTO link to check whether all linked
summaries were built with the same value of this flag. If not, an error
is issued when we detect a situation requiring whole program visibility
of the class hierarchy. This is the case when both of the following
conditions are met:
1) We are performing LowerTypeTests or Whole Program Devirtualization.
2) There are type tests or type checked loads in the code.

Note I have also changed the ThinLTOBitcodeWriter to also gate the
module splitting on the value of this flag.

Reviewers: pcc

Subscribers: ormris, mehdi_amini, Prazek, inglorion, eraman, steven_wu, dexonsmith, arphaman, dang, llvm-commits

Differential Revision: https://reviews.llvm.org/D53890

llvm-svn: 350948

3 files changed, 30 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
index e4dcd4d4dd7..87c65db0951 100644
--- a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
+++ b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
@@ -1702,6 +1702,13 @@ bool LowerTypeTestsModule::lower() {
       !ExportSummary && !ImportSummary)
     return false;
 
+  // If only some of the modules were split, we cannot correctly handle
+  // code that contains type tests.
+  if (TypeTestFunc && !TypeTestFunc->use_empty() &&
+      ((ExportSummary && ExportSummary->partiallySplitLTOUnits()) ||
+       (ImportSummary && ImportSummary->partiallySplitLTOUnits())))
+    report_fatal_error("inconsistent LTO Unit splitting with llvm.type.test");
+
   if (ImportSummary) {
     if (TypeTestFunc) {
       for (auto UI = TypeTestFunc->use_begin(), UE = TypeTestFunc->use_end();
diff --git a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
index a5382c4638d..510ecb516dc 100644
--- a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -418,8 +418,18 @@ void splitAndWriteThinLTOBitcode(
   }
 }
 
-// Returns whether this module needs to be split because it uses type metadata.
+// Returns whether this module needs to be split because splitting is
+// enabled and it uses type metadata.
 bool requiresSplit(Module &M) {
+  // First check if the LTO Unit splitting has been enabled.
+  bool EnableSplitLTOUnit = false;
+  if (auto *MD = mdconst::extract_or_null<ConstantInt>(
+          M.getModuleFlag("EnableSplitLTOUnit")))
+    EnableSplitLTOUnit = MD->getZExtValue();
+  if (!EnableSplitLTOUnit)
+    return false;
+
+  // Module only needs to be split if it contains type metadata.
   for (auto &GO : M.global_objects()) {
     if (GO.hasMetadata(LLVMContext::MD_type))
       return true;
@@ -431,7 +441,7 @@ bool requiresSplit(Module &M) {
 void writeThinLTOBitcode(raw_ostream &OS, raw_ostream *ThinLinkOS,
                          function_ref<AAResults &(Function &)> AARGetter,
                          Module &M, const ModuleSummaryIndex *Index) {
-  // See if this module has any type metadata. If so, we need to split it.
+  // Split module if splitting is enabled and it contains any type metadata.
   if (requiresSplit(M))
     return splitAndWriteThinLTOBitcode(OS, ThinLinkOS, AARGetter, M);
 
diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 37905daee4c..48bd0cda759 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -1563,6 +1563,17 @@ bool DevirtModule::run() {
       M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
   Function *AssumeFunc = M.getFunction(Intrinsic::getName(Intrinsic::assume));
 
+  // If only some of the modules were split, we cannot correctly handle
+  // code that contains type tests or type checked loads.
+  if ((ExportSummary && ExportSummary->partiallySplitLTOUnits()) ||
+      (ImportSummary && ImportSummary->partiallySplitLTOUnits())) {
+    if ((TypeTestFunc && !TypeTestFunc->use_empty()) ||
+        (TypeCheckedLoadFunc && !TypeCheckedLoadFunc->use_empty()))
+      report_fatal_error("inconsistent LTO Unit splitting with llvm.type.test "
+                         "or llvm.type.checked.load");
+    return false;
+  }
+
   // Normally if there are no users of the devirtualization intrinsics in the
   // module, this pass has nothing to do. But if we are exporting, we also need
   // to handle any users that appear only in the function summaries.