cxxrtl: disable optimization of debug_items().

Implementing outlining has greatly increased the amount of debug
information in a typical build, and consequently exposed performance
issues in C++ compilers, which are similar for both GCC and Clang;
the compile time of Minerva SoC SRAM increased almost twofold.

Although one would expect the slowdown to be caused by the increased
use of templates in `debug_eval()`, it is actually almost entirely
attributable to optimizations and codegen for `debug_items()`.

Fortunately, it is neither possible nor desirable to optimize
`debug_items()`: in most cases it is called exactly once, and its
body is a linear sequence of calls with unique arguments.

This commit turns off optimizations for `debug_items()` on GCC and
Clang, improving -Os compile time of Minerva SoC SRAM by ~40% (!)

backends/cxxrtl/cxxrtl.h

@@ -41,18 +41,29 @@
 
 #include <backends/cxxrtl/cxxrtl_capi.h>
 
+#ifndef __has_attribute
+#	define __has_attribute(x) 0
+#endif
+
 // CXXRTL essentially uses the C++ compiler as a hygienic macro engine that feeds an instruction selector.
 // It generates a lot of specialized template functions with relatively large bodies that, when inlined
 // into the caller and (for those with loops) unrolled, often expose many new optimization opportunities.
 // Because of this, most of the CXXRTL runtime must be always inlined for best performance.
-#ifndef __has_attribute
-#	define __has_attribute(x) 0
-#endif
 #if __has_attribute(always_inline)
 #define CXXRTL_ALWAYS_INLINE inline __attribute__((__always_inline__))
 #else
 #define CXXRTL_ALWAYS_INLINE inline
 #endif
+// Conversely, some functions in the generated code are extremely large yet very cold, with both of these
+// properties being extreme enough to confuse C++ compilers into spending pathological amounts of time
+// on a futile (the code becomes worse) attempt to optimize the least important parts of code.
+#if __has_attribute(optnone)
+#define CXXRTL_EXTREMELY_COLD __attribute__((__optnone__))
+#elif __has_attribute(optimize)
+#define CXXRTL_EXTREMELY_COLD __attribute__((__optimize__(0)))
+#else
+#define CXXRTL_EXTREMELY_COLD
+#endif
 
 // CXXRTL uses assert() to check for C++ contract violations (which may result in e.g. undefined behavior
 // of the simulation code itself), and CXXRTL_ASSERT to check for RTL contract violations (which may at

backends/cxxrtl/cxxrtl_backend.cc

@@ -2038,6 +2038,7 @@ struct CxxrtlWorker {
 				f << indent << "}\n";
 				f << "\n";
 			}
+			f << indent << "CXXRTL_EXTREMELY_COLD\n";
 			f << indent << "void " << mangle(module) << "::debug_info(debug_items &items, std::string path) {\n";
 			dump_debug_info_method(module);
 			f << indent << "}\n";