[llvm] Replace LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]] (NFC) (#163330)

This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]] where
we do not need to move the position of [[maybe_unused]] within
declarations.

Notes:

- [[maybe_unused]] is a standard feature of C++17.

- The compiler is far more lenient about the placement of
  __attribute__((unused)) than that of [[maybe_unused]].

I'll follow up with another patch to finish up the rest.

Author: kazutakahirata

llvm/include/llvm/CodeGen/LiveIntervals.h

@@ -229,8 +229,8 @@ class LiveIntervals {
   /// doing something wrong if you call pruneValue directly on a
   /// LiveInterval. Indeed, you are supposed to call pruneValue on the main
   /// LiveRange and all the LiveRanges of the subranges if any.
-  LLVM_ATTRIBUTE_UNUSED void pruneValue(LiveInterval &, SlotIndex,
-                                        SmallVectorImpl<SlotIndex> *) {
+  [[maybe_unused]] void pruneValue(LiveInterval &, SlotIndex,
+                                   SmallVectorImpl<SlotIndex> *) {
     llvm_unreachable(
         "Use pruneValue on the main LiveRange and on each subrange");
   }

llvm/lib/Analysis/MemorySSA.cpp

@@ -393,7 +393,7 @@ static bool isUseTriviallyOptimizableToLiveOnEntry(AliasAnalysisType &AA,
 /// \param AA        The AliasAnalysis we used for our search.
 /// \param AllowImpreciseClobber Always false, unless we do relaxed verify.
 
-LLVM_ATTRIBUTE_UNUSED static void
+[[maybe_unused]] static void
 checkClobberSanity(const MemoryAccess *Start, MemoryAccess *ClobberAt,
                    const MemoryLocation &StartLoc, const MemorySSA &MSSA,
                    const UpwardsMemoryQuery &Query, BatchAAResults &AA,

llvm/lib/CodeGen/AtomicExpandPass.cpp

@@ -770,7 +770,7 @@ struct PartwordMaskValues {
   Value *Inv_Mask = nullptr;
 };
 
-LLVM_ATTRIBUTE_UNUSED
+[[maybe_unused]]
 raw_ostream &operator<<(raw_ostream &O, const PartwordMaskValues &PMV) {
   auto PrintObj = [&O](auto *V) {
     if (V)

llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp

@@ -112,7 +112,7 @@ APInt GISelValueTracking::getKnownOnes(Register R) {
   return getKnownBits(R).One;
 }
 
-LLVM_ATTRIBUTE_UNUSED static void
+[[maybe_unused]] static void
 dumpResult(const MachineInstr &MI, const KnownBits &Known, unsigned Depth) {
   dbgs() << "[" << Depth << "] Compute known bits: " << MI << "[" << Depth
          << "] Computed for: " << MI << "[" << Depth << "] Known: 0x"

llvm/lib/CodeGen/ScheduleDAGInstrs.cpp

@@ -1551,7 +1551,7 @@ LLVM_DUMP_METHOD void ILPValue::dump() const {
   dbgs() << *this << '\n';
 }
 
-LLVM_ATTRIBUTE_UNUSED
+[[maybe_unused]]
 raw_ostream &llvm::operator<<(raw_ostream &OS, const ILPValue &Val) {
   Val.print(OS);
   return OS;

llvm/lib/Support/TextEncoding.cpp

@@ -54,9 +54,9 @@ static std::optional<TextEncoding> getKnownEncoding(StringRef Name) {
   return std::nullopt;
 }
 
-LLVM_ATTRIBUTE_UNUSED static void
-HandleOverflow(size_t &Capacity, char *&Output, size_t &OutputLength,
-               SmallVectorImpl<char> &Result) {
+[[maybe_unused]] static void HandleOverflow(size_t &Capacity, char *&Output,
+                                            size_t &OutputLength,
+                                            SmallVectorImpl<char> &Result) {
   // No space left in output buffer. Double the size of the underlying
   // memory in the SmallVectorImpl, adjust pointer and length and continue
   // the conversion.

llvm/lib/Support/UnicodeNameToCodepoint.cpp

@@ -476,7 +476,7 @@ nearestMatchesForCodepointName(StringRef Pattern, std::size_t MaxMatchesCount) {
       std::min(NormalizedName.size(), UnicodeNameToCodepointLargestNameSize) +
       1;
 
-  LLVM_ATTRIBUTE_UNUSED static std::size_t Rows =
+  [[maybe_unused]] static std::size_t Rows =
       UnicodeNameToCodepointLargestNameSize + 1;
 
   std::vector<char> Distances(

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -18168,7 +18168,7 @@ Align SITargetLowering::getPrefLoopAlignment(MachineLoop *ML) const {
   return CacheLineAlign;
 }
 
-LLVM_ATTRIBUTE_UNUSED
+[[maybe_unused]]
 static bool isCopyFromRegOfInlineAsm(const SDNode *N) {
   assert(N->getOpcode() == ISD::CopyFromReg);
   do {

llvm/lib/Target/CSKY/Disassembler/CSKYDisassembler.cpp

@@ -166,7 +166,7 @@ static DecodeStatus DecodeFPR64RegisterClass(MCInst &Inst, uint64_t RegNo,
 }
 
 // TODO
-LLVM_ATTRIBUTE_UNUSED
+[[maybe_unused]]
 static DecodeStatus DecodesFPR128RegisterClass(MCInst &Inst, uint64_t RegNo,
                                                uint64_t Address,
                                                const MCDisassembler *Decoder) {

llvm/lib/Target/Hexagon/Disassembler/HexagonDisassembler.cpp

@@ -667,11 +667,10 @@ static DecodeStatus DecodeHvxWRRegisterClass(MCInst &Inst, unsigned RegNo,
   return DecodeRegisterClass(Inst, RegNo, HvxWRDecoderTable);
 }
 
-LLVM_ATTRIBUTE_UNUSED // Suppress warning temporarily.
-    static DecodeStatus
-    DecodeHvxVQRRegisterClass(MCInst &Inst, unsigned RegNo,
-                              uint64_t /*Address*/,
-                              const MCDisassembler *Decoder) {
+[[maybe_unused]] // Suppress warning temporarily.
+static DecodeStatus DecodeHvxVQRRegisterClass(MCInst &Inst, unsigned RegNo,
+                                              uint64_t /*Address*/,
+                                              const MCDisassembler *Decoder) {
   static const MCPhysReg HvxVQRDecoderTable[] = {
       Hexagon::VQ0,  Hexagon::VQ1,  Hexagon::VQ2,  Hexagon::VQ3,
       Hexagon::VQ4,  Hexagon::VQ5,  Hexagon::VQ6,  Hexagon::VQ7};