[MC][DecoderEmitter] Fix build warning: explicit specialization cannot have a storage class (#156375)

Move `InsnBitWidth` template into anonymous namespace in the generated
code and move template specialization of `InsnBitWidth` to anonymous
namespace as well, and drop `static` for them. This makes `InsnBitWidth`
completely private to each target and fixes the "explicit specialization
cannot have a storage class" warning as well as any potential linker
errors if `InsnBitWidth` is kept in the `llvm::MCD` namespace.

Author: jurahul

llvm/include/llvm/MC/MCDecoder.h

@@ -72,13 +72,6 @@ insertBits(IntType &field, IntType bits, unsigned startBit, unsigned numBits) {
   field |= bits << startBit;
 }
 
-// InsnBitWidth is essentially a type trait used by the decoder emitter to query
-// the supported bitwidth for a given type. But default, the value is 0, making
-// it an invalid type for use as `InsnType` when instantiating the decoder.
-// Individual targets are expected to provide specializations for these based
-// on their usage.
-template <typename T> static constexpr uint32_t InsnBitWidth = 0;
-
 } // namespace llvm::MCD
 
 #endif // LLVM_MC_MCDECODER_H

llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp

@@ -447,13 +447,13 @@ static DecodeStatus decodeVersionImm(MCInst &Inst, unsigned Imm,
 
 #include "AMDGPUGenDisassemblerTables.inc"
 
+namespace {
 // Define bitwidths for various types used to instantiate the decoder.
-template <> inline constexpr uint32_t llvm::MCD::InsnBitWidth<uint32_t> = 32;
-template <> inline constexpr uint32_t llvm::MCD::InsnBitWidth<uint64_t> = 64;
-template <>
-inline constexpr uint32_t llvm::MCD::InsnBitWidth<std::bitset<96>> = 96;
-template <>
-inline constexpr uint32_t llvm::MCD::InsnBitWidth<std::bitset<128>> = 128;
+template <> constexpr uint32_t InsnBitWidth<uint32_t> = 32;
+template <> constexpr uint32_t InsnBitWidth<uint64_t> = 64;
+template <> constexpr uint32_t InsnBitWidth<std::bitset<96>> = 96;
+template <> constexpr uint32_t InsnBitWidth<std::bitset<128>> = 128;
+} // namespace
 
 //===----------------------------------------------------------------------===//
 //

llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp

@@ -701,11 +701,13 @@ static constexpr DecoderListEntry DecoderList32[]{
     {DecoderTableZdinxRV32Only32, {}, "RV32-only Zdinx (Double in Integer)"},
 };
 
+namespace {
 // Define bitwidths for various types used to instantiate the decoder.
-template <> inline constexpr uint32_t llvm::MCD::InsnBitWidth<uint16_t> = 16;
-template <> inline constexpr uint32_t llvm::MCD::InsnBitWidth<uint32_t> = 32;
+template <> constexpr uint32_t InsnBitWidth<uint16_t> = 16;
+template <> constexpr uint32_t InsnBitWidth<uint32_t> = 32;
 // Use uint64_t to represent 48 bit instructions.
-template <> inline constexpr uint32_t llvm::MCD::InsnBitWidth<uint64_t> = 48;
+template <> constexpr uint32_t InsnBitWidth<uint64_t> = 48;
+} // namespace
 
 DecodeStatus RISCVDisassembler::getInstruction32(MCInst &MI, uint64_t &Size,
                                                  ArrayRef<uint8_t> Bytes,

llvm/utils/TableGen/DecoderEmitter.cpp

@@ -2479,6 +2479,14 @@ void DecoderEmitter::run(raw_ostream &o) const {
 #include <assert.h>
 
 namespace {
+
+// InsnBitWidth is essentially a type trait used by the decoder emitter to query
+// the supported bitwidth for a given type. But default, the value is 0, making
+// it an invalid type for use as `InsnType` when instantiating the decoder.
+// Individual targets are expected to provide specializations for these based
+// on their usage.
+template <typename T> constexpr uint32_t InsnBitWidth = 0;
+
 )";
 
   // Do extra bookkeeping for variable-length encodings.