LibJS/JIT: Only preserve VM& when making native call to C++

Instead of pushing and popping every single caller-saved registers,
we can optimize code size (and speed!) by only pushing the one register
we actually care about: RDI (since it holds our VM&).

This means that native calls may clobber every other caller-saved
register, so this is something that you have to be aware of when
emitting native calls in the JIT.

This reduces code size on Kraken/ai-astar.js by 553 KiB and makes
execution time ~6% faster as well! :^)

Userland/Libraries/LibJS/JIT/Compiler.cpp

@@ -1006,28 +1006,21 @@ void Compiler::jump_to_exit()
 
 void Compiler::native_call(void* function_address, Vector<Assembler::Operand> const& stack_arguments)
 {
-    // push caller-saved registers on the stack
-    // (callee-saved registers: RBX, RSP, RBP, and R12–R15)
-    m_assembler.push(Assembler::Operand::Register(Assembler::Reg::RCX));
-    m_assembler.push(Assembler::Operand::Register(Assembler::Reg::RDX));
-    m_assembler.push(Assembler::Operand::Register(Assembler::Reg::RSI));
-    m_assembler.push(Assembler::Operand::Register(Assembler::Reg::RDI));
-    m_assembler.push(Assembler::Operand::Register(Assembler::Reg::R8));
-    m_assembler.push(Assembler::Operand::Register(Assembler::Reg::R9));
-    m_assembler.push(Assembler::Operand::Register(Assembler::Reg::R10));
-    m_assembler.push(Assembler::Operand::Register(Assembler::Reg::R11));
+    // Make sure we don't clobber the VM&.
+    m_assembler.push(Assembler::Operand::Register(ARG0));
 
+    // Align the stack pointer.
+    m_assembler.sub(Assembler::Operand::Register(STACK_POINTER), Assembler::Operand::Imm(8));
+
+    // NOTE: We don't preserve caller-saved registers when making a native call.
+    //       This means that they may have changed after we return from the call.
     m_assembler.native_call(function_address, stack_arguments);
 
-    // restore caller-saved registers from the stack
-    m_assembler.pop(Assembler::Operand::Register(Assembler::Reg::R11));
-    m_assembler.pop(Assembler::Operand::Register(Assembler::Reg::R10));
-    m_assembler.pop(Assembler::Operand::Register(Assembler::Reg::R9));
-    m_assembler.pop(Assembler::Operand::Register(Assembler::Reg::R8));
-    m_assembler.pop(Assembler::Operand::Register(Assembler::Reg::RDI));
-    m_assembler.pop(Assembler::Operand::Register(Assembler::Reg::RSI));
-    m_assembler.pop(Assembler::Operand::Register(Assembler::Reg::RDX));
-    m_assembler.pop(Assembler::Operand::Register(Assembler::Reg::RCX));
+    // Restore the stack pointer.
+    m_assembler.add(Assembler::Operand::Register(STACK_POINTER), Assembler::Operand::Imm(8));
+
+    // Restore our VM&.
+    m_assembler.pop(Assembler::Operand::Register(ARG0));
 }
 
 OwnPtr<NativeExecutable> Compiler::compile(Bytecode::Executable& bytecode_executable)