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use try_fold instead of try_for_each to reduce compile time #64885

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Oct 2, 2019
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use try_fold instead of try_for_each to reduce compile time #64885

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27 changes: 14 additions & 13 deletions src/libcore/iter/traits/iterator.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1859,14 +1859,13 @@ pub trait Iterator {
Self: Sized, F: FnMut(Self::Item) -> bool
{
#[inline]
fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut(T) -> LoopState<(), ()> {
move |x| {
fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> LoopState<(), ()> {
move |(), x| {
if f(x) { LoopState::Continue(()) }
else { LoopState::Break(()) }
}
}

self.try_for_each(check(f)) == LoopState::Continue(())
self.try_fold((), check(f)) == LoopState::Continue(())
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Thoughts on equality check vs pattern matching here, can it have an effect or none at all?

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@andjo403 andjo403 Oct 1, 2019 •
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made a quick diff in godbolt and there is less code to inline so that is something that I can do
ZN72$LT$example..LoopState$LT$C$C$B$GT$$u20$as$u20$core..cmp..PartialEq$GT$2eq17h37dbcaf2df999e09E is a lot to inline

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@scottmcm scottmcm Oct 1, 2019 •
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I would hope it has no effect, since LoopState<(),()> is an i1 in LLVM...

...and it is in -O, but very different in debug: https://rust.godbolt.org/z/LKOpZ7

Looks like the PartialEq::eq that gets generated is pretty bad, and it's still bad removing the generics: https://rust.godbolt.org/z/o6Nuaw Could there be a "this is a field-less enum so just compare the discriminants" path in the derive? It looks, unfortunately, like as u8 == 1 is the shortest-emitted-IR way to do these checks. And we're avoiding the derives in other places too, like

rust/src/libcore/cmp.rs

Lines 632 to 638 in 702b45e

#[stable(feature = "rust1", since = "1.0.0")]
impl Ord for Ordering {
#[inline]
fn cmp(&self, other: &Ordering) -> Ordering {
(*self as i32).cmp(&(*other as i32))
}
}

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@bluss bluss Oct 1, 2019 •
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Oh interesting. So we could improve here just by implementing PartialEq manually, or even adding a separate method for just discriminant comparison. But then pattern matching works well too. Like, just a method for ".is_continue()"

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but the pattern match avoids having a function to inline completely as long as a function is used the llvm-ir will contain a call and a function

}

/// Tests if any element of the iterator matches a predicate.
Expand Down Expand Up @@ -1913,14 +1912,14 @@ pub trait Iterator {
F: FnMut(Self::Item) -> bool
{
#[inline]
fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut(T) -> LoopState<(), ()> {
move |x| {
fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> LoopState<(), ()> {
move |(), x| {
if f(x) { LoopState::Break(()) }
else { LoopState::Continue(()) }
}
}

self.try_for_each(check(f)) == LoopState::Break(())
self.try_fold((), check(f)) == LoopState::Break(())
}

/// Searches for an element of an iterator that satisfies a predicate.
Expand Down Expand Up @@ -1972,14 +1971,16 @@ pub trait Iterator {
P: FnMut(&Self::Item) -> bool,
{
#[inline]
fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut(T) -> LoopState<(), T> {
move |x| {
fn check<T>(
mut predicate: impl FnMut(&T) -> bool
) -> impl FnMut((), T) -> LoopState<(), T> {
move |(), x| {
if predicate(&x) { LoopState::Break(x) }
else { LoopState::Continue(()) }
}
}

self.try_for_each(check(predicate)).break_value()
self.try_fold((), check(predicate)).break_value()
}

/// Applies function to the elements of iterator and returns
Expand All @@ -2004,14 +2005,14 @@ pub trait Iterator {
F: FnMut(Self::Item) -> Option<B>,
{
#[inline]
fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut(T) -> LoopState<(), B> {
move |x| match f(x) {
fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> LoopState<(), B> {
move |(), x| match f(x) {
Some(x) => LoopState::Break(x),
None => LoopState::Continue(()),
}
}

self.try_for_each(check(f)).break_value()
self.try_fold((), check(f)).break_value()
}

/// Searches for an element in an iterator, returning its index.
Expand Down