Previous Stacked Borrows diagnostics were missing a lot of information
about the state of the interpreter, and it was difficult to add
additional state because it was threaded through all the intervening
function signatures.
This change factors a lot of the arguments which used to be passed
individually to many stacked borrows functions into a single
`DiagnosticCx`, which is built in `Stacks::for_each`, and since it
wraps a handle to `AllocHistory`, we can now handle more nuanced
things like heterogeneous borrow of `!Freeze` types.
Previously, Item was a struct of a NonZeroU64, an Option which was
usually unset or irrelevant, and a 4-variant enum. So collectively, the
size of an Item was 24 bytes, but only 8 bytes were used for the most
part.
So this takes advantage of the fact that it is probably impossible to
exhaust the total space of SbTags, and steals 3 bits from it to pack the
whole struct into a single u64. This bit-packing means that we reduce
peak memory usage when Miri goes memory-bound by ~3x. We also get CPU
performance improvements of varying size, because not only are we simply
accessing less memory, we can now compare a Vec<Item> using a memcmp
because it does not have any padding.
This adds a very simple LRU-like cache which stores the locations of
often-used tags. While the implementation is very simple, the cache hit
rate is incredible at ~99.9% on most programs, and often the element at
position 0 in the cache has a hit rate of 90%. So the sub-optimality of
this cache basicaly vanishes into the noise in a profile.
Additionally, we keep a range which denotes where there might be an item
granting Unique permission in the stack, so that when we invalidate
Uniques we do not need to scan much of the stack, and often scan nothing
at all.
* Store the local crates in an Rc<[CrateNum]>
* Move all the allocation history into Stacks
* Clean up the implementation of get_logs_relevant_to a bit
* Pass a ThreadInfo down to grant/access to get the current span lazily
* Rename add_* to log_* for clarity
* Hoist borrow_mut calls out of loops by tweaking the for_each signature
* Explain the parameters of check_protector a bit more
