diff --git a/common/Assertions.h b/common/Assertions.h
index a7d841d07d..dadf468c45 100644
--- a/common/Assertions.h
+++ b/common/Assertions.h
@@ -155,8 +155,8 @@ extern pxDoAssertFnType* pxDoAssert;
 #define pxAssertMsg(cond, msg) (likely(cond))
 #define pxAssertDev(cond, msg) (likely(cond))
 
-#define pxAssumeMsg(cond, msg) (__assume(cond))
-#define pxAssumeDev(cond, msg) (__assume(cond))
+#define pxAssumeMsg(cond, msg) __assume(cond)
+#define pxAssumeDev(cond, msg) __assume(cond)
 
 #define pxFail(msg) \
 	do \
diff --git a/common/Pcsx2Defs.h b/common/Pcsx2Defs.h
index 2e4a8a80c0..c667ea51df 100644
--- a/common/Pcsx2Defs.h
+++ b/common/Pcsx2Defs.h
@@ -146,7 +146,7 @@ static const int __pagesize = PCSX2_PAGESIZE;
 	#define __aligned32 __attribute__((aligned(32)))
 	#define __pagealigned __attribute__((aligned(PCSX2_PAGESIZE)))
 
-	#define __assume(cond) ((void)0) // GCC has no equivalent for __assume
+	#define __assume(cond) do { if (!(cond)) __builtin_unreachable(); } while(0)
 	#define CALLBACK __attribute__((stdcall))
 
 	// Inlining note: GCC needs ((unused)) attributes defined on inlined functions to suppress
@@ -154,7 +154,11 @@ static const int __pagesize = PCSX2_PAGESIZE;
 	// happens *by design* like all the friggen time >_<)
 
 	#ifndef __fastcall
-		#define __fastcall __attribute__((fastcall))
+		#ifndef _M_X86_32
+			#define __fastcall // Attribute not available, and x86_32 is pretty much the only cc that passes literally everything in registers
+		#else
+			#define __fastcall __attribute__((fastcall))
+		#endif
 	#endif
 	#define __vectorcall __fastcall
 	#define _inline __inline__ __attribute__((unused))