Tidy up some extensions and remove some ifdefs

Since this codebase is already extremely nonportable, I've decided to
relax the obsessive ifdef-else-error usage around all the extensions.
From now on, if there's no alternative to using an extension, we can
just use that extension. If it's possible to do something in a
relatively portable way, we can still try to do that in order to make
the code somewhat reusable, in contexts where that makes sense.

I also decided to use langext.h for naked functions and tail calls. If
that's used in another codebase build with a different compiler, those
just won't work, but that's fine. The benefit is really just that
there's less ceremony in places where those are used, because it's
likely there'll be a few more such places in the future, and it gets
annoying reading all the double-underscore stuff all over the place.

I still kind of want to do something about all the _WIN32 ifdefs too,
but I've realised that doing so will lead to almost nothing actually
being built on Linux. Then again, none of it currently runs on Linux so
I guess that's a moot point. Will worry about it later, anyway.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/langext.h b/src/langext.h
index d2d9a23..0624b71 100644
--- a/src/langext.h
+++ b/src/langext.h
@@ -17,6 +17,7 @@
 #define unreachable __builtin_unreachable()
 #define assume(x) ((void)(!!(x) || (unreachable, 0)))
 #define cold __attribute((__cold__, __noinline__))
+#define asm_only __attribute((__naked__)) // N.B.: may not actually work in GCC?
 #else
 #define if_hot(x) if (x)
 #define if_cold(x) if (x)
@@ -25,11 +26,13 @@
 #define unreachable __assume(0)
 #define assume(x) ((void)(__assume(x), 0))
 #define cold __declspec(noinline)
+#define asm_only __declspec(naked)
 #else
 static inline _Noreturn void _invoke_ub() {}
 #define unreachable (_invoke_ub())
 #define assume(x) ((void)(!!(x) || (_invoke_ub(), 0)))
 #define cold
+//#define asm_only // Can't use this without Clang/GCC/MSVC. Too bad.
 #endif
 #endif
 
@@ -63,6 +66,17 @@ static inline _Noreturn void _invoke_ub() {}
 #endif
 #endif
 
+#ifdef __clang__
+#define tailcall \
+	/* Clang forces us to use void return and THEN warns about it ._. */ \
+	_Pragma("clang diagnostic push") \
+	_Pragma("clang diagnostic ignored \"-Wpedantic\"") \
+	__attribute((musttail)) return \
+	_Pragma("clang diagnostic pop")
+#else
+//#define tailcall // Can't use this without Clang.
+#endif
+
 #endif
 
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