1. The reason to do this is that an InteriorPointerOperand is a value type.
2. Sometimes one wants to be able to use the IntPtrOperand that is not a const &
and the value type formulation always works.
Functionality wise, this is NFCI.
...for handling borrow scopes:
- find[Extended]TransitiveGuaranteedUses
- BorrowingOperand::visit[Extended]ScopeEndingUses
- BorrowedValue BorrowingOperand::getBorrowIntroducingUserResult()
And document the logic.
Mostly NFC in this commit, but more RAUW cases should be correctly
handled now.
Particularly, ensure that we can cleanly handle all manner of
reborrows. This is necessary to ensure both CanonicalizeOSSA and
replace-all-uses higher-level utilities handle all cases.
This generalizes some of the logic in CanonicalizeOSSA so it can be
shared by other high-level OSSA utilities.
These utilities extend the fundamental BorrowingOperand and
BorrowedValue functionality that has been designed recently. It builds
on and replaces a mix of piece-meal functionality that was needed
during bootstrapping. These APIs are now consistent with the more
recently designed code. It should be obvious what they mean and how to
use them, should be very hard to use them incorrectly, and should be
as efficient as possible, since they're fundamental to other
higher-level utilities.
Most importantly, there were several very subtle correctness issues
that were not handled cleanly in a centralized way. There are now a
mix of higher-level utilities trying to use this underlying
functionality, but it was hard to tell if all those higher-level
utilities were covering all the subtle cases:
- checking for PointerEscapes everywhere we need to
- the differences between uses of borrow introducers and other
guaranteed values
- handling the uses of nested borrow scopes
- transitively handling reborrows
- the difference between nested and top-level reborrows
- forwarding destructures (which can cause exponential explosion)
In short, I was fundamentally confused and getting things wrong before
designing these utilities.
In OSSA, we enforce that addresses from interior pointer instructions are scoped
within a borrow scope. This means that it is invalid to use such an address
outside of its parent borrow scope and as a result one can not just RAUW an
address value by a dominating address value since the latter may be invalid at
the former. I foresee that I am going to have to solve this problem and so I
decided to write this API to handle the vast majority of cases.
The way this API works is that it:
1. Computes an access path with base for the new value. If we do not have a base
value and a valid access path with root, we bail.
2. Then we check if our base value is the result of an interior pointer
instruction. If it isn't, we are immediately done and can RAUW without further
delay.
3. If we do have an interior pointer instruction, we see if the immediate
guaranteed value we projected from has a single borrow introducer value. If not,
we bail. I think this is reasonable since with time, all guaranteed values will
always only have a single borrow introducing value (once struct, tuple,
destructure_struct, destructure_tuple become reborrows).
4. Then we gather up all inner uses of our access path. If for some reason that
fails, we bail.
5. Then we see if all of those uses are within our borrow scope. If so, we can
RAUW without any further worry.
6. Otherwise, we perform a copy+borrow of our interior pointer's operand value
at the interior pointer, create a copy of the interior pointer instruction upon
this new borrow and then RAUW oldValue with that instead. By construction all
uses of oldValue will be within this new interior pointer scope.
Instead, I just added some static helper methods that perform the same
operations without needing to deal with generics/etc on OwnershipForwardingMixin
itself. The reason why I did this is that this Mixin is not part of the SILNode
heirarchy so we shouldn't use utilities tied to the SILNode hierarchy.
This implies making -> and * return an Operand * instead of an
OwnershipForwardingInst. So one can thus do:
ForwardingOperand op;
op.myForwardingOperandMethod();
op->myOperandMethod();
Now that OperandOwnership determines the operand constraints, it
doesn't make sense to distinguish between Borrow and NestedBorrow at
this level. We want these uses to automatically convert between the
nested/non-nested state as the operand's ownership changes. The use
does not need to impose any constraint on the ownership of the
incoming value.
For algorithms that need to distinguish nested borrows, it's still
trivial to do so.
A NonUse operand does not use the value itself, so it ignores
ownership and does not require liveness. This is for operands that
represent dependence on a type but are not actually passed the value
of that type (e.g. they may refer an open_existential). This could be
used for other dependence-only operands in the future.
A TrivialUse operand has undefined ownership semantics aside from
requiring liveness. Therefore it is only legal to pass the use a value
with ownership None (a trivial value). Contrast this with things like
InstantaneousUse or BitwiseEscape, which just don't care about
ownership (i.e. they have no ownership semantics.
All of the explicitly listed operations in this category require
trivially typed operands. So the meaning is obvious to anyone
adding SIL operations and updating OperandOwnership.cpp, without
needing to decifer the value ownership kinds.
