Avoid needless allocations in `liveness_of_locals`.

[nnethercote]

We don't need to replace the heap-allocated bitset, we can just
overwrite its contents.

This speeds up most NLL benchmarks, the best by 1.5%.

r? @nikomatsakis

[nikomatsakis]

This method probably belongs on IdxSet rather than IdxSetBuf (note that IdxSetBuf derefs to IdxSet). The relationship between those two types is analogous to [] and Vec, so most operations that don't change the size or require allocation belong on IdxSet. There is however an existing clone_from method on that type that does the same thing:

https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/indexed_set/struct.IdxSet.html#method.clone_from

But maybe we should rename that method (IdxSet::clone_from) to overwrite? I wonder if we are having shadowing problems because of Clone::clone_from....

[nnethercote]

You are right about the shadowing. Here's the DHAT output:

==60110== -------------------- 1 of 500 --------------------
==60110== max-live:    217,728 in 1,701 blocks
==60110== tot-alloc:   77,545,592 in 1,330,065 blocks (avg size 58.30)
==60110== deaths:      1,330,065, at avg age 2,374,592 (0.00% of prog lifetime)
==60110== acc-ratios:  0.07 rd, 1.51 wr  (5,953,680 b-read, 117,537,792 b-written)
==60110==    at 0x4C2DECF: malloc (in /usr/lib/valgrind/vgpreload_exp-dhat-amd64-linux.so)
==60110==    by 0x7C016B0: alloc (alloc.rs:62)
==60110==    by 0x7C016B0: alloc (alloc.rs:123)
==60110==    by 0x7C016B0: allocate_in<usize,alloc::alloc::Global> (raw_vec.rs:103)
==60110==    by 0x7C016B0: with_capacity<usize> (raw_vec.rs:147)
==60110==    by 0x7C016B0: with_capacity<usize> (vec.rs:362)
==60110==    by 0x7C016B0: to_vec<usize> (slice.rs:167)
==60110==    by 0x7C016B0: to_vec<usize> (slice.rs:369)
==60110==    by 0x7C016B0: clone<usize> (vec.rs:1670)
==60110==    by 0x7C016B0: clone<rustc::mir::Local> (indexed_set.rs:36)
==60110==    by 0x7C016B0: clone_from<rustc_data_structures::indexed_set::IdxSetBuf<rustc::mir::Local>> (clone.rs:131)
==60110==    by 0x7C016B0: rustc_mir::util::liveness::liveness_of_locals (liveness.rs:144)
==60110==    by 0x7B22D4B: compute (liveness.rs:106)
==60110==    by 0x7B22D4B: rustc_mir::borrow_check::nll::compute_regions (mod.rs:101)
==60110==    by 0x7B33381: rustc_mir::borrow_check::do_mir_borrowck (mod.rs:238)

IdxSetBuf implements Clone, and therefore defines clone_from, which is a provided method that calls clone. IdxSet has a clone_from method, which is probably intended to be called, but the IdxSetBuf implementation is called instead.

I think renaming IdxSet::clone_from as IdxSet::overwrite is reasonable. I wonder if there are any other places like this, where the clone_from being called is not the one intended.

[nikomatsakis]

There aren't many definitions of clone_from, so it wouldn't be hard to audit.

[nnethercote]

I have updated. r? @nikomatsakis

[nikomatsakis]

rg 'fn clone_from\b' reports:

src/liballoc/binary_heap.rs:288:    fn clone_from(&mut self, source: &Self) {
src/liballoc/boxed.rs:302:    fn clone_from(&mut self, source: &Box<T>) {
src/liballoc/borrow.rs:173:    fn clone_from(&mut self, source: &Cow<'a, B>) {
src/liballoc/vec.rs:1682:    fn clone_from(&mut self, other: &Vec<T>) {
src/liballoc/string.rs:1688:    fn clone_from(&mut self, source: &Self) {
src/libcore/clone.rs:130:    fn clone_from(&mut self, source: &Self) {
src/libcore/mem.rs:1040:    fn clone_from(&mut self, source: &Self) {
src/librustc_data_structures/indexed_set.rs:236:    pub fn clone_from(&mut self, other: &IdxSet<T>) {

So...seems ok. =)

[nikomatsakis]

@bors r+

[bors]

📌 Commit 08683f0 has been approved by nikomatsakis

[bors]

⌛ Testing commit 08683f0 with merge e953e46...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing e953e46 to master...