[PATCH] D12618: Reserve a vendor reserved block ID for bitcode

[Rafael Espíndola via llvm-commits]

> What about when LLVM 4.0 will be here? What if some 4.0 bitcode is fed
> through LLVM 3.8?

It should not crash.

> Even if we don’t crash on the Bitcode *format*, which shouldn’t really
> happen, what about the optimizer generating intrinsics that are not
> supported by the backend?

Still, it should not crash. It should error saying it got an unknown intrinsic.

> I am confident we can get there by fuzz testing.
>
> Good idea!
>


> With what we have in trunk we should be able to implement errors like:
>
> Error: unknown linkage number XX. Input is likely from a newer llvm version.
>
> Would that be sufficient?
>
>
> Possibly, can you point me to the mechanism involved here?

Right now we have

static GlobalValue::LinkageTypes getDecodedLinkage(unsigned Val) {
  switch (Val) {
  default: // Map unknown/new linkages to external
  case 0:

Which is IMHO a really bad idea. It should look something like the
attached patch (which is missing the error propagation). With it
llvm-dis on the attached testcase prints

./bin/llvm-dis: error: unknown linkage number 20. Input is likely from
a newer llvm version.

Cheers,
Rafael
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diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp
index be39b88..99589a4 100644
--- a/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -527,9 +527,13 @@ static bool hasImplicitComdat(size_t Val) {
   }
 }
 
-static GlobalValue::LinkageTypes getDecodedLinkage(unsigned Val) {
+static GlobalValue::LinkageTypes
+getDecodedLinkage(DiagnosticHandlerFunction DiagnosticHandler, unsigned Val) {
   switch (Val) {
-  default: // Map unknown/new linkages to external
+  default:
+    error(DiagnosticHandler,
+          Twine("unknown linkage number ") + Twine(Val) +
+              ". Input is likely from a newer llvm version.");
   case 0:
     return GlobalValue::ExternalLinkage;
   case 2:
@@ -3006,7 +3010,8 @@ std::error_code BitcodeReader::parseModule(bool Resume,
       }
 
       uint64_t RawLinkage = Record[3];
-      GlobalValue::LinkageTypes Linkage = getDecodedLinkage(RawLinkage);
+      GlobalValue::LinkageTypes Linkage =
+          getDecodedLinkage(DiagnosticHandler, RawLinkage);
       unsigned Alignment;
       if (std::error_code EC = parseAlignmentValue(Record[4], Alignment))
         return EC;
@@ -3086,7 +3091,7 @@ std::error_code BitcodeReader::parseModule(bool Resume,
       Func->setCallingConv(static_cast<CallingConv::ID>(Record[1]));
       bool isProto = Record[2];
       uint64_t RawLinkage = Record[3];
-      Func->setLinkage(getDecodedLinkage(RawLinkage));
+      Func->setLinkage(getDecodedLinkage(DiagnosticHandler, RawLinkage));
       Func->setAttributes(getAttributes(Record[4]));
 
       unsigned Alignment;
@@ -3158,8 +3163,8 @@ std::error_code BitcodeReader::parseModule(bool Resume,
       if (!PTy)
         return error("Invalid type for value");
 
-      auto *NewGA =
-          GlobalAlias::create(PTy, getDecodedLinkage(Record[2]), "", TheModule);
+      auto *NewGA = GlobalAlias::create(
+          PTy, getDecodedLinkage(DiagnosticHandler, Record[2]), "", TheModule);
       // Old bitcode files didn't have visibility field.
       // Local linkage must have default visibility.
       if (Record.size() > 3 && !NewGA->hasLocalLinkage())
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