[IR] Add NODISCARD to attribute functions

[modocache]

Summary

Many functions on llvm::AttributeList and llvm::AttributeSet are
documented with "returns a new {list,set} because attribute
{lists,sets} are immutable." This documentation can be aided by the
addition of an attribute, LLVM_NODISCARD. Adding this prevents
unsuspecting users of the API from expecting
AttributeList::setAttributes from modifying the underlying list.

At the very least, it would have saved me a few hours of debugging, since I
had been doing just that! I had a bug in my program where I was calling
setAttributes but then passing in the unmutated AttributeList.
I tried adding LLVM_NODISCARD and confirmed that it would have made my bug
immediately obvious.

[rnk]

Looks good, assuming this doesn't trigger new warnings in LLVM & clang.

[modocache]

Thanks! Yup, I confirmed no new warnings when building check-llvm and check-clang.

[modocache]

Oops, sorry, I didn't check hard enough. There's code for AMDGPU that runs up against this warning, causing some buildbots to fail: http://lab.llvm.org:8011/builders/lld-perf-testsuite/builds/9637/steps/build-bin%2Flld/logs/stdio

I can revert this, build all targets and fix the warnings, and then re-commit.

include/llvm/IR/Attributes.h

public:
   static AttributeSet get(LLVMContext &C, const AttrBuilder &B);
   static AttributeSet get(LLVMContext &C, ArrayRef<Attribute> Attrs);
 
   bool operator==(const AttributeSet &O) const { return SetNode == O.SetNode; }
   bool operator!=(const AttributeSet &O) const { return !(*this == O); }
 
   /// Add an argument attribute. Returns a new set because attribute sets are
   /// immutable.
-  AttributeSet addAttribute(LLVMContext &C, Attribute::AttrKind Kind) const;
+  LLVM_NODISCARD AttributeSet addAttribute(LLVMContext &C,
+                                           Attribute::AttrKind Kind) const;
 
   /// Add a target-dependent attribute. Returns a new set because attribute sets
   /// are immutable.
-  AttributeSet addAttribute(LLVMContext &C, StringRef Kind,
+  LLVM_NODISCARD AttributeSet addAttribute(LLVMContext &C, StringRef Kind,
                                            StringRef Value = StringRef()) const;
 
   /// Add attributes to the attribute set. Returns a new set because attribute
   /// sets are immutable.
-  AttributeSet addAttributes(LLVMContext &C, AttributeSet AS) const;
+  LLVM_NODISCARD AttributeSet addAttributes(LLVMContext &C,
+                                            AttributeSet AS) const;
 
   /// Remove the specified attribute from this set. Returns a new set because
   /// attribute sets are immutable.
-  AttributeSet removeAttribute(LLVMContext &C, Attribute::AttrKind Kind) const;
+  LLVM_NODISCARD AttributeSet removeAttribute(LLVMContext &C,
+                                              Attribute::AttrKind Kind) const;
 
   /// Remove the specified attribute from this set. Returns a new set because
   /// attribute sets are immutable.
-  AttributeSet removeAttribute(LLVMContext &C, StringRef Kind) const;
+  LLVM_NODISCARD AttributeSet removeAttribute(LLVMContext &C,
+                                              StringRef Kind) const;
 
   /// Remove the specified attributes from this set. Returns a new set because
   /// attribute sets are immutable.
-  AttributeSet removeAttributes(LLVMContext &C,
-                                const AttrBuilder &AttrsToRemove) const;
+  LLVM_NODISCARD AttributeSet
+  removeAttributes(LLVMContext &C, const AttrBuilder &AttrsToRemove) const;
 
   /// Return the number of attributes in this set.
   unsigned getNumAttributes() const;
 
   /// Return true if attributes exists in this set.
   bool hasAttributes() const { return SetNode != nullptr; }
 
   /// Return true if the attribute exists in this set.
static AttributeList get(LLVMContext &C, unsigned Index,
                            ArrayRef<Attribute::AttrKind> Kinds);
   static AttributeList get(LLVMContext &C, unsigned Index,
                            ArrayRef<StringRef> Kind);
   static AttributeList get(LLVMContext &C, unsigned Index,
                            const AttrBuilder &B);
 
   /// Add an attribute to the attribute set at the given index.
   /// Returns a new list because attribute lists are immutable.
-  AttributeList addAttribute(LLVMContext &C, unsigned Index,
+  LLVM_NODISCARD AttributeList addAttribute(LLVMContext &C, unsigned Index,
                                             Attribute::AttrKind Kind) const;
 
