[llvm] Introduce LDBG_OS() macro as a variant of LDBG() (PR #157194)

[Andrzej Warzyński via llvm-commits]

================
@@ -76,29 +105,137 @@ namespace llvm {
 #define __LLVM_FILE_NAME__ ::llvm::impl::getShortFileName(__FILE__)
 #endif
 
-#define DEBUGLOG_WITH_STREAM_TYPE_FILE_AND_LINE(STREAM, LEVEL, TYPE, FILE,     \
-                                                LINE)                          \
-  for (bool _c =                                                               \
-           (::llvm::DebugFlag && ::llvm::isCurrentDebugType(TYPE, LEVEL));     \
+/// These macros are detecting if the DEBUG_TYPE or LDBG_DEBUG_STREAM macros are
+/// defined. They are using a combination of preprocessor tricks and C++17
+// used-defined string literals to achieve this.
+// For example, if DEBUG_TYPE is defined to "foo", the preprocessor will expand
+// the macro and then stringify the result to
+//  "foo"_LDBG_VARIABLE_CHECK
+// This dispatch to the user-defined string literal operator named
+// _LDBG_VARIABLE_CHECK which returns true. Otherwise it expands to
+// DEBUG_TYPE_LDBG_VARIABLE_CHECK which we define as a macro that returns false.
+#define LDBG_VARIABLE_CHECK_(VARIABLE, ...) VARIABLE##__VA_ARGS__
+#define LDBG_VARIABLE_CHECK(VARIABLE)                                          \
+  LDBG_VARIABLE_CHECK_(VARIABLE, _LDBG_VARIABLE_CHECK)
+// User-defined string literal operator for the LDBG_VARIABLE_CHECK macro.
+constexpr bool operator""_LDBG_VARIABLE_CHECK(const char *, std::size_t) {
+  return true;
+}
+
+#define IS_DEBUG_TYPE_DEFINED() LDBG_VARIABLE_CHECK(DEBUG_TYPE)
+#define DEBUG_TYPE_LDBG_VARIABLE_CHECK 0
+
+#define IS_LDBG_DEBUG_STREAM_DEFINED() LDBG_VARIABLE_CHECK(LDBG_DEBUG_STREAM)
+#define LDBG_DEBUG_STREAM_LDBG_VARIABLE_CHECK 0
+
+/// Helpers to get DEBUG_TYPE as a string literal, even when DEBUG_TYPE is not
+/// defined (in which case it expand to "DEBUG_TYPE")
+#define LDBG_GET_DEBUG_TYPE_STR__(X) #X##_LDBG_DEBUG_STRING
+#define LDBG_GET_DEBUG_TYPE_STR_(X) LDBG_GET_DEBUG_TYPE_STR__(X)
+#define LDBG_GET_DEBUG_TYPE_STR() LDBG_GET_DEBUG_TYPE_STR_(DEBUG_TYPE)
+/// If DEBUG_TYPE is defined, we get the string with the quotes, we need to
+/// remove them here.
----------------
banach-space wrote:

_Why_ do we "need" to remove quotes? I would just update the comment above (as suggested) and simply state _what_ this does. 
```suggestion
/// If DEBUG_TYPE is defined, it will expand to a name with quotes. This operator removes the quotes (e.g. "MY_PASS" -> MY_PASS).
```

https://github.com/llvm/llvm-project/pull/157194