block_of_code: Provide an alloc function to allocate space in the code block

src/backend_x64/block_of_code.cpp

@@ -4,6 +4,7 @@
  * General Public License version 2 or any later version.
  */
 
+#include <cstring>
 #include <limits>
 
 #include <xbyak.h>
@@ -227,6 +228,17 @@ void BlockOfCode::nop(size_t size) {
     }
 }
 
+void* BlockOfCode::alloc(size_t alloc_size) {
+    if (size_ + alloc_size >= maxSize_) {
+        throw Xbyak::Error(Xbyak::ERR_CODE_IS_TOO_BIG);
+    }
+
+    void* ret = getCurr<void*>();
+    size_ += alloc_size;
+    memset(ret, 0, alloc_size);
+    return ret;
+}
+
 void BlockOfCode::SetCodePtr(CodePtr code_ptr) {
     // The "size" defines where top_, the insertion point, is.
     size_t required_size = reinterpret_cast<const u8*>(code_ptr) - getCode();

src/backend_x64/block_of_code.h

@@ -127,6 +127,7 @@ public:
     void int3() { db(0xCC); }
     void nop(size_t size = 1);
 
+    void* alloc(size_t size);
     void SetCodePtr(CodePtr code_ptr);
     void EnsurePatchLocationSize(CodePtr begin, size_t size);