This removes the ambiguity when casting from a SingleValueInstruction to SILNode, which makes the code simpler. E.g. the "isRepresentativeSILNode" logic is not needed anymore.
Also, it reduces the size of the most used instruction class - SingleValueInstruction - by one pointer.
Conceptually, SILInstruction is still a SILNode. But implementation-wise SILNode is not a base class of SILInstruction anymore.
Only the two sub-classes of SILInstruction - SingleValueInstruction and NonSingleValueInstruction - inherit from SILNode. SingleValueInstruction's SILNode is embedded into a ValueBase and its relative offset in the class is the same as in NonSingleValueInstruction (see SILNodeOffsetChecker).
This makes it possible to cast from a SILInstruction to a SILNode without knowing which SILInstruction sub-class it is.
Casting to SILNode cannot be done implicitly, but only with an LLVM `cast` or with SILInstruction::asSILNode(). But this is a rare case anyway.
Add support for interleaved borrow scopes:
%b1 = begin_borrow %a
%c = copy
%b2 = begin_borrow %b1
end_borrow %b1
use %c
end_borrow %b2
Will be transformed to:
%c = copy %a
%b1 = begin_borrow %a
%b2 = begin_borrow %b1
end_borrow %b1
use %c
end_borrow %b2
This was the original intention but the implementation was incomplete.
This option can be enabled as soon as we need it for performance.
The intention is also to handle multi-block borrows, but that hasn't
been implemented.
I am trying to be more careful about this rather than letting someone make this
mistake in the future. I also added some comments and a DeadEndBlocks instance
to TempRValueElimination so that it gets full simplifications in OSSA.
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.
The reason why I am doing this is that I am building up more utilities based on
passing around this struct of context that do not want it for RAUWing
purposes. So it makes sense on a helper (OwnershipRAUWHelper) that composes with
its state.
Just a refactor, should be NFC.
Currently all of these places in the code base perform simplifyInstruction and
then a replaceAllSimplifiedUsesAndErase(...). This is a bad pattern since:
1. simplifyInstruction assumes its result will be passed to
replaceAllSimplifiedUsesAndErase. So by leaving these as separate things, we
allow for users to pointlessly make this mistake.
2. I am going to implement in a subsequent commit a utility that lifetime
extends interior pointer bases when replacing an address with an interior
pointer derived address. To do this efficiently, I want to reuse state I
compute during simplifyInstruction during the actual RAUW meaning that if the
two operations are split, that is difficult without extending the API. So by
removing this, I can make the transform and eliminate mistakes at the same
time.
Access scopes for enforcing exclusivity are currently the only
exception to our ability to canonicalize OSSA lifetime purely based on
the SSA value's known uses. This is because access scopes have
semantics relative to object deinitializers.
In general, deinitializers are asynchronous with respect to code that
is unrelated to the object's uses. Ignoring exclusivity, the optimizer
may always destroy objects as early as it wants, as long as the object
won't be used again. The optimizer may also extend the lifetime
(although in the future this lifetime extension should be limited by
"synchronization points").
The optimizer's freedom is however limited by exclusivity
enforcement. Optimization may never introduce new exclusivity
violations. Destroying an object within an access scope is an
exclusivity violation if the deinitializer accesses the same variable.
To handle this, OSSA canonicalization must detect access scopes that
overlap with the end of the pruned extended lifetime. Essentially:
%def
begin_access // access scope unrelated to def
use %def // pruned liveness ends here
end_access
destroy %def
Support for access scopes composes cleanly with the existing algorithm
without adding significant cost in the usual case. Overlapping access
scopes are unusual. A single CFG walk within the original extended
lifetime is normally sufficient. Only the blocks that are not already
LiveOut in the pruned liveness need to be visited. During this walk,
local overlapping access are detected by scanning for end_access
instructions after the last use point. Global overlapping accesses are
detected by checking NonLocalAccessBlockAnalysis. This avoids scanning
instructions in the common case. NonLocalAccessBlockAnalysis is a
trivial analysis that caches the rare occurence of nonlocal access
scopes. The analysis itself is a single linear scan over the
instruction stream. This analysis can be preserved across most
transformations and I expect it to be used to speed up other
optimizations related to access marker.
When an overlapping access is detected, pruned liveness is simply
extended to include the end_access as a new use point. Extending the
lifetime is iterative, but with each iteration, blocks that are now
marked LiveOut no longer need to be visited. Furthermore, interleaved
accessed scopes are not expected to happen in practice.
MandatoryCopyPropagation must be a separate pass in order to preserve
all debug_value instructions. CopyPropagation cannot preserve
debug_value because, as a rule, debug information cannot affect -O
behavior.
Canonicalizing OSSA provably minimizes the number of retains and
releases within the boundaries of that lifetime. This eliminates the
need for ad-hoc optimization of OSSA copies.
This initial implementation only canonicalizes owned values, but
canonicalizing guaranteed values is a simple extension.
This was originally part of the CopyPropagation prototype years
ago. Now OSSA is specified completely enough that it can be turned
into a simple utility instead.
CanonicalOSSALifetime uses PrunedLiveness to find the extended live
range and identify the consumes on the boundary. All other consumes
need their own copy. No other copies are needed.
By running this after other transformations that affect OSSA
lifetimes, we can avoid the need to run pattern-matching optimization
to SemanticARC to recover from suboptimal patterns, which is not
robust, maintainable, or efficient.
PR#34895 changed CSE to have common object for OpenedArchetypesTracker and Cloner.
CSE however frees and allocates instructions and this can result in the
Cloner having stale state for deleted instructions whose address may be
re-used while creating new instructions. Using a local object avoids
this.
