Auto merge of #26849 - bluss:read-to-end-memset, r=alexcrichton

Improve zerofill in Vec::resize and Read::read_to_end

We needed a more efficient way to zerofill the vector in read_to_end.
This to reduce the memory intialization overhead to a minimum.

Use the implementation of `std::vec::from_elem` (used for the vec![]
macro) for Vec::resize as well. For simple element types like u8, this
compiles to memset, so it makes Vec::resize much more efficient.

3 files changed, 12 insertions, 6 deletions

diff --git a/src/libstd/io/buffered.rs b/src/libstd/io/buffered.rs
index d389a0be117..98d8bc6bdaf 100644
--- a/src/libstd/io/buffered.rs
+++ b/src/libstd/io/buffered.rs
@@ -19,7 +19,6 @@ use error;
 use fmt;
 use io::{self, DEFAULT_BUF_SIZE, Error, ErrorKind, SeekFrom};
 use ptr;
-use iter;
 
 /// Wraps a `Read` and buffers input from it.
 ///
@@ -63,11 +62,9 @@ impl<R: Read> BufReader<R> {
     /// Creates a new `BufReader` with the specified buffer capacity.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn with_capacity(cap: usize, inner: R) -> BufReader<R> {
-        let mut buf = Vec::with_capacity(cap);
-        buf.extend(iter::repeat(0).take(cap));
         BufReader {
             inner: inner,
-            buf: buf,
+            buf: vec![0; cap],
             pos: 0,
             cap: 0,
         }
diff --git a/src/libstd/io/mod.rs b/src/libstd/io/mod.rs
index 9021f32fad0..50c44299dc7 100644
--- a/src/libstd/io/mod.rs
+++ b/src/libstd/io/mod.rs
@@ -16,7 +16,7 @@ use cmp;
 use rustc_unicode::str as core_str;
 use error as std_error;
 use fmt;
-use iter::{self, Iterator, Extend};
+use iter::{Iterator};
 use marker::Sized;
 use ops::{Drop, FnOnce};
 use option::Option::{self, Some, None};
@@ -106,7 +106,7 @@ fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize>
             if new_write_size < DEFAULT_BUF_SIZE {
                 new_write_size *= 2;
             }
-            buf.extend(iter::repeat(0).take(new_write_size));
+            buf.resize(len + new_write_size, 0);
         }
 
         match r.read(&mut buf[len..]) {
@@ -984,6 +984,14 @@ mod tests {
         let mut v = Vec::new();
         assert_eq!(c.read_to_end(&mut v).unwrap(), 1);
         assert_eq!(v, b"1");
+
+        let cap = 1024 * 1024;
+        let data = (0..cap).map(|i| (i / 3) as u8).collect::<Vec<_>>();
+        let mut v = Vec::new();
+        let (a, b) = data.split_at(data.len() / 2);
+        assert_eq!(Cursor::new(a).read_to_end(&mut v).unwrap(), a.len());
+        assert_eq!(Cursor::new(b).read_to_end(&mut v).unwrap(), b.len());
+        assert_eq!(v, data);
     }
 
     #[test]
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 73e45619774..caf3f497e10 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -146,6 +146,7 @@
 #![feature(unique)]
 #![feature(unsafe_no_drop_flag, filling_drop)]
 #![feature(vec_push_all)]
+#![feature(vec_resize)]
 #![feature(wrapping)]
 #![feature(zero_one)]
 #![cfg_attr(windows, feature(str_utf16))]