Clarify which uses are allowed to take Unowned values. Add enforcement
to ensure that Unowned values are not passed to other uses.
Operations that can take unowned are:
- copy_value
- apply/return @unowned argument
- aggregates (struct, tuple, destructure, phi)
- forwarding operations that are arbitrary type casts
Unowned values are currently borrowed within ObjC deinitializers
materialized by the Swift compiler. This will be banned as soon as
SILGen is fixed.
Migrating to this classification was made easy by the recent rewrite
of the OSSA constraint model. It's also consistent with
instruction-level abstractions for working with different kinds of
OperandOwnership that are being designed.
This classification vastly simplifies OSSA passes that rewrite OSSA
live ranges, making it straightforward to reason about completeness
and correctness. It will allow a simple utility to canonicalize OSSA
live ranges on-the-fly.
This avoids the need for OSSA-based utilities and passes to hard-code
SIL opcodes. This will allow several of those unmaintainable pieces of
code to be replaced with a trivial OperandOwnership check.
It's extremely important for SIL maintainers to see a list of all SIL
opcodes associated with a simple OSSA classification and set of
well-specified rules for each opcode class, without needing to guess
or reverse-engineer the meaning from the implementation. This
classification does that while eliminating a pile of unreadable
macros.
This classification system is the model that CopyPropagation was
initially designed to use. Now, rather than relying on a separate
pass, a simple, lightweight utility will canonicalize OSSA
live ranges.
The major problem with writing optimizations based on OperandOwnership
is that some operations don't follow structural OSSA requirements,
such as project_box and unchecked_ownership_conversion. Those are
classified as PointerEscape which prevents the compiler from reasoning
about, or rewriting the OSSA live range.
Functional Changes:
As a side effect, this corrects many operand constraints that should
in fact require trivial operand values.
This is a generic API that when ownership is enabled allows one to replace all
uses of a value with a value with a differing ownership by transforming/lifetime
extending as appropriate.
This API supports all pairings of ownership /except/ replacing a value with
OwnershipKind::None with a value without OwnershipKind::None. This is a more
complex optimization that we do not support today. As a result, we include on
our state struct a helper routine that callers can use to know if the two values
that they want to process can be handled by the algorithm.
My moticiation is to use this to to update InstSimplify and SILCombiner in a
less bug prone way rather than just turn stuff off.
Noting that this transformation inserts ownership instructions, I have made sure
to test this API in two ways:
1. With Mandatory Combiner alone (to make sure it works period).
2. With Mandatory Combiner + Semantic ARC Opts to make sure that we can
eliminate the extra ownership instructions it inserts.
As one can see from the tests, the optimizer today is able to handle all of
these transforms except one conditional case where I need to eliminate a dead
phi arg. I have a separate branch that hits that today but I have exposed unsafe
behavior in ClosureLifetimeFixup that I need to fix first before I can land
that. I don't want that to stop this PR since I think the current low level ARC
optimizer may be able to help me here since this is a simple transform it does
all of the time.
This commit is doing a few things:
1. It is centralizing all decisions about whether an operand's owner instruction
or a value's parent instruction is forwarding in each SILInstruction
itself. This will prevent this information from getting out of sync.
2. This allowed me to hide the low level queries in OwnershipUtils.h that
determined if a SILNodeKind was "forwarding". I tried to minimize the amount of
churn in this PR and thus didn't remove the
is{Owned,Ownership,Guaranteed}Forwarding{Use,Value} checks. Instead I left them
alone but added in asserts to make sure that if the old impl ever returns true,
the neew impl does as well. In a subsequent commit, I am going to remove the old
impl in favor of isa queries.
3. I also in the process discovered that there were some instructions that were
being inconsistently marked as forwarding. All of the asserts in the PR caught
these and I fixed these inconsistencies.
Specifically the optimization that is being performed here is the elimination of
lifetimes that hand off ownership in between two regions of code. Example:
```
%1 = copy_value %0
...
destroy_value %0
...
apply %consumingUser(%1)
```
We really want to handle this case since it is a natural thing that comes up in
programs and will let me eliminate the *evil* usage of emitDestroyValueOperation
in TypeLowering without needing to modify huge amounts of tests.
This originally trafficked in Instructions and for some reason the name was
never changed.
I also changed the result type to be a bool and added the ability for the passed
in closure to signal failure (and iteration stop) by returning false. This also
makes it possible to use visitLocalEndScopeUses in if statements which can be
useful.
