[PATCH] D20004: Implement a safer bitcode upgrade for DISubprogram

[Adrian Prantl via llvm-commits]

aprantl created this revision.
aprantl added reviewers: dexonsmith, joker.eph.
aprantl added a subscriber: llvm-commits.

The bitcode upgrade I added for DISubprogram in r266446 was based on the assumption that the CU node for the subprogram was already materialized by the time the DISubprogram is visited. This assumption may not hold true as future versions of LLVM may decide to write out bitcode in a different order. This patch corrects this by introducing a versioning bit next to the distinct flag to unambiguously differentiate the new from the old record layouts.

Note for people stabilizing LLVM out-of-tree: This patch introduces a bitcode incompatibility with llvm trunk revisions from r266446 — here. (But D19987 will ensure that it degrades gracefully).

http://reviews.llvm.org/D20004

Files:
  lib/Bitcode/Reader/BitcodeReader.cpp
  lib/Bitcode/Writer/BitcodeWriter.cpp

Index: lib/Bitcode/Writer/BitcodeWriter.cpp
===================================================================
--- lib/Bitcode/Writer/BitcodeWriter.cpp
+++ lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -1353,7 +1353,8 @@
 void ModuleBitcodeWriter::writeDISubprogram(const DISubprogram *N,
                                             SmallVectorImpl<uint64_t> &Record,
                                             unsigned Abbrev) {
-  Record.push_back(N->isDistinct());
+  uint64_t IsVersion3 = 1 << 1;
+  Record.push_back(N->isDistinct() | IsVersion3);
   Record.push_back(VE.getMetadataOrNullID(N->getScope()));
   Record.push_back(VE.getMetadataOrNullID(N->getRawName()));
   Record.push_back(VE.getMetadataOrNullID(N->getRawLinkageName()));
Index: lib/Bitcode/Reader/BitcodeReader.cpp
===================================================================
--- lib/Bitcode/Reader/BitcodeReader.cpp
+++ lib/Bitcode/Reader/BitcodeReader.cpp
@@ -2472,14 +2472,17 @@
         return error("Invalid record");
 
       IsDistinct =
-          Record[0] || Record[8]; // All definitions should be distinct.
+          (Record[0] & 1) || Record[8]; // All definitions should be distinct.
+      bool IsVersion3 = (Record[0] & (1 << 1)) == (1 << 1);
+      if (IsVersion3 && Record.size() != 19)
+        return error("Invalid record");
       // Version 1 has a Function as Record[15].
       // Version 2 has removed Record[15].
       // Version 3 has the Unit as Record[15].
       Metadata *CUorFn = getMDOrNull(Record[15]);
       unsigned Offset = Record.size() == 19 ? 1 : 0;
-      bool HasFn = Offset && dyn_cast_or_null<ConstantAsMetadata>(CUorFn);
-      bool HasCU = Offset && !HasFn;
+      bool HasFn = Offset && !IsVersion3;
+      bool HasCU = IsVersion3;
       DISubprogram *SP = GET_OR_DISTINCT(
           DISubprogram,
           (Context, getDITypeRefOrNull(Record[1]), getMDString(Record[2]),
@@ -2493,7 +2496,7 @@
 
       // Upgrade sp->function mapping to function->sp mapping.
       if (HasFn) {
-        if (auto *CMD = dyn_cast<ConstantAsMetadata>(CUorFn))
+        if (auto *CMD = dyn_cast_or_null<ConstantAsMetadata>(CUorFn))
           if (auto *F = dyn_cast<Function>(CMD->getValue())) {
             if (F->isMaterializable())
               // Defer until materialized; unmaterialized functions may not have


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