reserve rax for scratch

src/triples.rs

@@ -655,7 +655,6 @@ impl RegisterStore {
     }
     fn force_take(&mut self, reg: Registers) {
         self.registers[reg as usize] = None;
-        self.used.insert(reg);
     }
     fn free(&mut self, reg: Registers) {
         self.registers[reg as usize] = Some(reg);
@@ -709,7 +708,7 @@ impl Function {
     fn write<W: core::fmt::Write>(&self, w: &mut W) -> core::fmt::Result {
         writeln!(w, "{}:", self.name)?;
 
-        for reg in self.used_registers.iter().filter(|r| r != &&Registers::A) {
+        for reg in self.used_registers.iter() {
             writeln!(w, "push {}", reg.display(Width::QWord))?;
         }
 
@@ -732,12 +731,7 @@ impl Function {
         writeln!(w, "mov %rbp, %rsp")?;
         writeln!(w, "pop %rbp")?;
 
-        for reg in self
+        for reg in self.used_registers.iter().rev() {
-            .used_registers
-            .iter()
-            .rev()
-            .filter(|r| r != &&Registers::A)
-        {
             writeln!(w, "pop {}", reg.display(Width::QWord))?;
         }
 
@@ -796,6 +790,8 @@ impl<'a> Assembler<'a> {
         let mut allocas = HashMap::<Node, u32>::new();
 
         let mut register_store = RegisterStore::new();
+        // rax as scratch register
+        register_store.force_take(Registers::A);
         let mut registers = BTreeMap::<Registers, Node>::new();
         let mut stack_offset = 0;
         let mut func = Function::default();
@@ -940,6 +936,9 @@ impl<'a> Assembler<'a> {
                     let param_reg = Registers::sysv_param_idx(param_count).unwrap();
                     param_count += 1;
                     register_store.force_take(param_reg);
+                    writeln!(&mut func.entry, "push {}", param_reg.display(Width::QWord))?;
+                    stack_offset += 8;
+                    allocas.insert(node, stack_offset);
                     registers.insert(param_reg, node);
                 }
                 Inst::Add(ty) => {