From b8e47d5555f69c87080f1cefed80a87675517d86 Mon Sep 17 00:00:00 2001
From: Yascha Friesen <yascha@gmail.com>
Date: Thu, 28 Mar 2024 10:29:00 -0700
Subject: [PATCH] Add example code to show (unsafe) implementation of a handler
 that can be used to arbitrarily execute EVM calls before a given evm
 instruction.

---
 crates/revm/src/evm.rs                        | 30 +++++++++++++++----
 .../handler/handle_types/execution_loop.rs    | 16 +++++++++-
 crates/revm/src/handler/mainnet.rs            |  4 +--
 .../src/handler/mainnet/execution_loop.rs     | 14 +++++++--
 4 files changed, 54 insertions(+), 10 deletions(-)

diff --git a/crates/revm/src/evm.rs b/crates/revm/src/evm.rs
index ce0dd8d453..c959a92007 100644
--- a/crates/revm/src/evm.rs
+++ b/crates/revm/src/evm.rs
@@ -208,11 +208,31 @@ impl<EXT, DB: Database> Evm<'_, EXT, DB> {
 
         loop {
             // run interpreter
-            let action = stack_frame
-                .interpreter
-                .run(shared_memory, instruction_table, self);
-            // take shared memory back.
-            shared_memory = stack_frame.interpreter.take_memory();
+            let action = if let Some(action) = {
+                // I'm so sorry draganrakita
+                let mut boxed: Vec<Box<dyn Fn(&mut Interpreter, &mut Evm<'_, EXT, DB>) + 'static>> =
+                    Vec::new();
+                for ins in instruction_table.iter() {
+                    let ins: Box<dyn Fn(&mut Interpreter, &mut Self)> = Box::new(ins);
+                    let ins: Box<dyn Fn(&mut Interpreter, &mut Evm<'_, EXT, DB>) + 'static> =
+                        unsafe { std::mem::transmute(ins) };
+                    boxed.push(ins);
+                }
+
+                let boxed: [Box<dyn Fn(&mut Interpreter, &mut Evm<'_, EXT, DB>) + 'static>; 256] =
+                    unsafe { boxed.try_into().unwrap_unchecked() };
+
+                self.handler.execution_loop().pre_run_interpreter.clone()(self, &boxed, stack_frame)
+            } {
+                action
+            } else {
+                let res = stack_frame
+                    .interpreter
+                    .run(shared_memory, instruction_table, self);
+                // take shared memory back.
+                shared_memory = stack_frame.interpreter.take_memory();
+                res
+            };
 
             let new_frame = match action {
                 InterpreterAction::SubCall {
diff --git a/crates/revm/src/handler/handle_types/execution_loop.rs b/crates/revm/src/handler/handle_types/execution_loop.rs
index 41a2ae9065..9f1e30f194 100644
--- a/crates/revm/src/handler/handle_types/execution_loop.rs
+++ b/crates/revm/src/handler/handle_types/execution_loop.rs
@@ -4,10 +4,11 @@ use crate::{
         CallInputs, CreateInputs, Gas, InstructionResult, InterpreterResult, SharedMemory,
     },
     primitives::{db::Database, Env, Spec},
-    CallStackFrame, Context, FrameOrResult,
+    CallStackFrame, Context, Evm, FrameOrResult,
 };
 use alloc::{boxed::Box, sync::Arc};
 use core::ops::Range;
+use revm_interpreter::{Interpreter, InterpreterAction};
 
 /// Creates the first frame.
 pub type CreateFirstFrameHandle<'a, EXT, DB> =
@@ -57,6 +58,16 @@ pub type FrameSubCreateHandle<'a, EXT, DB> = Arc<
         + 'a,
 >;
 
