From 677e0234ff49641e92a0accbfa628ae7d8ec7d70 Mon Sep 17 00:00:00 2001 From: Chayim Refael Friedman Date: Thu, 20 Mar 2025 18:50:59 +0200 Subject: [PATCH] Use internal iteration in `Vec::extend_desugared()` Because LLVM is unable to optimize well external iteration with some iterator kinds (e.g. `chain()`). To do that I had to hoist the `size_hint()` call to the beginning of the loop (since I no longer have access to the iterator inside the loop), which might slightly pessimize certain iterators that are able to give more accurate size bounds during iteration (e.g. `flatten()`). However, the effect should not be big, and also, common iterators like these also suffer from the external iteration optimizibility problem (e.g. `flatten()`). --- library/alloc/src/vec/mod.rs | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs index 3782f9e95194c..ac263ccaca839 100644 --- a/library/alloc/src/vec/mod.rs +++ b/library/alloc/src/vec/mod.rs @@ -3543,11 +3543,12 @@ impl Vec { // for item in iterator { // self.push(item); // } - while let Some(element) = iterator.next() { + let (lower, _) = iterator.size_hint(); + let initial_len = self.len(); + iterator.for_each(move |element| { let len = self.len(); if len == self.capacity() { - let (lower, _) = iterator.size_hint(); - self.reserve(lower.saturating_add(1)); + self.reserve(lower.saturating_sub(len - initial_len).saturating_add(1)); } unsafe { ptr::write(self.as_mut_ptr().add(len), element); @@ -3556,7 +3557,7 @@ impl Vec { // NB can't overflow since we would have had to alloc the address space self.set_len(len + 1); } - } + }); } // specific extend for `TrustedLen` iterators, called both by the specializations