From aef97d440c4db5f87d0b7b8e043c6f218e0942a6 Mon Sep 17 00:00:00 2001
From: Maxime Chevalier-Boisvert <maximechevalierb@gmail.com>
Date: Sun, 26 Nov 2023 23:40:15 -0500
Subject: [PATCH] Add call_fp (call function pointer). Requested in #39.

---
 vm/src/asm.rs |  8 ++++++++
 vm/src/vm.rs  | 32 +++++++++++++++++++++++++++++---
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/vm/src/asm.rs b/vm/src/asm.rs
index a6eb349..d634de9 100644
--- a/vm/src/asm.rs
+++ b/vm/src/asm.rs
@@ -1154,6 +1154,7 @@ impl Assembler
                 self.code.push_u16(syscall_idx);
             }
 
+            // Call label
             "call" => {
                 let label_name = input.parse_ident()?;
                 input.expect_token(",")?;
@@ -1164,6 +1165,13 @@ impl Assembler
                 self.code.push_u8(argc);
             }
 
+            // Call function pointer
+            "call_fp" => {
+                let argc: u8 = self.parse_int_arg(input)?;
+                self.code.push_op(Op::call_fp);
+                self.code.push_u8(argc);
+            }
+
             "ret" => self.code.push_op(Op::ret),
             "exit" => self.code.push_op(Op::exit),
 
diff --git a/vm/src/vm.rs b/vm/src/vm.rs
index cbf3468..ba0c133 100644
--- a/vm/src/vm.rs
+++ b/vm/src/vm.rs
@@ -244,8 +244,8 @@ pub enum Op
     call,
 
     // Call a function pointer passed as argument
-    // call <num_args:u8> (f_ptr, arg0, arg1, ..., argN)
-    //call_fp,
+    // call <num_args:u8> (arg0, arg1, ..., argN, f_ptr)
+    call_fp,
 
     // Call into a host function
     // For example, to set up a device or to allocate more memory
@@ -1440,6 +1440,26 @@ impl VM
                     pc = ((pc as isize) + offset) as usize;
                 }
 
+                // call <num_args:u8> (arg0, arg1, ..., argN, f_ptr)
+                Op::call_fp => {
+                    // Absolute address of the function to call
+                    let fp = self.pop();
+
+                    // Argument count
+                    let num_args = self.code.read_pc::<u8>(&mut pc) as usize;
+                    assert!(num_args <= self.stack.len() - bp);
+
+                    self.frames.push(StackFrame {
+                        prev_bp: bp,
+                        ret_addr: pc,
+                        argc: num_args,
+                    });
+
+                    // The base pointer will point at the first local
+                    bp = self.stack.len();
+                    pc = fp.as_usize();
+                }
+
                 Op::syscall => {
                     let syscall_idx = self.code.read_pc::<u16>(&mut pc);
                     let syscall_fn = self.sys_state.get_syscall(syscall_idx);
@@ -1593,7 +1613,7 @@ mod tests
 
         // Keep track of how many short opcodes we have so far
         dbg!(Op::exit as usize);
-        assert!(Op::exit as usize <= 112);
+        assert!(Op::exit as usize <= 113);
     }
 
     #[test]
@@ -1681,6 +1701,12 @@ mod tests
         eval_i64("push 5; call foo, 0; pop; exit; foo: push 2; push 0; ret;", 5);
     }
 
+    #[test]
+    fn test_call_fp()
+    {
+        eval_i64(" push FN; call_fp 0; exit; FN: push_i8 33; ret;", 33);
+    }
+
     #[test]
     fn test_syscalls()
     {