+/// Handle interpreter pre-run.
+pub type PreRunInterpreterHandle<'a, EXT, DB> = Arc<
+    dyn Fn(
+            &mut Evm<'_, EXT, DB>,
+            &[Box<dyn Fn(&mut Interpreter, &mut Evm<'_, EXT, DB>)>; 256],
+            &mut Box<CallStackFrame>,
+        ) -> Option<InterpreterAction>
+        + 'a,
+>;
+
 /// Handles related to stack frames.
 pub struct ExecutionLoopHandler<'a, EXT, DB: Database> {
     /// Create Main frame
@@ -71,6 +82,8 @@ pub struct ExecutionLoopHandler<'a, EXT, DB: Database> {
     pub sub_call: FrameSubCallHandle<'a, EXT, DB>,
     /// Frame sub crate
     pub sub_create: FrameSubCreateHandle<'a, EXT, DB>,
+    /// Pre-run interpreter
+    pub pre_run_interpreter: PreRunInterpreterHandle<'a, EXT, DB>,
 }
 
 impl<'a, EXT: 'a, DB: Database + 'a> ExecutionLoopHandler<'a, EXT, DB> {
@@ -82,6 +95,7 @@ impl<'a, EXT: 'a, DB: Database + 'a> ExecutionLoopHandler<'a, EXT, DB> {
             frame_return: Arc::new(mainnet::frame_return::<SPEC, EXT, DB>),
             sub_call: Arc::new(mainnet::sub_call::<SPEC, EXT, DB>),
             sub_create: Arc::new(mainnet::sub_create::<SPEC, EXT, DB>),
+            pre_run_interpreter: Arc::new(mainnet::pre_run::<SPEC, EXT, DB>),
         }
     }
 }
diff --git a/crates/revm/src/handler/mainnet.rs b/crates/revm/src/handler/mainnet.rs
index cf92067938..4ab40ac9be 100644
--- a/crates/revm/src/handler/mainnet.rs
+++ b/crates/revm/src/handler/mainnet.rs
@@ -6,8 +6,8 @@ mod pre_execution;
 mod validation;
 
 pub use execution_loop::{
-    create_first_frame, first_frame_return, frame_return, frame_return_with_refund_flag, sub_call,
-    sub_create,
+    create_first_frame, first_frame_return, frame_return, frame_return_with_refund_flag, pre_run,
+    sub_call, sub_create,
 };
 pub use post_execution::{end, output, reimburse_caller, reward_beneficiary};
 pub use pre_execution::{deduct_caller, deduct_caller_inner, load, load_precompiles};
diff --git a/crates/revm/src/handler/mainnet/execution_loop.rs b/crates/revm/src/handler/mainnet/execution_loop.rs
index 0d6fc314e1..aa2a5a3be6 100644
--- a/crates/revm/src/handler/mainnet/execution_loop.rs
+++ b/crates/revm/src/handler/mainnet/execution_loop.rs
@@ -5,11 +5,11 @@ use crate::{
         InterpreterResult, SharedMemory,
     },
     primitives::{Env, Spec, TransactTo},
-    CallStackFrame, Context, FrameData, FrameOrResult,
+    CallStackFrame, Context, Evm, FrameData, FrameOrResult,
 };
 use alloc::boxed::Box;
 use core::ops::Range;
-use revm_interpreter::CallOutcome;
+use revm_interpreter::{CallOutcome, Interpreter, InterpreterAction};
 
 /// Creates first frame.
 #[inline]
@@ -161,6 +161,16 @@ pub fn sub_create<SPEC: Spec, EXT, DB: Database>(
     }
 }
 
+/// Handle intrepreter pre-run.
+#[inline]
+pub fn pre_run<SPEC: Spec, EXT, DB: Database>(
+    _evm: &mut Evm<'_, EXT, DB>,
+    _ins: &[Box<dyn Fn(&mut Interpreter, &mut Evm<'_, EXT, DB>)>; 256],
+    _frame: &mut Box<CallStackFrame>,
+) -> Option<InterpreterAction> {
+    None
+}
+
 #[cfg(test)]
 mod tests {
     use revm_interpreter::primitives::CancunSpec;