refactor(profiler): remove from optimized builds

While the overhead of profiling is so small that it is difficult to
measure, an optimized build should not be slowed down. Remove the
corresponding code from optimized builds using conditional compilation.

It is possible to opt-in to profiling by disabling `default-features`.

triton-vm/Cargo.toml

@@ -49,6 +49,10 @@ proptest-arbitrary-interop.workspace = true
 serde_json.workspace = true
 test-strategy.workspace = true
 
+[features]
+default = ["no_profile"]
+no_profile = [] # see `profiler.rs` for an explanation of this seemingly backwards feature
+
 [lints]
 workspace = true
 

triton-vm/src/profiler.rs

@@ -8,8 +8,36 @@
 //! This allows multiple threads to run in parallel without interfering with
 //! each other's profiling data.
 //!
+//! # Enabling Profiling
+//!
+//! In release builds, profiling is disabled by default to allow for the fastest
+//! possible proof generation. To enable profiling, either make sure that
+//! `debug_assertions` is set, or add the following to your `Cargo.toml`:
+//!
+//! ```toml
+//! [dependencies]
+//! triton-vm = { version = "x.y.z", default-features = false }
+//! ```
+//!
+//! ### A note on the `no_profile` feature design decision
+//!
+//! The feature `no_profile` _disables_ profiling, and is enabled by default.
+//! This seems backwards. However, it is an expression of how Triton VM favors
+//! performance over profiling. Note [how Cargo resolves dependencies][deps]:
+//! if some dependency is transitively declared multiple times, the _union_ of
+//! all features will be enabled.
+//!
+//! Imagine some dependency `foo` enables a hypothetical `do_profile` feature.
+//! If another dependency `bar` requires the most efficient proof generation,
+//! it would be slowed down by `foo` and could do nothing about it. Instead,
+//! Disabling profiling by <i>en</i>abling the feature `no_profile` allows `bar`
+//! to dictate. This:
+//! 1. makes the profile reports of `foo` disappear, which is sad, but
+//! 1. lets `bar` be fast, which is more important for Triton VM.
+//!
 //! [proving]: crate::stark::Stark::prove
 //! [verifying]: crate::stark::Stark::verify
+//! [deps]: https://doc.rust-lang.org/cargo/reference/features.html#feature-unification
 
 use std::cell::RefCell;
 use std::cmp::max;
@@ -38,17 +66,27 @@ thread_local! {
 
 /// Start profiling. If the profiler is already running, this function cancels
 /// the current profiling session and starts a new one.
+///
+/// See the module-level documentation for information on how to enable profiling.
 pub fn start(profile_name: impl Into<String>) {
-    PROFILER.replace(Some(TritonProfiler::new(profile_name)));
+    if cfg!(any(debug_assertions, not(feature = "no_profile"))) {
+        PROFILER.replace(Some(TritonProfiler::new(profile_name)));
+    }
 }
 
 /// Stop the current profiling session and generate a [`Report`]. If the
-/// profiler is not running, an empty [`Report`] is returned.
+/// profiler is disabled or not running, an empty [`Report`] is returned.
+///
+/// See the module-level documentation for information on how to enable profiling.
 pub fn finish() -> Report {
-    PROFILER
-        .take()
-        .map(|mut profiler| profiler.report())
-        .unwrap_or_default()
+    if cfg!(any(debug_assertions, not(feature = "no_profile"))) {
+        PROFILER
+            .take()
+            .map(|mut profiler| profiler.report())
+            .unwrap_or_default()
+    } else {
+        Report::default()
+    }
 }
 
 #[derive(Debug, Clone, Eq, PartialEq, Hash)]
@@ -92,6 +130,7 @@ impl TritonProfiler {
         }
     }
 
+    #[cfg(any(debug_assertions, not(feature = "no_profile")))]
     fn ignoring(&self) -> bool {
         self.stack
             .last()
@@ -205,12 +244,14 @@ impl TritonProfiler {
         }
     }
 
+    #[cfg(any(debug_assertions, not(feature = "no_profile")))]
     pub fn start(&mut self, name: &str, category: Option<String>) {
         if !self.ignoring() {
             self.plain_start(name, TaskType::Generic, category);
         }
     }
 
+    #[cfg(any(debug_assertions, not(feature = "no_profile")))]
     fn plain_start(
         &mut self,
         name: impl Into<String>,
@@ -235,6 +276,7 @@ impl TritonProfiler {
         });
     }
 
+    #[cfg(any(debug_assertions, not(feature = "no_profile")))]
     pub fn iteration_zero(&mut self, name: &str, category: Option<String>) {
         if self.ignoring() {
             return;
@@ -297,6 +339,7 @@ impl TritonProfiler {
         }
     }
 
+    #[cfg(any(debug_assertions, not(feature = "no_profile")))]
     pub fn stop(&mut self, name: &str) {
         assert!(
             !self.stack.is_empty(),
@@ -423,7 +466,12 @@ impl Report {
 
 impl Default for Report {
     fn default() -> Self {
-        let name = "__empty__".to_string();
+        let name = if cfg!(feature = "no_profile") {
+            "Triton VM's profiler is disabled through feature `no_profile`.".to_string()
+        } else {
+            "__empty__".to_string()
+        };
+
         Self {
             name,
             tasks: vec![],
@@ -583,13 +631,15 @@ impl Display for Report {
 /// The second, optional argument is a task category.
 macro_rules! prof_start {
     ($s:expr, $c:expr) => {{
+        #[cfg(any(debug_assertions, not(feature = "no_profile")))]
         crate::profiler::PROFILER.with_borrow_mut(|profiler| {
             if let Some(profiler) = profiler.as_mut() {
                 profiler.start($s, Some($c.to_string()));
             }
         })
     }};
     ($s:expr) => {{
+        #[cfg(any(debug_assertions, not(feature = "no_profile")))]
         crate::profiler::PROFILER.with_borrow_mut(|profiler| {
             if let Some(profiler) = profiler.as_mut() {
                 profiler.start($s, None);
@@ -606,6 +656,7 @@ pub(crate) use prof_start;
 /// match to prevent the accidental stopping of a different task.
 macro_rules! prof_stop {
     ($s:expr) => {{
+        #[cfg(any(debug_assertions, not(feature = "no_profile")))]
         crate::profiler::PROFILER.with_borrow_mut(|profiler| {
             if let Some(profiler) = profiler.as_mut() {
                 profiler.stop($s);
@@ -622,13 +673,15 @@ pub(crate) use prof_stop;
 /// the loop has to be stopped with [`prof_stop`] after the loop.
 macro_rules! prof_itr0 {
     ($s:expr, $c:expr) => {{
+        #[cfg(any(debug_assertions, not(feature = "no_profile")))]
         crate::profiler::PROFILER.with_borrow_mut(|profiler| {
             if let Some(profiler) = profiler.as_mut() {
                 profiler.iteration_zero($s, Some($c.to_string()));
             }
         })
     }};
     ($s:expr) => {{
+        #[cfg(any(debug_assertions, not(feature = "no_profile")))]
         crate::profiler::PROFILER.with_borrow_mut(|profiler| {
             if let Some(profiler) = profiler.as_mut() {
                 profiler.iteration_zero($s, None);