[Attributes] Clean up handling of UB implying attributes (NFC)

Rather than adding methods for dropping these attributes in
various places, add a function that returns an AttrBuilder with
these attributes, which can then be used with existing methods
for dropping attributes. This is with an eye on D104641, which
also needs to drop them from returns, not just parameters.

Also be more explicit about the semantics of the method in the
documentation. Refer to UB rather than Undef, which is what this
is actually about.

include/llvm/IR/Attributes.h

@@ -546,12 +546,6 @@ public:
     return removeAttributes(C, ArgNo + FirstArgIndex, AttrsToRemove);
   }
 
-  /// Remove noundef attribute and other attributes that imply undefined
-  /// behavior if a `undef` or `poison` value is passed from this attribute
-  /// list. Returns a new list because attribute lists are immutable.
-  LLVM_NODISCARD AttributeList
-  removeParamUndefImplyingAttributes(LLVMContext &C, unsigned ArgNo) const;
-
   /// Remove all attributes at the specified arg index from this
   /// attribute list. Returns a new list because attribute lists are immutable.
   LLVM_NODISCARD AttributeList removeParamAttributes(LLVMContext &C,
@@ -1038,6 +1032,13 @@ namespace AttributeFuncs {
 /// Which attributes cannot be applied to a type.
 AttrBuilder typeIncompatible(Type *Ty);
 
+/// Get param/return attributes which imply immediate undefined behavior if an
+/// invalid value is passed. For example, this includes noundef (where undef
+/// implies UB), but not nonnull (where null implies poison). It also does not
+/// include attributes like nocapture, which constrain the function
+/// implementation rather than the passed value.
+AttrBuilder getUBImplyingAttributes();
+
 /// \returns Return true if the two functions have compatible target-independent
 /// attributes for inlining purposes.
 bool areInlineCompatible(const Function &Caller, const Function &Callee);

include/llvm/IR/InstrTypes.h

@@ -1558,15 +1558,6 @@ public:
     setAttributes(PAL);
   }
 
-  /// Removes noundef and other attributes that imply undefined behavior if a
-  /// `undef` or `poison` value is passed from the given argument.
-  void removeParamUndefImplyingAttrs(unsigned ArgNo) {
-    assert(ArgNo < getNumArgOperands() && "Out of bounds");
-    AttributeList PAL = getAttributes();
-    PAL = PAL.removeParamUndefImplyingAttributes(getContext(), ArgNo);
-    setAttributes(PAL);
-  }
-
   /// adds the dereferenceable attribute to the list of attributes.
   void addDereferenceableAttr(unsigned i, uint64_t Bytes) {
     AttributeList PAL = getAttributes();

lib/IR/Attributes.cpp

@@ -1335,16 +1335,6 @@ AttributeList AttributeList::removeAttributes(LLVMContext &C,
   return getImpl(C, AttrSets);
 }
 
-AttributeList
-AttributeList::removeParamUndefImplyingAttributes(LLVMContext &C,
-                                                  unsigned ArgNo) const {
-  AttrBuilder B;
-  B.addAttribute(Attribute::NoUndef);
-  B.addDereferenceableAttr(1);
-  B.addDereferenceableOrNullAttr(1);
-  return removeParamAttributes(C, ArgNo, B);
-}
-
 AttributeList AttributeList::addDereferenceableAttr(LLVMContext &C,
                                                     unsigned Index,
                                                     uint64_t Bytes) const {
@@ -1926,6 +1916,14 @@ AttrBuilder AttributeFuncs::typeIncompatible(Type *Ty) {
   return Incompatible;
 }
 
+AttrBuilder AttributeFuncs::getUBImplyingAttributes() {
+  AttrBuilder B;
+  B.addAttribute(Attribute::NoUndef);
+  B.addDereferenceableAttr(1);
+  B.addDereferenceableOrNullAttr(1);
+  return B;
+}
+
 template<typename AttrClass>
 static bool isEqual(const Function &Caller, const Function &Callee) {
   return Caller.getFnAttribute(AttrClass::getKind()) ==

lib/IR/Function.cpp

@@ -601,12 +601,6 @@ void Function::removeParamAttrs(unsigned ArgNo, const AttrBuilder &Attrs) {
   setAttributes(PAL);
 }
 
-void Function::removeParamUndefImplyingAttrs(unsigned ArgNo) {
-  AttributeList PAL = getAttributes();
-  PAL = PAL.removeParamUndefImplyingAttributes(getContext(), ArgNo);
-  setAttributes(PAL);
-}
-
 void Function::addDereferenceableAttr(unsigned i, uint64_t Bytes) {
   AttributeList PAL = getAttributes();
   PAL = PAL.addDereferenceableAttr(getContext(), i, Bytes);

lib/Transforms/IPO/DeadArgumentElimination.cpp

@@ -287,6 +287,7 @@ bool DeadArgumentEliminationPass::RemoveDeadArgumentsFromCallers(Function &Fn) {
   SmallVector<unsigned, 8> UnusedArgs;
   bool Changed = false;
 
+  AttrBuilder UBImplyingAttributes = AttributeFuncs::getUBImplyingAttributes();
   for (Argument &Arg : Fn.args()) {
     if (!Arg.hasSwiftErrorAttr() && Arg.use_empty() &&
         !Arg.hasPassPointeeByValueCopyAttr()) {
@@ -295,7 +296,7 @@ bool DeadArgumentEliminationPass::RemoveDeadArgumentsFromCallers(Function &Fn) {
         Changed = true;
       }
       UnusedArgs.push_back(Arg.getArgNo());
-      Fn.removeParamUndefImplyingAttrs(Arg.getArgNo());
+      Fn.removeParamAttrs(Arg.getArgNo(), UBImplyingAttributes);
     }
   }
 
@@ -313,7 +314,7 @@ bool DeadArgumentEliminationPass::RemoveDeadArgumentsFromCallers(Function &Fn) {
 
       Value *Arg = CB->getArgOperand(ArgNo);
       CB->setArgOperand(ArgNo, UndefValue::get(Arg->getType()));
-      CB->removeParamUndefImplyingAttrs(ArgNo);
+      CB->removeParamAttrs(ArgNo, UBImplyingAttributes);
 
       ++NumArgumentsReplacedWithUndef;
       Changed = true;