Silence MSVC warnings about unsigned wrapping without UB

Of course, casting an unsigned value too large for 'int' is UB. So,
write out the ternary. LLVM folds it to ADD anyway.

Fixes the warning from r303693 a different way.

Thanks to Erich Keane for pointing this out!

llvm-svn: 315406

1 files changed, 5 insertions, 7 deletions

diff --git a/llvm/lib/IR/Attributes.cpp b/llvm/lib/IR/Attributes.cpp
index 54b9761bd03..c8f1aaaccee 100644
--- a/llvm/lib/IR/Attributes.cpp
+++ b/llvm/lib/IR/Attributes.cpp
@@ -790,14 +790,12 @@ std::string AttributeSetNode::getAsString(bool InAttrGrp) const {
 // AttributeListImpl Definition
 //===----------------------------------------------------------------------===//
 
-/// Map from AttributeList index to the internal array index. Adding one works:
-///   FunctionIndex: ~0U -> 0
-///   ReturnIndex:    0  -> 1
-///   FirstArgIndex: 1.. -> 2..
+/// Map from AttributeList index to the internal array index. Adding one happens
+/// to work, but it relies on unsigned integer wrapping. MSVC warns about
+/// unsigned wrapping in constexpr functions, so write out the conditional. LLVM
+/// folds it to add anyway.
 static constexpr unsigned attrIdxToArrayIdx(unsigned Index) {
-  // MSVC warns about '~0U + 1' wrapping around when this is called on
-  // FunctionIndex, so cast to int first.
-  return static_cast<int>(Index) + 1;
+  return Index == AttributeList::FunctionIndex ? 0 : Index + 1;
 }
 
 AttributeListImpl::AttributeListImpl(LLVMContext &C,