From 0d47f50f572951450f80815277503d41363934cf Mon Sep 17 00:00:00 2001
From: MerryMage <MerryMage@users.noreply.github.com>
Date: Wed, 19 Apr 2017 18:58:36 +0100
Subject: [PATCH] block_of_code: Implement farcode

---
 src/backend_x64/block_of_code.cpp | 30 +++++++++++++++++++++++++++---
 src/backend_x64/block_of_code.h   | 12 +++++++++++-
 2 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/src/backend_x64/block_of_code.cpp b/src/backend_x64/block_of_code.cpp
index 7148bd40..29791982 100644
--- a/src/backend_x64/block_of_code.cpp
+++ b/src/backend_x64/block_of_code.cpp
@@ -18,8 +18,11 @@
 namespace Dynarmic {
 namespace BackendX64 {
 
+constexpr size_t TOTAL_CODE_SIZE = 128 * 1024 * 1024;
+constexpr size_t FAR_CODE_OFFSET = 100 * 1024 * 1024;
+
 BlockOfCode::BlockOfCode(UserCallbacks cb, LookupBlockCallback lookup_block, void* lookup_block_arg)
-        : Xbyak::CodeGenerator(128 * 1024 * 1024)
+        : Xbyak::CodeGenerator(TOTAL_CODE_SIZE)
         , cb(cb)
         , lookup_block(lookup_block)
         , lookup_block_arg(lookup_block_arg)
@@ -28,11 +31,16 @@ BlockOfCode::BlockOfCode(UserCallbacks cb, LookupBlockCallback lookup_block, voi
     GenRunCode();
     GenMemoryAccessors();
     unwind_handler.Register(this);
-    user_code_begin = getCurr<CodePtr>();
+    near_code_begin = getCurr();
+    far_code_begin = getCurr() + FAR_CODE_OFFSET;
+    ClearCache();
 }
 
 void BlockOfCode::ClearCache() {
-    SetCodePtr(user_code_begin);
+    in_far_code = false;
+    near_code_ptr = near_code_begin;
+    far_code_ptr = far_code_begin;
+    SetCodePtr(near_code_begin);
 }
 
 size_t BlockOfCode::RunCode(JitState* jit_state, size_t cycles_to_run) const {
@@ -183,6 +191,22 @@ Xbyak::Address BlockOfCode::MConst(u64 constant) {
     return constant_pool.GetConstant(constant);
 }
 
+void BlockOfCode::SwitchToFarCode() {
+    ASSERT(!in_far_code);
+    in_far_code = true;
+    near_code_ptr = getCurr();
+    SetCodePtr(far_code_ptr);
+
+    ASSERT_MSG(near_code_ptr < far_code_begin, "Near code has overwritten far code!");
+}
+
+void BlockOfCode::SwitchToNearCode() {
+    ASSERT(in_far_code);
+    in_far_code = false;
+    far_code_ptr = getCurr();
+    SetCodePtr(near_code_ptr);
+}
+
 void BlockOfCode::nop(size_t size) {
     switch (size) {
     case 0:
diff --git a/src/backend_x64/block_of_code.h b/src/backend_x64/block_of_code.h
index ca1bce9d..4a9a1cbb 100644
--- a/src/backend_x64/block_of_code.h
+++ b/src/backend_x64/block_of_code.h
@@ -59,6 +59,11 @@ public:
 
     Xbyak::Address MConst(u64 constant);
 
+    /// Far code sits far away from the near code. Execution remains primarily in near code.
+    /// "Cold" / Rarely executed instructions sit in far code, so the CPU doesn't fetch them unless necessary.
+    void SwitchToFarCode();
+    void SwitchToNearCode();
+
     const void* GetReturnFromRunCodeAddress() const {
         return return_from_run_code[0];
     }
@@ -127,10 +132,15 @@ private:
     LookupBlockCallback lookup_block;
     void* lookup_block_arg;
 
-    CodePtr user_code_begin;
+    CodePtr near_code_begin;
+    CodePtr far_code_begin;
 
     ConstantPool constant_pool;
 
+    bool in_far_code = false;
+    CodePtr near_code_ptr;
+    CodePtr far_code_ptr;
+
     using RunCodeFuncType = void(*)(JitState*);
     RunCodeFuncType run_code = nullptr;
     static constexpr size_t NO_SWITCH_MXCSR = 1 << 0;