From c5fa7776dff913dd75fed6f4e7ed483f0e75e367 Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Sun, 20 Sep 2015 11:04:01 +0300
Subject: [PATCH 1/3] Do not drop_in_place elements of Vec<T> if T doesn't need
 dropping

With -O2, LLVM's inliner can remove this code, but this does not happen
with -O1 and lower. As a result, dropping Vec<u8> was linear with length,
resulting in abysmal performance for large buffers.
---
 src/libcollections/vec.rs | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/libcollections/vec.rs b/src/libcollections/vec.rs
index c99460a55c952..fac846179430e 100644
--- a/src/libcollections/vec.rs
+++ b/src/libcollections/vec.rs
@@ -65,7 +65,7 @@ use alloc::heap::EMPTY;
 use core::cmp::Ordering;
 use core::fmt;
 use core::hash::{self, Hash};
-use core::intrinsics::{arith_offset, assume, drop_in_place};
+use core::intrinsics::{arith_offset, assume, drop_in_place, needs_drop};
 use core::iter::FromIterator;
 use core::mem;
 use core::ops::{Index, IndexMut, Deref};
@@ -1322,8 +1322,10 @@ impl<T> Drop for Vec<T> {
         // OK because exactly when this stops being a valid assumption, we
         // don't need unsafe_no_drop_flag shenanigans anymore.
         if self.buf.unsafe_no_drop_flag_needs_drop() {
-            for x in self.iter_mut() {
-                unsafe { drop_in_place(x); }
+            if unsafe { needs_drop::<T>() } {
+                for x in self.iter_mut() {
+                    unsafe { drop_in_place(x); }
+                }
             }
         }
         // RawVec handles deallocation

From 6beb4ba1aaecb59fdfdc676434b5a3b17a4b5bfc Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Sun, 20 Sep 2015 22:15:39 +0300
Subject: [PATCH 2/3] Fix style.

---
 src/libcollections/vec.rs | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/libcollections/vec.rs b/src/libcollections/vec.rs
index fac846179430e..ddab16493f013 100644
--- a/src/libcollections/vec.rs
+++ b/src/libcollections/vec.rs
@@ -1322,9 +1322,11 @@ impl<T> Drop for Vec<T> {
         // OK because exactly when this stops being a valid assumption, we
         // don't need unsafe_no_drop_flag shenanigans anymore.
         if self.buf.unsafe_no_drop_flag_needs_drop() {
-            if unsafe { needs_drop::<T>() } {
-                for x in self.iter_mut() {
-                    unsafe { drop_in_place(x); }
+            unsafe {
+                if needs_drop::<T>() {
+                    for x in self.iter_mut() {
+                        drop_in_place(x);
+                    }
                 }
             }
         }

From 77f5da77a91af88130fcff3e726931934c71ba3f Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Sun, 20 Sep 2015 22:19:30 +0300
Subject: [PATCH 3/3] Add comment.

---
 src/libcollections/vec.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/libcollections/vec.rs b/src/libcollections/vec.rs
index ddab16493f013..4110faa41b324 100644
--- a/src/libcollections/vec.rs
+++ b/src/libcollections/vec.rs
@@ -1323,6 +1323,8 @@ impl<T> Drop for Vec<T> {
         // don't need unsafe_no_drop_flag shenanigans anymore.
         if self.buf.unsafe_no_drop_flag_needs_drop() {
             unsafe {
+                // The branch on needs_drop() is an -O1 performance optimization.
+                // Without the branch, dropping Vec<u8> takes linear time.
                 if needs_drop::<T>() {
                     for x in self.iter_mut() {
                         drop_in_place(x);