[llvm] A Unified LTO Bitcode Frontend

Here's a high level summary of the changes in this patch. For more
information on rational, see the RFC.
(https://discourse.llvm.org/t/rfc-a-unified-lto-bitcode-frontend/61774).

  - Add config parameter to LTO backend, specifying which LTO mode is
    desired when using unified LTO.
  - Add unified LTO flag to the summary index for efficiency. Unified
    LTO modules can be detected without parsing the module.
  - Make sure that the ModuleID is generated by incorporating more types
    of symbols.

Differential Revision: https://reviews.llvm.org/D123803

2 files changed, 38 insertions, 22 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index ebc88c8..4f6c5f0 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -8033,14 +8033,17 @@ Expected<std::unique_ptr<ModuleSummaryIndex>> BitcodeModule::getSummary() {
   return std::move(Index);
 }
 
-static Expected<bool> getEnableSplitLTOUnitFlag(BitstreamCursor &Stream,
-                                                unsigned ID) {
+static Expected<std::pair<bool, bool>>
+getEnableSplitLTOUnitAndUnifiedFlag(BitstreamCursor &Stream,
+                                                 unsigned ID,
+                                                 BitcodeLTOInfo &LTOInfo) {
   if (Error Err = Stream.EnterSubBlock(ID))
     return std::move(Err);
   SmallVector<uint64_t, 64> Record;
 
   while (true) {
     BitstreamEntry Entry;
+    std::pair<bool, bool> Result = {false,false};
     if (Error E = Stream.advanceSkippingSubblocks().moveInto(Entry))
       return std::move(E);
 
@@ -8048,10 +8051,10 @@ static Expected<bool> getEnableSplitLTOUnitFlag(BitstreamCursor &Stream,
     case BitstreamEntry::SubBlock: // Handled for us already.
     case BitstreamEntry::Error:
       return error("Malformed block");
-    case BitstreamEntry::EndBlock:
-      // If no flags record found, conservatively return true to mimic
-      // behavior before this flag was added.
-      return true;
+    case BitstreamEntry::EndBlock: {
+      // If no flags record found, set both flags to false.
+      return Result;
+    }
     case BitstreamEntry::Record:
       // The interesting case.
       break;
@@ -8068,9 +8071,13 @@ static Expected<bool> getEnableSplitLTOUnitFlag(BitstreamCursor &Stream,
     case bitc::FS_FLAGS: { // [flags]
       uint64_t Flags = Record[0];
       // Scan flags.
-      assert(Flags <= 0x1ff && "Unexpected bits in flag");
+      assert(Flags <= 0x2ff && "Unexpected bits in flag");
+
+      bool EnableSplitLTOUnit = Flags & 0x8;
+      bool UnifiedLTO = Flags & 0x200;
+      Result = {EnableSplitLTOUnit, UnifiedLTO};
 
-      return Flags & 0x8;
+      return Result;
     }
     }
   }
@@ -8096,25 +8103,31 @@ Expected<BitcodeLTOInfo> BitcodeModule::getLTOInfo() {
       return error("Malformed block");
     case BitstreamEntry::EndBlock:
       return BitcodeLTOInfo{/*IsThinLTO=*/false, /*HasSummary=*/false,
-                            /*EnableSplitLTOUnit=*/false};
+                            /*EnableSplitLTOUnit=*/false, /*UnifiedLTO=*/false};
 
     case BitstreamEntry::SubBlock:
       if (Entry.ID == bitc::GLOBALVAL_SUMMARY_BLOCK_ID) {
-        Expected<bool> EnableSplitLTOUnit =
-            getEnableSplitLTOUnitFlag(Stream, Entry.ID);
-        if (!EnableSplitLTOUnit)
-          return EnableSplitLTOUnit.takeError();
-        return BitcodeLTOInfo{/*IsThinLTO=*/true, /*HasSummary=*/true,
-                              *EnableSplitLTOUnit};
+        BitcodeLTOInfo LTOInfo;
+        Expected<std::pair<bool, bool>> Flags =
+            getEnableSplitLTOUnitAndUnifiedFlag(Stream, Entry.ID, LTOInfo);
+        if (!Flags)
+          return Flags.takeError();
+        std::tie(LTOInfo.EnableSplitLTOUnit, LTOInfo.UnifiedLTO) = Flags.get();
+        LTOInfo.IsThinLTO = true;
+        LTOInfo.HasSummary = true;
+        return LTOInfo;
       }
 
       if (Entry.ID == bitc::FULL_LTO_GLOBALVAL_SUMMARY_BLOCK_ID) {
-        Expected<bool> EnableSplitLTOUnit =
-            getEnableSplitLTOUnitFlag(Stream, Entry.ID);
-        if (!EnableSplitLTOUnit)
-          return EnableSplitLTOUnit.takeError();
-        return BitcodeLTOInfo{/*IsThinLTO=*/false, /*HasSummary=*/true,
-                              *EnableSplitLTOUnit};
+        BitcodeLTOInfo LTOInfo;
+        Expected<std::pair<bool, bool>> Flags =
+            getEnableSplitLTOUnitAndUnifiedFlag(Stream, Entry.ID, LTOInfo);
+        if (!Flags)
+          return Flags.takeError();
+        std::tie(LTOInfo.EnableSplitLTOUnit, LTOInfo.UnifiedLTO) = Flags.get();
+        LTOInfo.IsThinLTO = false;
+        LTOInfo.HasSummary = true;
+        return LTOInfo;
       }
 
       // Ignore other sub-blocks.
diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index bedae66..f2bfb4e 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -4086,6 +4086,9 @@ void ModuleBitcodeWriterBase::writePerModuleGlobalValueSummary() {
   // Bits 1-3 are set only in the combined index, skip them.
   if (Index->enableSplitLTOUnit())
     Flags |= 0x8;
+  if (Index->hasUnifiedLTO())
+    Flags |= 0x200;
+
   Stream.EmitRecord(bitc::FS_FLAGS, ArrayRef<uint64_t>{Flags});
 
   if (Index->begin() == Index->end()) {
@@ -4112,7 +4115,7 @@ void ModuleBitcodeWriterBase::writePerModuleGlobalValueSummary() {
   auto Abbv = std::make_shared<BitCodeAbbrev>();
   Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE_PROFILE));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // valueid
-  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));   // flags
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // flags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // instcount
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // fflags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // numrefs