[SVE] Fix ubsan issues in DecodeIITType

[david-arm]

Summary

In an earlier patch I removed the need for
IITDescriptor::ScalableVecArgument, which involved changing
DecodeIITType to pull out the last IIT_Info from the list. However,
it turns out this is unsafe and causes ubsan failures. I've tried to
fix this a different way by simply passing the last IIT_Info as an
additional argument to DecodeIITType.

[dsanders]

LGTM

llvm/lib/IR/Function.cpp

enum IIT_Info {
   IIT_SUBDIVIDE2_ARG = 44,
   IIT_SUBDIVIDE4_ARG = 45,
   IIT_VEC_OF_BITCASTS_TO_INT = 46,
   IIT_V128 = 47,
   IIT_BF16 = 48
 };
 
 static void DecodeIITType(unsigned &NextElt, ArrayRef<unsigned char> Infos,
+                      IIT_Info LastInfo,
                       SmallVectorImpl<Intrinsic::IITDescriptor> &OutputTable) {
   using namespace Intrinsic;
 
-  bool IsScalableVector = false;
-  if (NextElt > 0) {
-    IIT_Info LastInfo = IIT_Info(Infos[NextElt - 1]);
-    IsScalableVector = (LastInfo == IIT_SCALABLE_VEC);
-  }
+  bool IsScalableVector = (LastInfo == IIT_SCALABLE_VEC);
 
   IIT_Info Info = IIT_Info(Infos[NextElt++]);
   unsigned StructElts = 2;
 
   switch (Info) {
   case IIT_Done:
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::Void, 0));
     return;
static void DecodeIITType(unsigned &NextElt, ArrayRef<unsigned char> Infos,
   case IIT_I64:
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::Integer, 64));
     return;
   case IIT_I128:
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::Integer, 128));
     return;
   case IIT_V1:
     OutputTable.push_back(IITDescriptor::getVector(1, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_V2:
     OutputTable.push_back(IITDescriptor::getVector(2, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_V4:
     OutputTable.push_back(IITDescriptor::getVector(4, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_V8:
     OutputTable.push_back(IITDescriptor::getVector(8, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_V16:
     OutputTable.push_back(IITDescriptor::getVector(16, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_V32:
     OutputTable.push_back(IITDescriptor::getVector(32, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_V64:
     OutputTable.push_back(IITDescriptor::getVector(64, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_V128:
     OutputTable.push_back(IITDescriptor::getVector(128, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_V512:
     OutputTable.push_back(IITDescriptor::getVector(512, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_V1024:
     OutputTable.push_back(IITDescriptor::getVector(1024, IsScalableVector));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_PTR:
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::Pointer, 0));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   case IIT_ANYPTR: {  // [ANYPTR addrspace, subtype]
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::Pointer,
                                              Infos[NextElt++]));
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   }
   case IIT_ARG: {
     unsigned ArgInfo = (NextElt == Infos.size() ? 0 : Infos[NextElt++]);
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::Argument, ArgInfo));
     return;
   }
   case IIT_EXTEND_ARG: {
static void DecodeIITType(unsigned &NextElt, ArrayRef<unsigned char> Infos,
   case IIT_STRUCT6: ++StructElts; LLVM_FALLTHROUGH;
   case IIT_STRUCT5: ++StructElts; LLVM_FALLTHROUGH;
   case IIT_STRUCT4: ++StructElts; LLVM_FALLTHROUGH;
   case IIT_STRUCT3: ++StructElts; LLVM_FALLTHROUGH;
   case IIT_STRUCT2: {
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::Struct,StructElts));
 
     for (unsigned i = 0; i != StructElts; ++i)
-      DecodeIITType(NextElt, Infos, OutputTable);
+      DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   }
   case IIT_SUBDIVIDE2_ARG: {
     unsigned ArgInfo = (NextElt == Infos.size() ? 0 : Infos[NextElt++]);
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::Subdivide2Argument,
                                              ArgInfo));
     return;
   }
   case IIT_SUBDIVIDE4_ARG: {
     unsigned ArgInfo = (NextElt == Infos.size() ? 0 : Infos[NextElt++]);
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::Subdivide4Argument,
                                              ArgInfo));
     return;
   }
   case IIT_VEC_ELEMENT: {
     unsigned ArgInfo = (NextElt == Infos.size() ? 0 : Infos[NextElt++]);
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::VecElementArgument,
                                              ArgInfo));
     return;
   }
   case IIT_SCALABLE_VEC: {
-    DecodeIITType(NextElt, Infos, OutputTable);
+    DecodeIITType(NextElt, Infos, Info, OutputTable);
     return;
   }
   case IIT_VEC_OF_BITCASTS_TO_INT: {
     unsigned ArgInfo = (NextElt == Infos.size() ? 0 : Infos[NextElt++]);
     OutputTable.push_back(IITDescriptor::get(IITDescriptor::VecOfBitcastsToInt,
                                              ArgInfo));
     return;
   }
do {
       TableVal >>= 4;
     } while (TableVal);
 
     IITEntries = IITValues;
     NextElt = 0;
   }
 
   // Okay, decode the table into the output vector of IITDescriptors.
-  DecodeIITType(NextElt, IITEntries, T);
+  DecodeIITType(NextElt, IITEntries, IIT_Done, T);
   while (NextElt != IITEntries.size() && IITEntries[NextElt] != 0)
-    DecodeIITType(NextElt, IITEntries, T);
+    DecodeIITType(NextElt, IITEntries, IIT_Done, T);
 }
 
 static Type *DecodeFixedType(ArrayRef<Intrinsic::IITDescriptor> &Infos,
                              ArrayRef<Type*> Tys, LLVMContext &Context) {
   using namespace Intrinsic;
 
   IITDescriptor D = Infos.front();
   Infos = Infos.slice(1);