Reintroduce `Undef` and properly check constant value sizes
r? @RalfJung
cc @eddyb
basically all kinds of silent failures that never occurred are assertions now
introduce universes to NLL type check
This branch aims to fix#48071 and also advance chalk integration a bit at the same time. It re-implements the subtyping/type-equating check so that NLL doesn't "piggy back" on the subtyping code of the old type checker.
This new code uses the "universe-based" approach to handling higher-ranked lifetimes, which sidesteps some of the limitations of the current "leak-based" scheme. This avoids the ICE in #48071.
At the same time, I aim for this to potentially be a kind of optimization. This NLL code is (currently) not cached, but it also generates constraints without doing as much instantiation, substitution, and folding. Right now, though, it still piggy backs on the `relate_tys` trait, which is a bit unfortunate -- it means we are doing more hashing and things than we have to. I want to measure the see the perf. Refactoring that trait is something I'd prefer to leave for follow-up work.
r? @pnkfelix -- but I want to measure perf etc first
Add unaligned volatile intrinsics
Surprisingly enough, it turns out that unaligned volatile loads are actually useful for certain (very niche) types of lock-free code. I included unaligned volatile stores for completeness, but I currently do not know of any use cases for them.
These are only exposed as intrinsics for now. If they turn out to be useful in practice, we can work towards stabilizing them.
r? @alexcrichton
nll experiment: compute SCCs instead of iterative region solving
This is an attempt to speed up region solving by replacing the current iterative dataflow with a SCC computation. The idea is to detect cycles (SCCs) amongst region constraints and then compute just one value per cycle. The graph with all cycles removed is of course a DAG, so we can then solve constraints "bottom up" once the liveness values are known.
I kinda ran out of time this morning so the last commit is a bit sloppy but I wanted to get this posted, let travis run on it, and maybe do a perf run, before I clean it up.
Store scalar pair bools as i8 in memory
We represent `bool` as `i1` in a `ScalarPair`, unlike other aggregates,
to optimize IR for checked operators and the like. With this patch, we
still do so when the pair is an immediate value, but we use the `i8`
memory type when the value is loaded or stored as an LLVM aggregate.
So `(bool, bool)` looks like an `{ i1, i1 }` immediate, but `{ i8, i8 }`
in memory. When a pair is a direct function argument, `PassMode::Pair`,
it is still passed using the immediate `i1` type, but as a return value
it will use the `i8` memory type. Also, `bool`-like` enum tags will now
use scalar pairs when possible, where they were previously excluded due
to optimization issues.
Fixes#51516.
Closes#51566.
r? @eddyb
cc @nox
We represent `bool` as `i1` in a `ScalarPair`, unlike other aggregates,
to optimize IR for checked operators and the like. With this patch, we
still do so when the pair is an immediate value, but we use the `i8`
memory type when the value is loaded or stored as an LLVM aggregate.
So `(bool, bool)` looks like an `{ i1, i1 }` immediate, but `{ i8, i8 }`
in memory. When a pair is a direct function argument, `PassMode::Pair`,
it is still passed using the immediate `i1` type, but as a return value
it will use the `i8` memory type. Also, `bool`-like` enum tags will now
use scalar pairs when possible, where they were previously excluded due
to optimization issues.
(This is just the data structure changes and some boilerplate match
code that followed from it; the actual emission of these statements
comes in a follow-up commit.)
