jit: make sure RSP is 16 byte aligned when we call into rust code (#523)

The System V ABI requires $rsp to be 16 bytes aligned. Internally we
don't emit any instructions that require alignment but when we call out
to rustc generated code we must align.

src/jit.rs

@@ -923,13 +923,23 @@ impl<'a, C: ContextObject> JitCompiler<'a, C> {
         for reg in saved_registers.iter() {
             self.emit_ins(X86Instruction::push(*reg, None));
         }
-
+
+        // Align RSP to 16 bytes
+        self.emit_ins(X86Instruction::push(RSP, None));
+        self.emit_ins(X86Instruction::push(RSP, Some(X86IndirectAccess::OffsetIndexShift(0, RSP, 0))));
+        self.emit_ins(X86Instruction::alu(OperandSize::S64, 0x81, 4, RSP, -16, None));
+
+        let stack_arguments = arguments.len().saturating_sub(ARGUMENT_REGISTERS.len()) as i64;
+        if stack_arguments % 2 != 0 {
+            // If we're going to pass an odd number of stack args we need to pad
+            // to preserve alignment
+            self.emit_ins(X86Instruction::alu(OperandSize::S64, 0x81, 5, RSP, 8, None));
+        }
+
         // Pass arguments
-        let mut stack_arguments = 0;
         for argument in arguments {
             let is_stack_argument = argument.index >= ARGUMENT_REGISTERS.len();
             let dst = if is_stack_argument {
-                stack_arguments += 1;
                 R11
             } else {
                 ARGUMENT_REGISTERS[argument.index]
@@ -997,7 +1007,10 @@ impl<'a, C: ContextObject> JitCompiler<'a, C> {
         }
 
         // Restore registers from stack
-        self.emit_ins(X86Instruction::alu(OperandSize::S64, 0x81, 0, RSP, stack_arguments * 8, None));
+        self.emit_ins(X86Instruction::alu(OperandSize::S64, 0x81, 0, RSP,
+            if stack_arguments % 2 != 0 { stack_arguments + 1 } else { stack_arguments } * 8, None));
+        self.emit_ins(X86Instruction::load(OperandSize::S64, RSP, RSP, X86IndirectAccess::OffsetIndexShift(8, RSP, 0)));
+
         for reg in saved_registers.iter().rev() {
             self.emit_ins(X86Instruction::pop(*reg));
         }