I think what was happening here was that we were using one of the superclass
classofs and were getting lucky since in the place I was using this I was
guaranteed to have single value instructions and that is what I wrote as my
first case X ).
I also added more robust checks tieing the older isGuaranteed...* APIs to the
ForwardingOperand API. I also eliminated the notion of Branch being an owned
forwarding instruction. We only used this in one place in the compiler (when
finding owned value introducers), yet we treat a phi as an introducer, so we
would never hit a branch in our search since we would stop at the phi argument.
The bigger picture here is that this means that all "forwarding instructions"
either forward ownership for everything or for everything but owned/unowned.
And for those listening in, I did find one instruction that was from an
ownership forwarding subclass but was not marked as forwarding:
DifferentiableFunctionInst. With this change, we can no longer by mistake have
such errors enter the code base.
A reborrow occurs when a Borrowing Operand ends the lifetime of a borrowed value
and propagates forward a new guaranteed value that continues the guaranteed
lifetime of the value.
This makes it easier to understand conceptually why a ValueOwnershipKind with
Any ownership is invalid and also allowed me to explicitly document the lattice
that relates ownership constraints/value ownership kinds.
This makes it clearer that isConsumingUse() is not an owned oriented API and
returns also for instructions that end the lifetime of guaranteed values like
end_borrow.
This updates how we model reborrow's lifetimes for ownership verification.
Today we follow and combine a borrow's lifetime through phi args as well.
Owned values lifetimes end at a phi arg. This discrepency in modeling
lifetimes leads to the OwnershipVerifier raising errors incorrectly for
cases such as this, where the borrow and the base value do not dominate
the end_borrow:
bb0:
cond_br undef, bb1, bb2
bb1:
%copy0 = copy_value %0
%borrow0 = begin_borrow %copy0
br bb3(%borrow0, %copy0)
bb2:
%copy1 = copy_value %1
%borrow1 = begin_borrow %copy1
br bb3(%borrow1, %copy1)
bb3(%borrow, %baseVal):
end_borrow %borrow
destroy_value %baseVal
This PR adds a new ReborrowVerifier. The ownership verifier collects borrow's
lifetime ending users and populates the worklist of the ReborrowVerifier
with reborrows and the corresponding base value.
ReborrowVerifier then verifies that the lifetime of the reborrow is
within the lifetime of the base value.
My upcoming SILGen change introduces more stores directly into
the result of a ref_element_addr or struct_element_addr in some
cases. Make sure we handle the failing combinations flagged by
the ownership verifier.
I am currently working on updating SimplifyCFG for ownership. One thing that I
am finding that I need is the ability to compute the lifetime of a guaranteed
argument from a checked_cast_br/switch_enum in order to convert said
instructions to a br. The issue comes from br acting as a consuming (lifetime
ending) use of a borrowed value, unlike checked_cast_br/switch_enum (which are
treated like forwarding instructions). This means that during the conversion we
need to insert a begin_borrow on the value before the new br instruction and
then create an end_borrow at the end of the successor block's argument's
lifetime.
By refactoring out this code from the ownership verifier, I can guarantee that
said lifetime computation will always match what the ownership verifier does
internally preventing them from getting out of sync.
Today unchecked_bitwise_cast returns a value with ObjCUnowned ownership. This is
important to do since the instruction can truncate memory meaning we want to
treat it as a new object that must be copied before use.
This means that in OSSA we do not have a purely ossa forwarding unchecked
layout-compatible assuming cast. This role is filled by unchecked_value_cast.
Just like br inst, structs are phis in the ownership graph that one can induce
on top of the def-use graph. In this commit, I basically fill in the relevant
blanks in the ADT for such phis for struct so that the optimization for branches
is generalized onto structs.
<rdar://problem/63950481>
checked_cast_br may take an additional operands for the source and
target types. I'm sure the compiler forgets to check this in many
places. In this case, it was just a harmless assert.
Fixes <rdar://61122253> Assertion failed:
(termInst->getNumOperands() == 1 && "Transformation terminators should only have single operands")
Specifically, I split it into 3 initial categories: IR, Utils, Verifier. I just
did this quickly, we can always split it more later if we want.
I followed the model that we use in SILOptimizer: ./lib/SIL/CMakeLists.txt vends
a macro (sil_register_sources) to the sub-folders that register the sources of
the subdirectory with a global state variable that ./lib/SIL/CMakeLists.txt
defines. Then after including those subdirs, the parent cmake declares the SIL
library. So the output is the same, but we have the flexibility of having
subdirectories to categorize source files.