   /// Add an attribute to the attribute set at the given index.
   /// Returns a new list because attribute lists are immutable.
-  AttributeList addAttribute(LLVMContext &C, unsigned Index, StringRef Kind,
-                             StringRef Value = StringRef()) const;
+  LLVM_NODISCARD AttributeList
+  addAttribute(LLVMContext &C, unsigned Index, StringRef Kind,
 
   /// Add an attribute to the attribute set at the given index.
   /// Returns a new list because attribute lists are immutable.
-  AttributeList addAttribute(LLVMContext &C, unsigned Index, Attribute A) const;
+  LLVM_NODISCARD AttributeList addAttribute(LLVMContext &C, unsigned Index,
+                                            Attribute A) const;
 
   /// Add attributes to the attribute set at the given index.
   /// Returns a new list because attribute lists are immutable.
-  AttributeList addAttributes(LLVMContext &C, unsigned Index,
+  LLVM_NODISCARD AttributeList addAttributes(LLVMContext &C, unsigned Index,
                                              const AttrBuilder &B) const;
 
   /// Add an argument attribute to the list. Returns a new list because
   /// attribute lists are immutable.
-  AttributeList addParamAttribute(LLVMContext &C, unsigned ArgNo,
-                                  Attribute::AttrKind Kind) const {
+  LLVM_NODISCARD AttributeList addParamAttribute(
+      LLVMContext &C, unsigned ArgNo, Attribute::AttrKind Kind) const {
     return addAttribute(C, ArgNo + FirstArgIndex, Kind);
   }
 
   /// Add an argument attribute to the list. Returns a new list because
   /// attribute lists are immutable.
-  AttributeList addParamAttribute(LLVMContext &C, unsigned ArgNo,
-                                  StringRef Kind,
-                                  StringRef Value = StringRef()) const {
+  LLVM_NODISCARD AttributeList
+  addParamAttribute(LLVMContext &C, unsigned ArgNo, StringRef Kind,
     return addAttribute(C, ArgNo + FirstArgIndex, Kind, Value);
   }
 
   /// Add an attribute to the attribute list at the given arg indices. Returns a
   /// new list because attribute lists are immutable.
-  AttributeList addParamAttribute(LLVMContext &C, ArrayRef<unsigned> ArgNos,
+  LLVM_NODISCARD AttributeList addParamAttribute(LLVMContext &C,
+                                                 ArrayRef<unsigned> ArgNos,
                                                  Attribute A) const;
 
   /// Add an argument attribute to the list. Returns a new list because
   /// attribute lists are immutable.
-  AttributeList addParamAttributes(LLVMContext &C, unsigned ArgNo,
+  LLVM_NODISCARD AttributeList addParamAttributes(LLVMContext &C,
+                                                  unsigned ArgNo,
                                                   const AttrBuilder &B) const {
     return addAttributes(C, ArgNo + FirstArgIndex, B);
   }
 
   /// Remove the specified attribute at the specified index from this
   /// attribute list. Returns a new list because attribute lists are immutable.
-  AttributeList removeAttribute(LLVMContext &C, unsigned Index,
+  LLVM_NODISCARD AttributeList removeAttribute(LLVMContext &C, unsigned Index,
                                                Attribute::AttrKind Kind) const;
 
   /// Remove the specified attribute at the specified index from this
   /// attribute list. Returns a new list because attribute lists are immutable.
-  AttributeList removeAttribute(LLVMContext &C, unsigned Index,
+  LLVM_NODISCARD AttributeList removeAttribute(LLVMContext &C, unsigned Index,
                                                StringRef Kind) const;
 
   /// Remove the specified attributes at the specified index from this
   /// attribute list. Returns a new list because attribute lists are immutable.
-  AttributeList removeAttributes(LLVMContext &C, unsigned Index,
-                                 const AttrBuilder &AttrsToRemove) const;
+  LLVM_NODISCARD AttributeList removeAttributes(
+      LLVMContext &C, unsigned Index, const AttrBuilder &AttrsToRemove) const;
 
   /// Remove all attributes at the specified index from this
   /// attribute list. Returns a new list because attribute lists are immutable.
-  AttributeList removeAttributes(LLVMContext &C, unsigned Index) const;
+  LLVM_NODISCARD AttributeList removeAttributes(LLVMContext &C,
+                                                unsigned Index) const;
 
   /// Remove the specified attribute at the specified arg index from this
   /// attribute list. Returns a new list because attribute lists are immutable.
-  AttributeList removeParamAttribute(LLVMContext &C, unsigned ArgNo,
-                                     Attribute::AttrKind Kind) const {
+  LLVM_NODISCARD AttributeList removeParamAttribute(
+      LLVMContext &C, unsigned ArgNo, Attribute::AttrKind Kind) const {
     return removeAttribute(C, ArgNo + FirstArgIndex, Kind);
   }
 
