Evict registers from the cache based on LRU.

The old method would always evict the first suitable register, i.e. the
same register every time once the cache got full.  The cache doesn't get
terribly often, but the result is pathological...

Source/Core/Core/PowerPC/Jit64/JitRegCache.cpp

@@ -11,7 +11,7 @@
 using namespace Gen;
 using namespace PowerPC;
 
-RegCache::RegCache() : emit(nullptr)
+RegCache::RegCache() : emit(nullptr), cur_use_quantum(0)
 {
 }
 
@@ -29,6 +29,7 @@ void RegCache::Start()
 		regs[i].location = GetDefaultLocation(i);
 		regs[i].away = false;
 		regs[i].locked = false;
+		regs[i].last_used_quantum = 0;
 	}
 
 	// todo: sort to find the most popular regs
@@ -110,18 +111,29 @@ X64Reg RegCache::GetFreeXReg()
 	//Okay, not found :( Force grab one
 
 	//TODO - add a pass to grab xregs whose ppcreg is not used in the next 3 instructions
+	u32 last_used = 0xFFFFFFFF;
+	X64Reg last_used_xr = INVALID_REG;
+	size_t last_used_preg = 0;
 	for (size_t i = 0; i < aCount; i++)
 	{
 		X64Reg xr = (X64Reg)aOrder[i];
 		if (xregs[xr].locked)
 			continue;
 		size_t preg = xregs[xr].ppcReg;
-		if (!regs[preg].locked)
+		if (!regs[preg].locked && regs[preg].last_used_quantum < last_used)
 		{
-			StoreFromRegister(preg);
-			return xr;
+			last_used = regs[preg].last_used_quantum;
+			last_used_xr = xr;
+			last_used_preg = preg;
 		}
 	}
+
+	if (last_used_xr != INVALID_REG)
+	{
+		StoreFromRegister(last_used_preg);
+		return last_used_xr;
+	}
+
 	//Still no dice? Die!
 	_assert_msg_(DYNA_REC, 0, "Regcache ran out of regs");
 	return INVALID_REG;
@@ -170,6 +182,7 @@ void RegCache::DiscardRegContentsIfCached(size_t preg)
 		xregs[xr].ppcReg = INVALID_REG;
 		regs[preg].away = false;
 		regs[preg].location = GetDefaultLocation(preg);
+		regs[preg].last_used_quantum = 0;
 	}
 }
 
@@ -251,6 +264,7 @@ void RegCache::BindToRegister(size_t i, bool doLoad, bool makeDirty)
 		}
 		regs[i].away = true;
 		regs[i].location = ::Gen::R(xr);
+		regs[i].last_used_quantum = ++cur_use_quantum;
 	}
 	else
 	{
@@ -293,6 +307,7 @@ void RegCache::StoreFromRegister(size_t i, FlushMode mode)
 		{
 			regs[i].location = newLoc;
 			regs[i].away = false;
+			regs[i].last_used_quantum = 0;
 		}
 	}
 }
@@ -348,4 +363,6 @@ void RegCache::Flush(FlushMode mode)
 			}
 		}
 	}
+
+	cur_use_quantum = 0;
 }

Source/Core/Core/PowerPC/Jit64/JitRegCache.h

@@ -20,6 +20,7 @@ struct PPCCachedReg
 	Gen::OpArg location;
 	bool away;  // value not in source register
 	bool locked;
+	u32 last_used_quantum;
 };
 
 struct X64CachedReg
@@ -45,6 +46,8 @@ protected:
 
 	Gen::XEmitter *emit;
 
+	u32 cur_use_quantum;
+
 public:
 	RegCache();
 	virtual ~RegCache() {}