remove bounds check

rust2/src/codegen.rs

@@ -13,6 +13,9 @@
 //!
 //! technically, the `JumpIfNotZero` would be an unconditional Jmp to the `JmpIfZero`, but that's
 //! a needless indirection.
+//!
+//! this module must not produce out of bounds jumps and always put the `End` instruction at the
+//! end
 
 use crate::opts::{Ir, Stmt as IrStmt, StmtKind};
 use crate::parse::Span;
@@ -34,8 +37,18 @@ pub enum Stmt {
 
 #[derive(Debug, Clone)]
 pub struct Code<'c> {
-    pub stmts: Vec<Stmt, &'c Bump>,
-    pub debug: Vec<Span, &'c Bump>,
+    stmts: Vec<Stmt, &'c Bump>,
+    debug: Vec<Span, &'c Bump>,
+}
+
+impl Code<'_> {
+    pub fn stmts(&self) -> &[Stmt] {
+        &self.stmts
+    }
+
+    pub fn debug(&self) -> &[Span] {
+        &self.debug
+    }
 }
 
 pub fn generate<'c>(alloc: &'c Bump, ir: &Ir<'_>) -> Code<'c> {
@@ -70,7 +83,7 @@ fn ir_to_stmt<'c>(code: &mut Code<'c>, ir_stmt: &IrStmt<'_>) {
         StmtKind::SetNull => Stmt::SetNull,
         StmtKind::Loop(instr) => {
             let skip_jmp_idx = code.stmts.len();
-            code.stmts.push(Stmt::JmpIfZero(usize::MAX)); // placeholder
+            code.stmts.push(Stmt::JmpIfZero(0)); // placeholder
             code.debug.push(ir_stmt.span);
 
             // compile the loop body now

rust2/src/codegen_interpreter.rs

@@ -30,13 +30,22 @@ where
         mem: [Wrapping(0u8); MEM_SIZE],
     };
 
-    interpreter.execute();
+    // SAFETY: `Code` can only be produced by the `crate::codegen` module, which is trusted to not
+    // produce out of bounds jumps and put the `End` at the end
+    unsafe {
+        interpreter.execute();
+    }
 }
 
 impl<'c, W: Write, R: Read> Interpreter<'c, W, R> {
-    fn execute(&mut self) {
+    unsafe fn execute(&mut self) {
+        let stmts = self.code.stmts();
         loop {
-            let instr = self.code.stmts[self.ip];
+            // SAFETY: If the code ends with an `End` and there are no out of bounds jumps,
+            // `self.ip` will never be out of bounds
+            // Removing this bounds check speeds up execution by about 40%
+            debug_assert!(self.ip < stmts.len());
+            let instr = unsafe { *stmts.get_unchecked(self.ip) };
             self.ip += 1;
             match instr {
                 Stmt::Add(n) => {

rust2/src/lib.rs

@@ -1,4 +1,5 @@
 #![feature(allocator_api, let_else)]
+#![deny(unsafe_op_in_unsafe_fn)]
 #![warn(rust_2018_idioms)]
 #![allow(dead_code)]