[nll] share buffer for `liveness::simulate_block`

[nikomatsakis]

The simulate_block helper takes the set of live variables at the exit of the block and computes the live variables at each point in the block:

rust/src/librustc_mir/util/liveness.rs

Lines 161 to 167 in 764232c

	/// Walks backwards through the statements/terminator in the given
	/// basic block `block`. At each point within `block`, invokes
	/// the callback `op` with the current location and the set of
	/// variables that are live on entry to that location.
	pub fn simulate_block<'tcx, OP>(&self, mir: &Mir<'tcx>, block: BasicBlock, mut callback: OP)
	where
	OP: FnMut(Location, &LocalSet),

It works by creating a temporary bitset that contains the final value from the end of the block to start:

rust/src/librustc_mir/util/liveness.rs

Lines 171 to 172 in 764232c

	// Get a copy of the bits on exit from the block.
	let mut bits = self.outs[block].clone();

Rather than re-allocating this buffer, it would be nice to share it across invocations of simulate_block. This makes sense because simulate_block is really only invoked from one place (apart from debugging output), which is the method add_liveness_constraint. That method is invoked from a simple loop over all basic blocks:

https://github.com/rust-lang/rust/blob/master/src/librustc_mir/borrow_check/nll/type_check/liveness.rs#L52-L54

We could therefore have simulate_block take some extra "buffer" parameter (which would be stored on the stack and passed to add_liveness_constraint).

According to recent profiles, this fn accounts for about 2% of MIR borrowck, though, so this may not be worth it. Still, we're at the point where "every little bit helps!"

[nikomatsakis]

I'm adding this to the Preview 2 milestone, but it is relatively low priority I think. If @spastorino doesn't get to it, we can just defer it.

[nikomatsakis]

@spastorino implemented this but it didn't seem to be much of a win:

https://perf.rust-lang.org/compare.html?start=860d169474acabdc53b9a698f8ce02eba7e0daeb&end=00bcc44fb92c28465c727881355c3235a56a4045&stat=instructions%3Au