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std: add a fixme to note performance issues in vec::from_elem. #7137

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std: add a fixme to note performance issues in vec::from_elem. #7137

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6 changes: 4 additions & 2 deletions src/libstd/vec.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -155,8 +155,10 @@ pub fn from_fn<T>(n_elts: uint, op: old_iter::InitOp<T>) -> ~[T] {
* to the value `t`.
*/
pub fn from_elem<T:Copy>(n_elts: uint, t: T) -> ~[T] {
// hack: manually inline from_fn for 2x plus speedup (sadly very important, from_elem is a
// bottleneck in borrowck!)
// FIXME (#7136): manually inline from_fn for 2x plus speedup (sadly very
// important, from_elem is a bottleneck in borrowck!). Unfortunately it
// still is substantially slower than using the unsafe
// vec::with_capacity/ptr::set_memory for primitive types.
unsafe {
let mut v = with_capacity(n_elts);
do as_mut_buf(v) |p, _len| {
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