[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.

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/utils/TableGen/DecoderEmitter.cpp b/llvm/utils/TableGen/DecoderEmitter.cpp
index 354c2a7..fcaf4339 100644
--- a/llvm/utils/TableGen/DecoderEmitter.cpp
+++ b/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.