From 845fae3462c98b1cd28f933e03f5bc4850670737 Mon Sep 17 00:00:00 2001
From: mrmr1993 <mr_1993@hotmail.co.uk>
Date: Tue, 5 Dec 2023 22:36:18 +0000
Subject: [PATCH] Implement `srl`

---
 optimism/src/mips/interpreter.rs | 26 +++++++++++++++++++++++++-
 optimism/src/mips/witness.rs     | 11 +++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/optimism/src/mips/interpreter.rs b/optimism/src/mips/interpreter.rs
index 1688798677..8f55f38c04 100644
--- a/optimism/src/mips/interpreter.rs
+++ b/optimism/src/mips/interpreter.rs
@@ -526,6 +526,19 @@ pub trait InterpreterEnv {
         position: Self::Position,
     ) -> Self::Variable;
 
+    /// Return the result of shifting `x` by `by`, storing the result in `position`.
+    ///
+    /// # Safety
+    ///
+    /// There are no constraints on the returned value; callers must assert the relationship with
+    /// the source variable `x` and the shift amount `by`.
+    unsafe fn shift_right(
+        &mut self,
+        x: &Self::Variable,
+        by: &Self::Variable,
+        position: Self::Position,
+    ) -> Self::Variable;
+
     /// Returns 1 if `x` is 0, or 0 otherwise, storing the result in `position`.
     ///
     /// # Safety
@@ -665,7 +678,18 @@ pub fn interpret_rtype<Env: InterpreterEnv>(env: &mut Env, instr: RTypeInstructi
             env.set_next_instruction_pointer(next_instruction_pointer + Env::constant(4u32));
             return;
         }
-        RTypeInstruction::ShiftRightLogical => (),
+        RTypeInstruction::ShiftRightLogical => {
+            let rt = env.read_register(&rt);
+            // FIXME: Constrain this value
+            let shifted = unsafe {
+                let pos = env.alloc_scratch();
+                env.shift_right(&rt, &shamt, pos)
+            };
+            env.write_register(&rd, shifted);
+            env.set_instruction_pointer(next_instruction_pointer.clone());
+            env.set_next_instruction_pointer(next_instruction_pointer + Env::constant(4u32));
+            return;
+        }
         RTypeInstruction::ShiftRightArithmetic => (),
         RTypeInstruction::ShiftLeftLogicalVariable => (),
         RTypeInstruction::ShiftRightLogicalVariable => (),
diff --git a/optimism/src/mips/witness.rs b/optimism/src/mips/witness.rs
index eeea858713..795a647d65 100644
--- a/optimism/src/mips/witness.rs
+++ b/optimism/src/mips/witness.rs
@@ -227,6 +227,17 @@ impl<Fp: Field> InterpreterEnv for Env<Fp> {
         res
     }
 
+    unsafe fn shift_right(
+        &mut self,
+        x: &Self::Variable,
+        by: &Self::Variable,
+        position: Self::Position,
+    ) -> Self::Variable {
+        let res = x >> by;
+        self.write_column(position, res.into());
+        res
+    }
+
     unsafe fn test_zero(&mut self, x: &Self::Variable, position: Self::Position) -> Self::Variable {
         let res = if *x == 0 { 1 } else { 0 };
         self.write_column(position, res.into());