[Attributes] Fix a bug in AttributeList::get so it can handle a mix FunctionIndex and ReturnIndex/arg indices at the same time

[craig.topper]

Summary

The code uses the index of the last element in the sorted array to determine the maximum size needed for the vector. But if the last index is a FunctionIndex(~0), attrIdxToArrayIdx will return 0 and the vector will have size 1. If there are any indices before FunctionIndex, those values would return a value larger than 0 from attrIdxToArrayIdx. So in this case we need to look in front of the FunctionIndex to get the true size needed.

This was found working in our internal fork so I don't have a test case. I'll look into creating a unit test.

[craig.topper]

Added unittest

[rnk]

Yikes. Thanks for the fix, looks good.

llvm/trunk/lib/IR/Attributes.cpp

assert(std::is_sorted(Attrs.begin(), Attrs.end(),
          "Misordered Attributes list!");
   assert(llvm::none_of(Attrs,
                        [](const std::pair<unsigned, AttributeSet> &Pair) {
                          return !Pair.second.hasAttributes();
                        }) &&
          "Pointless attribute!");
 
   unsigned MaxIndex = Attrs.back().first;
+  // If the MaxIndex is FunctionIndex and there are other indices in front
+  // of it, we need to use the largest of those to get the right size.
+  if (MaxIndex == FunctionIndex && Attrs.size() > 1)
+    MaxIndex = Attrs[Attrs.size() - 2].first;
 
   SmallVector<AttributeSet, 4> AttrVec(attrIdxToArrayIdx(MaxIndex) + 1);
   for (const auto Pair : Attrs)
     AttrVec[attrIdxToArrayIdx(Pair.first)] = Pair.second;
 
   return getImpl(C, AttrVec);
 }
 

llvm/trunk/unittests/IR/AttributesTest.cpp

 
 TEST(Attributes, EmptyGet) {
   LLVMContext C;
   AttributeList EmptyLists[] = {AttributeList(), AttributeList()};
   AttributeList AL = AttributeList::get(C, EmptyLists);
   EXPECT_TRUE(AL.isEmpty());
 }
 
+TEST(Attributes, OverflowGet) {
+  LLVMContext C;
+  std::pair<unsigned, Attribute> Attrs[] = { { AttributeList::ReturnIndex, Attribute::get(C, Attribute::SExt) },
+                                             { AttributeList::FunctionIndex, Attribute::get(C, Attribute::ReadOnly) } };
+  AttributeList AL = AttributeList::get(C, Attrs);
+  EXPECT_EQ(2U, AL.getNumAttrSets());
+}
+
 } // end anonymous namespace