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Dead-end blocks are blocks from which there is no path to the function exit (`return`, `throw` or unwind). These are blocks which end with an unreachable instruction and blocks from which all paths end in "unreachable" blocks.
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Utilities for SIL optimizer passes
This document lists a set of SIL utilities to be used by the Swift implementation of the SIL optimizer.
Some of the utilities presented in this document are still in the design phase and are not implemented yet (see Status).
Goals for utilities
We want to avoid a situation like we have in the C++ SIL optimizer, where a huge amount of (partly) redundant utilities exist. Many of those utilities have overlapping functionality, are difficult to discover and are poorly designed. We want to do better in the Swift SIL optimizer.
Basic Data-Structures
Stack
An allocation-free, array like data structure. To be used instead of Swift.Array wherever random-access is not needed.
Related C++ utilities: llvm::SmallVector, Stack
Status: done
BasicBlockSet
An extremely efficient implementation of a set of basic blocks.
Related C++ utilities: BasicBlockSet
Status: done
ValueSet
An extremely efficient implementation of a set of values.
Related C++ utilities: ValueSet
Status: done
InstructionSet
An extremely efficient implementation of a set of instructions.
Related C++ utilities: InstructionSet
Status: done
BasicBlockWorklist
To be used for all kind of basic-block work-list algorithms.
Uses: Stack, BasicBlockSet
Related C++ utilities: BasicBlockWorklist
Status: done
SIL data structures
SmallProjectionPath
Describes a path of projections.
Related C++ utilities: AccessPath, ProjectionPath
Status: done
ProjectedValue
A projected value is defined by the original value and a projection path.
Related C++ utilities: AccessPath
Status: done
Building SIL
static Builder.insert(after:, insertFunc: (Builder) -> ())
Useful to insert instructions after a (potential) terminator instruction.
Related C++ utilities: SILBuilderWithScope::insertAfter()
Status: done
SSA Traversal Utilities
Walk Utils
This consists of four protocols, which can be implemented to walk up or down the SSA graph:
ValueDefUseWalkerAddressDefUseWalkerValueUseDefWalkerAddressUseDefWalker
Uses: instruction classifications
Related C++ utilities: AccessPath, RCIdentityAnalysis, various def-use/use-def walkers in optimization passes.
Status: done
Escape Utilities
Escape analysis, which is used e.g. in stack promotion or alias analysis.
Escape analysis is usable through the following methods of ProjectedValue and Value:
isEscaping()isAddressEscaping()visit()visitAddress()
Uses: Walk Utilities, ProjectedValue
Related C++ utilities: EscapeAnalysis, various def-use walkers in optimization passes.
Status: done
Access Utils
A set of utilities for analyzing memory accesses. It defines the following concepts:
AccessBase: represents the base address of a memory access.AccessPath: a pair of anAccessBaseandSmallProjectionPathwith the path describing the specific address (in terms of projections) of the access.- Access storage path (which is of type
ProjectedValue): identifies the reference - or a value which contains a reference - an address originates from.
Uses: Walk utils
Related C++ utilities: AccessPath and other access utilities.
Status: done
Control- and Dataflow
DeadEndBlocks
A utility for finding dead-end blocks.
Dead-end blocks are blocks from which there is no path to the function exit (return, throw or unwind).
These are blocks which end with an unreachable instruction and blocks from which all paths end in "unreachable" blocks.
Uses: BasicBlockWorklist
Related C++ utilities: DeadEndBlocks
Status: done
BasicBlockRange
Defines a range from a dominating "begin" block to one or more "end" blocks. To be used for all kind of backward block reachability analysis.
Uses: Stack, BasicBlockSet, BasicBlockWorklist
Related C++ utilities: PrunedLiveBlocks, findJointPostDominatingSet()
Status: done
InstructionRange
Like BasicBlockRange, but at the granularity of instructions.
Uses: BasicBlockRange
Related C++ utilities: PrunedLiveness, ValueLifetimeAnalysis
Status: done
Utilities for Inter-procedural Analysis
FunctionUses
Provides a list of instructions, which reference a function. This utility performs an analysis of all functions in the module and collects instructions which reference other functions. It can be used to do inter-procedural caller-analysis.
Related C++ utilities: CallerAnalysis
Status: done
OSSA Utilities
Value.makeAvailable() and Value.copy(at:)
To be used where a value is copied in one block and used in another block.
Uses: BasicBlockRange
Related C++ utilities: makeValueAvailable(), OwnershipLifetimeExtender
Status: done
Instruction classifications
We want to classify certain instructions into common groups so that passes can deal deal with the group instead of individual instruction opcodes. Optimization passes then don't need to be updated if new instructions are added to a group.
In Swift we can easily do this by introducing protocols to which instruction classes can conform to.
ApplySite
Related C++ utilities: ApplySite
Status: exists; to-do: complete member variables/functions
CastInstruction
Details need to be decided.
Conforming instructions: UpcastInst, UncheckedRefCastInst, etc.
Members: ownership behavior, e.g. forwarding, etc.
Status: to-do
AddressProjectionInstruction
Details need to be decided.
Conforming instructions: StructElementAddrInst, TupleElementAddrInst
Members: var fieldIndex: Int
Related C++ utilities: skipAddrProjections
Status: to-do
AggregateInstruction
Details need to be decided.
Conforming instructions: StructInst, TupleInst
Status: to-do
ExtractInstruction
Details need to be decided.
Conforming instructions: StructExtractInst, TupleExtractInst
Status: to-do
BorrowScope
Details need to be decided.
Maybe it's better to not do this as a protocol but just add an extension function Value.introducesBorrowScope. Reason: an Argument can be guaranteed or owned.
Conforming instructions: BeginBorrowInst, Argument with guaranteed ownership (however will do that), LoadBorrowInst
Status: to-do