backend/x64: use mmap for all code allocations on Linux

src/dynarmic/backend/x64/block_of_code.cpp

@@ -52,6 +52,41 @@ constexpr size_t CONSTANT_POOL_SIZE = 2 * 1024 * 1024;
 
 class CustomXbyakAllocator : public Xbyak::Allocator {
 public:
+#ifndef _WIN32
+    static constexpr size_t PAGE_SIZE = 4096;
+
+    // Can't subclass Xbyak::MmapAllocator because it is not a pure interface
+    // and doesn't expose its construtor
+    uint8_t* alloc(size_t size) override {
+        // Waste a page to store the size
+        size += PAGE_SIZE;
+
+#    if defined(MAP_ANONYMOUS)
+        int mode = MAP_PRIVATE | MAP_ANONYMOUS;
+#    elif defined(MAP_ANON)
+        int mode = MAP_PRIVATE | MAP_ANON;
+#    else
+#        error "not supported"
+#    endif
+#    ifdef MAP_JIT
+        mode |= MAP_JIT;
+#    endif
+
+        void* p = mmap(nullptr, size, PROT_READ | PROT_WRITE, mode, -1, 0);
+        if (p == MAP_FAILED) {
+            throw Xbyak::Error(Xbyak::ERR_CANT_ALLOC);
+        }
+        std::memcpy(p, &size, sizeof(size_t));
+        return static_cast<uint8_t*>(p) + PAGE_SIZE;
+    }
+
+    void free(uint8_t* p) override {
+        size_t size;
+        std::memcpy(&size, p - PAGE_SIZE, sizeof(size_t));
+        munmap(p - PAGE_SIZE, size);
+    }
+#endif
+
 #ifdef DYNARMIC_ENABLE_NO_EXECUTE_SUPPORT
     bool useProtect() const override { return false; }
 #endif