   /// Remove the specified attribute at the specified arg index from this
   /// attribute list. Returns a new list because attribute lists are immutable.
-  AttributeList removeParamAttribute(LLVMContext &C, unsigned ArgNo,
+  LLVM_NODISCARD AttributeList removeParamAttribute(LLVMContext &C,
+                                                    unsigned ArgNo,
                                                     StringRef Kind) const {
     return removeAttribute(C, ArgNo + FirstArgIndex, Kind);
   }
 
   /// Remove the specified attribute at the specified arg index from this
   /// attribute list. Returns a new list because attribute lists are immutable.
-  AttributeList removeParamAttributes(LLVMContext &C, unsigned ArgNo,
-                                      const AttrBuilder &AttrsToRemove) const {
+  LLVM_NODISCARD AttributeList removeParamAttributes(
+      LLVMContext &C, unsigned ArgNo, const AttrBuilder &AttrsToRemove) const {
     return removeAttributes(C, ArgNo + FirstArgIndex, AttrsToRemove);
   }
 
   /// Remove all attributes at the specified arg index from this
   /// attribute list. Returns a new list because attribute lists are immutable.
-  AttributeList removeParamAttributes(LLVMContext &C, unsigned ArgNo) const {
+  LLVM_NODISCARD AttributeList removeParamAttributes(LLVMContext &C,
+                                                     unsigned ArgNo) const {
     return removeAttributes(C, ArgNo + FirstArgIndex);
   }
 
   /// \brief Add the dereferenceable attribute to the attribute set at the given
   /// index. Returns a new list because attribute lists are immutable.
-  AttributeList addDereferenceableAttr(LLVMContext &C, unsigned Index,
+  LLVM_NODISCARD AttributeList addDereferenceableAttr(LLVMContext &C,
+                                                      unsigned Index,
                                                       uint64_t Bytes) const;
 
   /// \brief Add the dereferenceable attribute to the attribute set at the given
   /// arg index. Returns a new list because attribute lists are immutable.
-  AttributeList addDereferenceableParamAttr(LLVMContext &C, unsigned ArgNo,
-                                            uint64_t Bytes) const {
+  LLVM_NODISCARD AttributeList addDereferenceableParamAttr(
+      LLVMContext &C, unsigned ArgNo, uint64_t Bytes) const {
     return addDereferenceableAttr(C, ArgNo + FirstArgIndex, Bytes);
   }
 
   /// Add the dereferenceable_or_null attribute to the attribute set at
   /// the given index. Returns a new list because attribute lists are immutable.
-  AttributeList addDereferenceableOrNullAttr(LLVMContext &C, unsigned Index,
-                                             uint64_t Bytes) const;
+  LLVM_NODISCARD AttributeList addDereferenceableOrNullAttr(
+      LLVMContext &C, unsigned Index, uint64_t Bytes) const;
 
   /// Add the dereferenceable_or_null attribute to the attribute set at
   /// the given arg index. Returns a new list because attribute lists are
   /// immutable.
-  AttributeList addDereferenceableOrNullParamAttr(LLVMContext &C,
-                                                  unsigned ArgNo,
-                                                  uint64_t Bytes) const {
+  LLVM_NODISCARD AttributeList addDereferenceableOrNullParamAttr(
+      LLVMContext &C, unsigned ArgNo, uint64_t Bytes) const {
     return addDereferenceableOrNullAttr(C, ArgNo + FirstArgIndex, Bytes);
   }
 
   /// Add the allocsize attribute to the attribute set at the given index.
   /// Returns a new list because attribute lists are immutable.
-  AttributeList addAllocSizeAttr(LLVMContext &C, unsigned Index,
-                                 unsigned ElemSizeArg,
-                                 const Optional<unsigned> &NumElemsArg);
+  LLVM_NODISCARD AttributeList
+  addAllocSizeAttr(LLVMContext &C, unsigned Index, unsigned ElemSizeArg,
 
   /// Add the allocsize attribute to the attribute set at the given arg index.
   /// Returns a new list because attribute lists are immutable.
-  AttributeList addAllocSizeParamAttr(LLVMContext &C, unsigned ArgNo,
-                                      unsigned ElemSizeArg,
-                                      const Optional<unsigned> &NumElemsArg) {
+  LLVM_NODISCARD AttributeList
+  addAllocSizeParamAttr(LLVMContext &C, unsigned ArgNo, unsigned ElemSizeArg,
     return addAllocSizeAttr(C, ArgNo + FirstArgIndex, ElemSizeArg, NumElemsArg);
   }
 
   //===--------------------------------------------------------------------===//
   // AttributeList Accessors
   //===--------------------------------------------------------------------===//
 
   /// Retrieve the LLVM context.