From 42320d815e74bfb77933f6ee758f9ea09dfb11b7 Mon Sep 17 00:00:00 2001
From: Nicholas Ward <npward@berkeley.edu>
Date: Fri, 30 Sep 2022 09:13:56 -0700
Subject: [PATCH] fix

---
 evm/Cargo.toml                    |  1 -
 evm/src/cpu/kernel/asm/modexp.asm | 76 +++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+), 1 deletion(-)
 create mode 100644 evm/src/cpu/kernel/asm/modexp.asm

diff --git a/evm/Cargo.toml b/evm/Cargo.toml
index 7df7edd5..5ee3b1ff 100644
--- a/evm/Cargo.toml
+++ b/evm/Cargo.toml
@@ -8,7 +8,6 @@ edition = "2021"
 plonky2 = { path = "../plonky2", default-features = false, features = ["rand", "timing"] }
 plonky2_util = { path = "../util" }
 eth-trie-utils = { git = "https://github.com/mir-protocol/eth-trie-utils.git", rev = "dd3595b4ba7923f8d465450d210f17a2b4e20f96" }
-maybe_rayon = { path = "../maybe_rayon" }
 anyhow = "1.0.40"
 env_logger = "0.9.0"
 ethereum-types = "0.14.0"
diff --git a/evm/src/cpu/kernel/asm/modexp.asm b/evm/src/cpu/kernel/asm/modexp.asm
new file mode 100644
index 00000000..f1b448cb
--- /dev/null
+++ b/evm/src/cpu/kernel/asm/modexp.asm
@@ -0,0 +1,76 @@
+/// Recursive implementation of exp.
+/// Equivalent to:
+///     def exp(x, e):
+///         if e == 0:
+///             # The path where JUMPI does not jump to `step_case`
+///             return 1
+///         else:
+///             # This is under the `step_case` label
+///             return (x if e % 2 else 1) * exp(x * x, e // 2)
+/// Note that this correctly handles exp(0, 0) == 1.
+
+global modexp:
+    // stack: x, e, retdest
+    dup2
+    // stack: e, x, e, retdest
+    %jumpi(step_case)
+    // stack: x, e, retdest
+    pop
+    // stack: e, retdest
+    pop
+    // stack: retdest
+    push 1
+    // stack: 1, retdest
+    swap1
+    // stack: retdest, 1
+    jump
+
+step_case:
+    // stack: x, e, retdest
+    push recursion_return
+    // stack: recursion_return, x, e, retdest
+    push 2
+    // stack: 2, recursion_return, x, e, retdest
+    dup4
+    // stack: e, 2, recursion_return, x, e, retdest
+    div
+    // stack: e / 2, recursion_return, x, e, retdest
+    dup3
+    // stack: x, e / 2, recursion_return, x, e, retdest
+    %square
+    // stack: x * x, e / 2, recursion_return, x, e, retdest
+    %jump(exp)
+recursion_return:
+    // stack: exp(x * x, e / 2), x, e, retdest
+    push 2
+    // stack: 2, exp(x * x, e / 2), x, e, retdest
+    dup4
+    // stack: e, 2, exp(x * x, e / 2), x, e, retdest
+    mod
+    // stack: e % 2, exp(x * x, e / 2), x, e, retdest
+    push 1
+    // stack: 1, e % 2, exp(x * x, e / 2), x, e, retdest
+    dup4
+    // stack: x, 1, e % 2, exp(x * x, e / 2), x, e, retdest
+    sub
+    // stack: x - 1, e % 2, exp(x * x, e / 2), x, e, retdest
+    mul
+    // stack: (x - 1) * (e % 2), exp(x * x, e / 2), x, e, retdest
+    push 1
+    // stack: 1, (x - 1) * (e % 2), exp(x * x, e / 2), x, e, retdest
+    add
+    // stack: 1 + (x - 1) * (e % 2), exp(x * x, e / 2), x, e, retdest
+    mul
+    // stack: (1 + (x - 1) * (e % 2)) * exp(x * x, e / 2), x, e, retdest
+    swap3
+    // stack: retdest, x, e, (1 + (x - 1) * (e % 2)) * exp(x * x, e / 2)
+    swap2
+    // stack: e, x, retdest, (1 + (x - 1) * (e % 2)) * exp(x * x, e / 2)
+    pop
+    // stack: x, retdest, (1 + (x - 1) * (e % 2)) * exp(x * x, e / 2)
+    pop
+    // stack: retdest, (1 + (x - 1) * (e % 2)) * exp(x * x, e / 2)
+    jump
+
+global sys_exp:
+    PANIC