Use BasicBlockBitfield to record per-block liveness state. This has
been the intention since BasicBlockBitfield was first introduced.
Remove the per-field bitfield from PrunedLiveBlocks. This
(re)specializes the data structure for scalar liveness and drastically
simplifies the implementation.
This utility is fundamental to all ownership utilities. It will be on
the critical path in many areas of the compiler, including at
-Onone. It needs to be minimal and as easy as possible for compiler
engineers to understand, investigate, and debug.
This is in preparation for fixing bugs related to multi-def liveness
as used by the move checker.
When devirtualizing a `begin_apply`, it was passing the token's
use list to the conversion function when trying to convert the
yielded result. It's suppose to be the yielded result's list.
This became apparent when it encountered an access of a
`@_borrowed` property and we hit an assertion about an empty
use-list of a guaranteed value, when it was in fact the wrong list!
Although nonescaping closures are representationally trivial pointers to their
on-stack context, it is useful to model them as borrowing their captures, which
allows for checking correct use of move-only values across the closure, and
lets us model the lifetime dependence between a closure and its captures without
an ad-hoc web of `mark_dependence` instructions.
During ownership elimination, We eliminate copy/destroy_value instructions and
end the partial_apply's lifetime with an explicit dealloc_stack as before,
for compatibility with existing IRGen and non-OSSA aware passes.
When salvaging debug info, it is not always permissible to create the
new debug info instruction at the same location as the old instruction:
the old instruction may occur after the end of the lifetime of the new
debug_value operand.
This instruction can be inserted by Onone optimizations as a replacement for deleted instructions to
ensure that it's possible to single step on its location.
This allows dynamically indexing into tuples. IRGen not yet
implemented.
I think I'm going to need a type_refine_addr instruction in
order to handle substitutions into the operand type that
eliminate the outer layer of tuple-ness. Gonna handle that
in a follow-up commit.
This API is the inverse of visitEnclosingDefs when called on a phi.
This replaces the visitAdjacentReborrowsOfPhi algorithm with a small
loop that simply checks all the phis in the current block.
This should all be fairly efficient once SILArgument has a "reborrow"
flag.
For those who are unaware, CanonicalizeOSSALifetime::canonicalizeValueLifetime()
is really a high level driver routine for the functionality of
CanonicalizeOSSALifetime that computes liveness and then rewrites copies using
boundary information. This change introduces splits the implementation of
canonicalizeValueLifetime into two parts: a first part called computeLiveness
and a second part called rewriteLifetimes. Internally canonicalizeValueLifetime
still just calls these two methods.
The reason why I am doing this is that it lets the move only object checker use
the raw liveness information computed before the rewriting mucks with the
analysis information. This information is used by the checker to compute the raw
liveness boundary of a value and use that information to determine the list of
consuming uses not on the boundary, consuming uses on the boundary, and
non-consuming uses on the boundary. This is then used by later parts of the
checker to emit our errors.
Some additional benefits of doing this are:
1. I was able to eliminate callbacks in the rewriting stage of
CanonicalOSSALifetimes which previously gave the checker this information.
2. Previously the move checker did not have access to the non-consuming boundary
uses causing us to always fail appropriately, but sadly not emit a note showing
the non-consuming use. I am going to wire this up in a subsequent commit.
The other change to the implementation of the move checker that this caused is
that I needed to add an extra diagnostic check for instructions that consume the
value twice or consume the value and use the value. The reason why this must be
done is that liveness does not distinguish in between different operands on the
same instruction meaning such an error would be lost.
Having added these, I'm not entirely sure we couldn't just use
alloc_stack and dealloc_stack. Well, if we find ourselves adding
a lot of redundancy with those instructions (e.g. around DI), we
can always go back and rip these out.
- SILPackType carries whether the elements are stored directly
in the pack, which we're not currently using in the lowering,
but it's probably something we'll want in the final ABI.
Having this also makes it clear that we're doing the right
thing with substitution and element lowering. I also toyed
with making this a scalar type, which made it necessary in
various places, although eventually I pulled back to the
design where we always use packs as addresses.
- Pack boundaries are a core ABI concept, so the lowering has
to wrap parameter pack expansions up as packs. There are huge
unimplemented holes here where the abstraction pattern will
need to tell us how many elements to gather into the pack,
but a naive approach is good enough to get things off the
ground.
- Pack conventions are related to the existing parameter and
result conventions, but they're different on enough grounds
that they deserve to be separated.
For most uses, some access scopes must be "respected"--if an extended
value's original lifetime originally extends beyond an access scope, its
canonicalized lifetime must not end _within_ such scopes (although
ending before them is fine). Currently, to be conservative, the utility
applies this behavior to all access scopes.
For move-only values, however, lifetimes end at final consumes without
regard to access scopes.
Allow this behavior to be controlled by whether or not a
NonLocalAccessBlockAnalysis is provided to the utility in its
constructor.
rdar://104635319
When encountering inside a borrow scope a non-lexical move_value or a
move_value [lexical] where the borrowed value is itself already lexical,
delete the move_value and regard its uses as uses of the moved-from
value.
Previously, `begin_borrow [lexical]` were created during SILGen for
@owned arguments. Such borrows could be deleted if trivially dead,
which was the original reason why @owned arguments were considered
lexical and could not have their destroys hoisted.
Those borrows were however important during inlining because they would
maintain the lifetime of the owned argument. Unless of course the
borrow scope was trivially dead. In which case the owned argument's
lifetime would not be maintained. And if the caller's value was
non-lexical, destroys of the value could be hoisted over deinit
barriers.
Here, during inlining, `move_value [lexical]`s are introduced during
inlining whever the caller's value is non-lexical. This maintains the
lifetime of the owned argument even after inlining.
If a guaranted function argument is lexical, its lifetime must be
mainted during inlining. If the caller's value is already lexical,
no work is required to do the inlining.
If the callee is a non-generic thunk which calls a (not inlinable) generic function in the defining module,
it's more efficient to not devirtualize, but call the non-generic thunk - even though it's done through the witness table.
Example:
```
protocol P {
func f(x: [Int]) // not generic
}
struct S: P {
func f(x: some RandomAccessCollection<Int>) { ... } // generic
}
```
In the defining module, the generic conformance can be fully specialized (which is not possible in the client module, because it's not inlinable).
rdar://102623022
