part: Removed unneeded 2nd thread spawn in `SearchPar`"s `search_combined()`: Backwards searching is done on main thread, forward searching is done on background thread.

Main thread yields once before starting its search loop.

Fortune for reverse's current commit: Curse − 凶

src/ext.rs

@@ -0,0 +1,162 @@
+//! Extension traits, global functions + macros.
+#![allow(unused)]
+use super::*;
+use std::convert::Infallible;
+
+/// The default bottom type.
+///
+/// To use the `unwrap_infallible()`-like interface, functions that return `-> !` should be changed to `-> Never`.
+/// When `unstable` is enabled, this is an alias to `!` and `-> !` is not special cased.
+///
+/// # As return argument
+/// When feature `unstable` is enabled, `into_unreachable()` may not be required to ensure propogation to `!` from a function returning `-> Never`.
+#[cfg(feature="unstable")] 
+pub type Never = !;
+
+/// The default bottom type.
+///
+/// To use the `unwrap_infallible()`-like interface, functions special cased to `-> !` should be changed to `-> Never`.
+///
+/// # As return argument
+/// When feature `unstable` is not enabled, `into_unreachable()` may be required to be used when dealing with return bottom types other than the special case `-> !`.
+/// This is a current limitation of the type system.
+#[cfg(not(feature="unstable"))] 
+pub type Never = Infallible;
+
+/// Contractually ensures this type cannot exist (i.e. it is a bottom type.)
+///
+/// # Safety
+/// Instances of the impl type **cannot exist**.
+/// They must be bottom types (i.e. empty enums, types contatining an `Infallible` / `!` object, etc.)
+///
+/// # Auto-impl
+/// This trait is not intended to be implemented on any user-defined type other than empty enums.
+///
+/// By default it is implemented for the following types:
+/// - `core::convert::Infallible`
+/// - `!` (**feature**: `unstable`)
+/// - `Box<T>` *where* `T: ?Sized + Unreachable`
+pub unsafe trait Unreachable {
+    /// Force control flow to terminate type checking here.
+    ///
+    /// # Note
+    /// This function will never be executed, it is used to terminate the value's existence in the type system, by converting it from any `Unreachable` type into the bottom return type `!`.
+    /// If this function ever **can** be called at all, it is undefined behaviour.
+    #[inline]
+    #[cold]
+    fn into_unreachable(self) -> !
+    where Self: Sized
+    {
+	if cfg!(debug_assertions) {
+	    unreachable!("Unreachable conversion from {}!", std::any::type_name::<Self>())
+	} else {
+	    // SAFETY: Contractually enforced by the trait impl itself.
+	    unsafe {
+		std::hint::unreachable_unchecked()
+	    }
+	}
+    }
+}
+
+unsafe impl Unreachable for Infallible {
+    #[inline(always)]
+    #[cold]
+    fn into_unreachable(self) -> ! {
+	match self {}
+    }
+}
+#[cfg(feature="unstable")] 
+unsafe impl Unreachable for ! {
+    #[inline(always)]
+    #[cold]
+    fn into_unreachable(self) -> ! {
+	match self {}
+    }
+}
+
+unsafe impl<T: ?Sized + Unreachable> Unreachable for Box<T> {}
+
+pub trait UnwrapPanicExt<T, E> {
+    /// Unwrap the result `Ok` value or panic as described by the non-returning function `F`, with the `Unreachable` bottom type `N`.
+    /// This will usually be an `-> !` function, (or an `-> Never` function using the `Unreachable` interface.)
+    ///
+    /// # Panic usage
+    /// `func` must not return. It should panic, resume a panic, or exit the thread/program, trap, or terminate in an infinite loop.
+    ///
+    /// It does not *have* to call `panic!()` if it terminates in another way that is not a panic, however.
+    fn unwrap_or_panic<N: Unreachable, F: FnOnce(E) -> N>(self, func: F) -> T;
+    
+    /// Unwrap the result `Ok` value or panic as described by the non-returning function `func` with the default bottom type `Never`.
+    #[inline(always)] 
+    fn unwrap_or_panic_unreachable<F: FnOnce(E) -> Never>(self, func: F) -> T
+    where Self: Sized {
+	self.unwrap_or_panic::<Never, _>(func)
+    }
+}
+
+pub trait UnwrapInfallibleExt<T> {
+    /// Unwrapping is infallible and therefore safe to do so without checking.
+    fn unwrap_infallible(self) -> T;
+}
+
+pub trait UnwrapPanicResumeExt<T> {
+    /// Unwrap or resume a previous unwind, with the unwind payload in the `Err` variant.
+    fn unwrap_or_resume(self) -> T;
+}
+
+impl<T, E> UnwrapPanicExt<T, E> for Result<T, E>
+{
+    #[inline] 
+    fn unwrap_or_panic<N: Unreachable, F: FnOnce(E) -> N>(self, func: F) -> T {
+	#[inline(never)]
+	#[cold]
+	fn _do_panic<Nn: Unreachable, Ee, Ff: FnOnce(Ee) -> Nn>(error: Ee, func: Ff) -> !
+	{
+	    func(error).into_unreachable()
+	}
+
+	match self {
+	    Ok(v) => v,
+	    Err(e) => _do_panic(e, func)
+	}
+    }
+}
+
+impl<T, E: Unreachable> UnwrapInfallibleExt<T> for Result<T, E>
+{
+    #[inline] 
+    fn unwrap_infallible(self) -> T {
+	match self {
+	    Ok(v) => v,
+	    Err(e) => if cfg!(debug_assertions) {
+		e.into_unreachable()
+	    } else {
+		// SAFETY: Contract bound of `E: Unreachable` ensures this path will never be taken.
+		unsafe {
+		    std::hint::unreachable_unchecked()
+		}
+	    }
+	}
+    }
+}
+
+/// The type of a caught unwind payload.
+pub type UnwindPayload = Box<dyn std::any::Any + Send>;
+
+#[cold]
+#[inline(never)]
+fn _resume_unwind<E: Into<UnwindPayload>>(e: E) -> !
+{
+    std::panic::resume_unwind(e.into())   
+}
+
+impl<T, E: Into<UnwindPayload>> UnwrapPanicResumeExt<T> for Result<T, E>
+{
+    #[inline] 
+    fn unwrap_or_resume(self) -> T {
+	match self {
+	    Ok(v) => v,
+	    Err(e) => _resume_unwind(e),
+	}
+    }
+}

