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.

7 files changed, 21 insertions, 30 deletions

diff --git a/src/inputhud.c b/src/inputhud.c
index fbcbe79..6cff487 100644
--- a/src/inputhud.c
+++ b/src/inputhud.c
@@ -150,16 +150,12 @@ static inline int bsf(uint x) {
 	// doing a straight bsf (e.g. via BitScanForward or __builtin_ctz) creates
 	// a false dependency on many CPUs, which compilers don't understand somehow
 	int ret = 0;
-#if defined(__GNUC__) || defined(__clang__)
 	__asm volatile (
 		"bsf %0, %1\n"
 		: "+r" (ret)
 		: "r" (x)
 	);
 	return ret;
-#else
-#error need some sort of inline asm, or a non-broken(!) bitscan intrinsic
-#endif
 }
 
 // IMPORTANT: these things must all match the button order in engineapi.h
diff --git a/src/l4d1democompat.c b/src/l4d1democompat.c
index 4b84dd3..eff6ad0 100644
--- a/src/l4d1democompat.c
+++ b/src/l4d1democompat.c
@@ -114,14 +114,7 @@ static void VCALLCONV hook_ReadDemoHeader(struct CDemoFile *this) {
 	orig_ReadDemoHeader(this);
 }
 
-#if defined(__clang__)
-__attribute((naked))
-#elif defined(_MSC_VER)
-#error Inadequate inline assembly syntax, use Clang instead.
-#else
-#error No way to do naked functions! We only support Clang at the moment.
-#endif
-static int hook_midpoint() {
+static asm_only int hook_midpoint() {
 	__asm volatile (
 		"push eax\n"
 		"mov eax, %1\n"
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
 
 // vi: sw=4 ts=4 noet tw=80 cc=80
diff --git a/src/mem.h b/src/mem.h
index 726018f..86d310e 100644
--- a/src/mem.h
+++ b/src/mem.h
@@ -49,11 +49,8 @@ static inline s64 mem_loads64(const void *p) {
 
 /* Retrieves a pointer from an unaligned pointer-to-pointer. */
 static inline void *mem_loadptr(const void *p) {
-#if defined(_WIN64) || defined(__x86_64__)
-	return (void *)mem_loadu64(p);
-#else
+	if (sizeof(void *) == 8) return (void *)mem_loadu64(p);
 	return (void *)mem_loadu32(p);
-#endif
 }
 
 /* Retrieves a signed size/offset value from an unaligned pointer. */
diff --git a/src/sst.c b/src/sst.c
index 25e396c..7fd0275 100644
--- a/src/sst.c
+++ b/src/sst.c
@@ -504,15 +504,7 @@ static void hook_plugin_unload_cbv2(const struct con_cmdargs *args) {
 		case UNLOAD_SKIP:
 			return;
 		case UNLOAD_SELF:
-#ifdef __clang__
-			// thanks clang for forcing use of return here and ALSO warning!
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wpedantic"
-			__attribute((musttail)) return orig_plugin_unload_cb.v2(args);
-#pragma clang diagnostic pop
-#else
-#error We are tied to clang without an assembly solution for this!
-#endif
+			tailcall orig_plugin_unload_cb.v2(args);
 		case UNLOAD_OTHER:
 			orig_plugin_unload_cb.v2(args);
 			EMIT_PluginUnloaded();
diff --git a/src/vcall.h b/src/vcall.h
index 20cde7e..f91a6bd 100644
--- a/src/vcall.h
+++ b/src/vcall.h
@@ -23,13 +23,8 @@
  */
 
 #ifdef _WIN32
-#if defined(__GNUC__) || defined(__clang__)
 #define VCALLCONV __thiscall
 #else
-// XXX: could support MSVC via __fastcall and dummy param, but is there a point?
-#error C __thiscall support requires Clang or GCC
-#endif
-#else
 #define VCALLCONV
 #endif
 
diff --git a/src/wincrt.c b/src/wincrt.c
index 9e8b9f5..9a0326b 100644
--- a/src/wincrt.c
+++ b/src/wincrt.c
@@ -10,6 +10,10 @@
 //
 // Is it actually reasonable to have to do any of this? Of course not.
 
+// Note: these functions have ifdefs with non-asm fallbacks just in case this
+// file is ever useful somewhere else, but generally we assume this codebase
+// will be built with Clang.
+
 int memcmp(const void *restrict x, const void *restrict y, unsigned int sz) {
 #if defined(__GNUC__) || defined(__clang__)
 	int a, b;