BlockOfCode: Detect space remaining

We also clear the code cache when we run out of space.

This closes #111.

src/backend_x64/block_of_code.cpp

@@ -57,6 +57,25 @@ void BlockOfCode::ClearCache() {
     SetCodePtr(near_code_begin);
 }
 
+size_t BlockOfCode::SpaceRemaining() const {
+    // This function provides an underestimate of near-code-size but that's okay.
+    // (Why? The maximum size of near code should be measured from near_code_begin, not top_.)
+    // These are offsets from Xbyak::CodeArray::top_.
+    std::size_t far_code_offset, near_code_offset;
+    if (in_far_code) {
+        near_code_offset = static_cast<const u8*>(near_code_ptr) - getCode();
+        far_code_offset = getCurr() - getCode();
+    } else {
+        near_code_offset = getCurr() - getCode();
+        far_code_offset = static_cast<const u8*>(far_code_ptr) - getCode();
+    }
+    if (far_code_offset > TOTAL_CODE_SIZE)
+        return 0;
+    if (near_code_offset > FAR_CODE_OFFSET)
+        return 0;
+    return std::min(TOTAL_CODE_SIZE - far_code_offset, FAR_CODE_OFFSET - near_code_offset);
+}
+
 size_t BlockOfCode::RunCode(JitState* jit_state, size_t cycles_to_run) const {
     constexpr size_t max_cycles_to_run = static_cast<size_t>(std::numeric_limits<decltype(jit_state->cycles_remaining)>::max());
     ASSERT(cycles_to_run <= max_cycles_to_run);

src/backend_x64/block_of_code.h

@@ -28,6 +28,8 @@ public:
 
     /// Clears this block of code and resets code pointer to beginning.
     void ClearCache();
+    /// Calculates how much space is remaining to use. This is the minimum of near code and far code.
+    size_t SpaceRemaining() const;
 
     /// Runs emulated code for approximately `cycles_to_run` cycles.
     size_t RunCode(JitState* jit_state, size_t cycles_to_run) const;

src/backend_x64/interface_x64.cpp

@@ -141,6 +141,12 @@ private:
         if (block)
             return *block;
 
+        constexpr size_t MINIMUM_REMAINING_CODESIZE = 1 * 1024 * 1024;
+        if (block_of_code.SpaceRemaining() < MINIMUM_REMAINING_CODESIZE) {
+            invalidate_entire_cache = true;
+            PerformCacheInvalidation();
+        }
+
         IR::Block ir_block = Arm::Translate(descriptor, callbacks.memory.ReadCode);
         Optimization::GetSetElimination(ir_block);
         Optimization::DeadCodeElimination(ir_block);