src/main.rs

@@ -1,4 +1,7 @@
+#![cfg_attr(feature="unstable", feature(never_type))] // See `Never`.
 
+
+#[macro_use] mod ext; use ext::*;
 mod part;
 
 //#[inline] 

src/part.rs

@@ -33,13 +33,13 @@ impl MidpointFBSearcher<u8> for SearchSeq
     #[inline] 
     fn search_combined<'a>(&self, haystack: &'a [u8], begin: usize, needle: u8) -> Option<&'a u8> {
 	let max_cap = match get_max_pivot_search_area(haystack.len() > (DEFAULT_PIVOT_MAX_SEARCH_AREA * DEFAULT_MEM_DETECT_HUGE_SIZE_PAGES)){ // Assume huge-page memory if len is larger than 4 pages.
-		
-		// On debug builds, cap search area to one system page only.
-		_ignore if cfg!(debug_assertions) && (*REAL_PAGE_SIZE) > 0 =>
-		// SAFETY: We have checked if `*REAL_PAGE_SIZE` is non-zero above.
-		    Some(unsafe { NonZeroUsize::new_unchecked(*REAL_PAGE_SIZE as usize) }),
-		// Otherwise, use the detected value.
-		cap => cap,
+	    
+	    // On debug builds, cap search area to one system page only.
+	    _ignore if cfg!(debug_assertions) && (*REAL_PAGE_SIZE) > 0 =>
+	    // SAFETY: We have checked if `*REAL_PAGE_SIZE` is non-zero above.
+		Some(unsafe { NonZeroUsize::new_unchecked(*REAL_PAGE_SIZE as usize) }),
+	    // Otherwise, use the detected value.
+	    cap => cap,
 	};
 
 	
@@ -236,25 +236,31 @@ impl MidpointFBSearcher<u8> for SearchPar
 	    };
 	    // Cap the cap to `max_cap` if there is a max cap.
 	    let cap = if let Some(max) = max_cap.as_ref() {
-		    std::cmp::min(max.get(), self.cap_start)
-		} else {
-		    self.cap_start
-		};
+		std::cmp::min(max.get(), self.cap_start)
+	    } else {
+		self.cap_start
+	    };
 
