From 18d317b9cf2f859206e4b3ca939ed799e7aef294 Mon Sep 17 00:00:00 2001
From: Robin Salen <salenrobin@gmail.com>
Date: Wed, 5 Apr 2023 06:35:52 -0400
Subject: [PATCH] Implement sdiv in interpreter.

---
 evm/src/cpu/kernel/interpreter.rs | 108 +++++++++++++++++++++++++++++-
 1 file changed, 107 insertions(+), 1 deletion(-)

diff --git a/evm/src/cpu/kernel/interpreter.rs b/evm/src/cpu/kernel/interpreter.rs
index 3f699260..49ff2222 100644
--- a/evm/src/cpu/kernel/interpreter.rs
+++ b/evm/src/cpu/kernel/interpreter.rs
@@ -1,5 +1,6 @@
 //! An EVM interpreter for testing and debugging purposes.
 
+use core::cmp::Ordering;
 use std::collections::HashMap;
 use std::ops::Range;
 
@@ -305,7 +306,7 @@ impl<'a> Interpreter<'a> {
             0x02 => self.run_mul(),                                     // "MUL",
             0x03 => self.run_sub(),                                     // "SUB",
             0x04 => self.run_div(),                                     // "DIV",
-            0x05 => todo!(),                                            // "SDIV",
+            0x05 => self.run_sdiv(),                                    // "SDIV",
             0x06 => self.run_mod(),                                     // "MOD",
             0x07 => todo!(),                                            // "SMOD",
             0x08 => self.run_addmod(),                                  // "ADDMOD",
@@ -467,6 +468,41 @@ impl<'a> Interpreter<'a> {
         self.push(if y.is_zero() { U256::zero() } else { x / y });
     }
 
+    fn run_sdiv(&mut self) {
+        let mut x = self.pop();
+        let mut y = self.pop();
+
+        let y_is_zero = y.is_zero();
+
+        if y_is_zero {
+            self.push(U256::zero());
+        } else if y.eq(&MINUS_ONE) && x.eq(&MIN_VALUE) {
+            self.push(MIN_VALUE);
+        } else {
+            let x_is_pos = x.eq(&(x & SIGN_MASK));
+            let y_is_pos = y.eq(&(y & SIGN_MASK));
+
+            // We compute the absolute quotient first,
+            // then adapt its sign based on the operands.
+            if !x_is_pos {
+                x = two_complement(x);
+            }
+            if !y_is_pos {
+                y = two_complement(y);
+            }
+            let div = x / y;
+            if div.eq(&U256::zero()) {
+                self.push(U256::zero());
+            }
+
+            self.push(if x_is_pos == y_is_pos {
+                div
+            } else {
+                two_complement(div)
+            });
+        }
+    }
+
     fn run_mod(&mut self) {
         let x = self.pop();
         let y = self.pop();
@@ -837,6 +873,76 @@ impl<'a> Interpreter<'a> {
     }
 }
 
+// Computes the two's complement of the given integer.
+fn two_complement(x: U256) -> U256 {
+    let flipped_bits = x ^ MINUS_ONE;
+    flipped_bits.overflowing_add(U256::one()).0
+}
+
+fn signed_cmp(x: U256, y: U256) -> Ordering {
+    let x_is_zero = x.is_zero();
+    let y_is_zero = y.is_zero();
+
+    if x_is_zero && y_is_zero {
+        return Ordering::Equal;
+    }
+
+    let x_is_pos = x.eq(&(x & SIGN_MASK));
+    let y_is_pos = y.eq(&(y & SIGN_MASK));
+
+    if x_is_zero {
+        if y_is_pos {
+            return Ordering::Less;
+        } else {
+            return Ordering::Greater;
+        }
+    };
+
+    if y_is_zero {
+        if x_is_pos {
+            return Ordering::Greater;
+        } else {
+            return Ordering::Less;
+        }
+    };
+
+    match (x_is_pos, y_is_pos) {
+        (true, true) => x.cmp(&y),
+        (true, false) => Ordering::Greater,
+        (false, true) => Ordering::Less,
+        (false, false) => x.cmp(&y).reverse(),
+    }
+}
+
+const MINUS_ZERO: U256 = U256([
+    0x0000000000000001,
+    0x0000000000000000,
+    0x0000000000000000,
+    0x8000000000000000,
+]);
+
+const MINUS_ONE: U256 = U256([
+    0xffffffffffffffff,
+    0xffffffffffffffff,
+    0xffffffffffffffff,
+    0xffffffffffffffff,
+]);
+
+/// -2^255 in two's complement representation consists in the MSB set to 1.
+const MIN_VALUE: U256 = U256([
+    0x0000000000000000,
+    0x0000000000000000,
+    0x0000000000000000,
+    0x8000000000000000,
+]);
+
+const SIGN_MASK: U256 = U256([
+    0xffffffffffffffff,
+    0xffffffffffffffff,
+    0xffffffffffffffff,
+    0x7fffffffffffffff,
+]);
+
 /// Return the (ordered) JUMPDEST offsets in the code.
 fn find_jumpdests(code: &[u8]) -> Vec<usize> {
     let mut offset = 0;