 	    let forward = if hf.len() > 0 {
 		let cap = cap;
 		let sf = &self;
 		let complete = &complete;
-		Some(s.spawn(move || -> Option<_> {
+		// Background thread: Forward search (`forward-searcher`.)
+		Some(std::thread::Builder::new().name("forward-searcher".into()).spawn_scoped(s, move || -> Option<_> {
 		    let mut cap = std::cmp::min(cap, hf.len());
 		    let len = hf.len();
+		    
+		    // Check completion before starting loop too.
+		    if complete.load() {
+			return None;
+		    }
 		    while cap <= len {
 			// If `cap` is larger than the buffer `hf`, truncate it.
 			cap = std::cmp::min(cap, hf.len());
 			// Search forward in `hf` up to `cap` bytes.
 			if let /*v @ */Some(x) = sf.search_forward(&hf[..cap], needle) {
+			    // Tell other operation we have found something.
 			    complete.store(true);
-			    //complete.call_once(|| complete_val = true);
 			    return Some(x);
 			} else if complete.load() {
 			    break;
@@ -265,24 +271,34 @@ impl MidpointFBSearcher<u8> for SearchPar
 			cap = max_cap.map(|max| std::cmp::min(max.get(), cap << 1)).unwrap_or_else(|| cap << 1);
 		    }
 		    None::<&'a u8>
-		}))
+		}).expect("Failed to spawn forward-searcher thread"))
 	    } else {
 		None
 	    };
+	    //NOTE: There is no need to spawn another thread for the 2nd operation, since they are both join()'d at the end regardless and both already communicate completion.
 	    let backward = if hb.len() > 0 {
 		let cap = cap;
 		let sf = &self;
 		let complete = &complete;
-		Some(s.spawn(move || -> Option<_> {
+
+		// Main thread: Backwards search.
+		move || -> Option<_> {
 		    let mut cap = std::cmp::min(cap, hb.len());
 		    let len = hb.len();
+
+		    // Check completion before starting loop too.
+		    if complete.load() {
+			return None;
+		    } else {
+			// Allow previous thread to run if it is not.
+			std::thread::yield_now(); 
+		    }
 		    while cap <= len {
 			// If `cap` is larger than the buffer `hb`, truncate it.
 			cap = std::cmp::min(cap, hb.len());
 			// Search backwards in `hb` up to `cap` bytes.
 			if let /*v @ */Some(x) = sf.search_backward(&hb[(hb.len()-cap)..], needle) {
 			    complete.store(true);
-			    //complete.call_once(|| complete_val = true);
 			    return Some(x);
 			} else if complete.load() {
 			    break;
@@ -293,17 +309,32 @@ impl MidpointFBSearcher<u8> for SearchPar
 			cap = max_cap.map(|max| std::cmp::min(max.get(), cap << 1)).unwrap_or_else(|| cap << 1);
 		    }
 		    None::<&'a u8>
-		}))
+		}()
 	    } else {
 		None
 	    };
+	    if backward.is_some() && forward.as_ref().map(|th| !th.is_finished()).unwrap_or(false) {
+		// `backward` found something, `forward` is still running.
+		debug_assert_ne!(complete.load(), false, "Complete has not been set! (main thread waiting for forward-searcher thread");
+		complete.store(true);
+	    }
+
+	    #[cold]
+	    #[inline(never)]
+	    fn _resume_unwind(e: Box<dyn std::any::Any + Send>) -> Never
+	    {
+		if cfg!(debug_assertions) {
+		    panic!("forward-searcher thread panic")
+		} else {
+		    std::panic::resume_unwind(e)
+		}
+	    }
 
 	    match (forward, backward) {
 		(None, None) => None,
-		(None, Some(back)) => back.join().unwrap_or(None),
-		(Some(forward), None) => forward.join().unwrap_or(None),
-		(Some(forward), Some(backward)) => forward.join().unwrap_or(None).
-		    or_else(move || backward.join().unwrap_or(None)),
+		(None, back @ Some(_)) => back,
+		(Some(forward), backward) => backward.or_else(move || forward.join().unwrap_or_panic(_resume_unwind)),
+		//(Some(forward), Some(_)) => Handled ^
 	    }
 
 	})
@@ -338,17 +369,17 @@ mod test
     }
 
     //TODO: Thread-reusing parallel `MidpointFBSearcher` (SearchSeq is thread-*spawning*; heavy.) This may require we use async and tasks. If it does, we should also create a `SearchAsync` partitioner (XXX: MidpointFBSearcher is currently a synchonous-only interface; a pure-async pivot finder may require a refactor.)
-	#[cfg(all(feature="threads-async", feature = "threads"))] 
-	#[test]
-	fn partition_par_light()
-	{
-	    unimplemented!("A light (thread-*reusing*) parallel searcher has not yet been implemented")
-	}
+    #[cfg(all(feature="threads-async", feature = "threads"))] 
+    #[test]
+    fn partition_par_light()
+    {
+	unimplemented!("A light (thread-*reusing*) parallel searcher has not yet been implemented")
+    }
 
-	#[cfg(feature="threads-async")] 
-	#[/*tokio::*/test]
-	fn partition_par_async()
-	{
-	    unimplemented!("A pure async parallel searcher has not yet been implemented")
-	}
+    #[cfg(feature="threads-async")] 
+    #[/*tokio::*/test]
+    fn partition_par_async()
+    {
+	unimplemented!("A pure async parallel searcher has not yet been implemented")
+    }
 }