Several optimization improvements, mainly for `begin_borrow`, `load` and `load_borrow` #86113

eeckstein · 2025-12-17T18:09:46Z

handle debug_value in begin_borrow simplification
ignore type-dependent operands when converting borrowed -> owned
remove borrow scopes which are borrowing an already guaranteed value
allow optimizing lexical begin_borrows outside the mandatory pipeline
SimplifyLoad/SimplifyLoadBorrow: replace address casts of heap objects
SimplifyLoadBorrow: replace a load_borrow of a store_borrow with a begin_borrow
convert destructure_tuple/destructure_struct with guaranteed ownership to to tuple_extract/struct_extract

The improvements for begin_borrow simplification allows to remove the now obsolete BorrowScope optimization from SemanticArcOpts.

This PR also fixes a few small bugs which I run into while working on the optimizations:

fix operand ownership for taskAddCancellationHandler and taskAddPriorityEscalationHandler builtins
LoadableByAddress: handle the unchecked_bitwise_cast instruction
don't remove begin_borrow [lexical] of a thin_to_thick_function in the mandatory pipeline
OwnershipOptUtils: fix a wrong debug location when creating destroy_value
CopyToBorrowOptimization: need to update borrowed-from instructions when changes are made
SILCombine: disable unchecked_bitwise_cast -> unchekced_ref_cast conversion in OSSA
SILVerifier: print a readable error message when referencing an undefined SIL function
SILVerifier: don't crash on verifier errors in the function header

For details see the commit messages

eeckstein · 2025-12-17T18:11:35Z

@swift-ci test

eeckstein · 2025-12-17T18:11:45Z

@swift-ci apple silicon benchmark

meg-gupta · 2025-12-17T19:10:26Z

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyLoadBorrow.swift

+    return true
+  }
+
+  private func tryForwardStoreBorrow(_ context: SimplifyContext) {


Is there a unit test for store borrow forwarding?

good catch! I added a test and also a source comment

Nice. I think this maybe the first transform assuming store_borrow result immutability within its scope. It maybe good to verify this like we verify load_borrow source immutability via verifyNoMutatingUsesInLiverange in the future..

It maybe good to verify this like we verify load_borrow source immutability via verifyNoMutatingUsesInLiverange in the future..

good idea!

meg-gupta · 2025-12-17T19:18:33Z

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyLoad.swift

+  /// ```
+  ///   %1 = load [copy] %0
+  ///   %2 = unchecked_ref_cast %1 : $SomeClass to $OtherClass
+  /// ```


Can you comment why this transform is useful?

eeckstein · 2025-12-18T16:43:54Z

@swift-ci test

eeckstein · 2025-12-18T20:11:45Z

@swift-ci test

meg-gupta · 2025-12-18T21:18:35Z

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyBeginBorrow.swift

+///   bb0(%0 : @guaranteed $T):
+///     // ... uses of %0
+/// ```
+private func tryReplaceInnerBorrowScope(beginBorrow: BeginBorrowInst, _ context: SimplifyContext) -> Bool {


Do we need a lexical check here? Can we remove an inner lexical borrow with an outer non lexical borrow?

The lexical check is only done in the mandatory pipeline. So, when running in the optimization pipeline, lexical begin_borrrows will be removed, anyway.

Can we remove an inner lexical borrow with an outer non lexical borrow?

I don't think so (in the mandatory pipeline) because then the lexical flag gets lost and the outer, non-lexical, borrow scope could be removed as well

meg-gupta · 2025-12-18T21:47:43Z

I have a another comment - #86113 (comment)

Otherwise LGTM

eeckstein · 2025-12-19T07:12:44Z

@swift-ci test linux

eeckstein · 2025-12-19T07:54:02Z

@swift-ci test linux

…orrow.swift and SimplifyLoadBorrow.swift

* rename `lookThroughSingleForwardingUses` -> `lookThroughOwnedConvertibaleForwardingChain` * fix the comment of `replaceGuaranteed`

When trying to remove a borrow scope by replacing all guaranteed uses with owned uses, don't bail for values which have debug_value uses. Also make sure that the debug_value instructions are located within the owned value's lifetime.

…orrowed -> owned

* remove borrow scopes which are borrowing an already guaranteed value * allow optimizing lexical `begin_borrows` outside the mandatory pipeline * fix: don't remove `begin_borrow [lexical]` of a `thin_to_thick_function` in the mandatory pipeline

…now covered by SimplifyBeginBorrow

…alue When taking the location from the builder's insertion point, we must make sure it's not a return location. Fixes an assertion failure. I found this during my work on simplifying borrow scopes

* Replace address casts of heap objects ``` %1 = unchecked_addr_cast %0 : $*SomeClass to $*OtherClass %2 = load [copy] %1 ``` with ref-casts of the loaded value ``` %1 = load [copy] %0 %2 = unchecked_ref_cast %1 : $SomeClass to $OtherClass ``` * Replace a `load_borrow` of a `store_borrow` with a `begin_borrow`: ``` %1 = alloc_stack $T %2 = store_borrow %0 to %1 ... %3 = load_borrow %2 // ... uses of %3 end_borrow %3 ``` -> ``` %1 = alloc_stack $T %2 = store_borrow %0 to %1 ... %3 = begin_borrow %0 // ... uses of %3 end_borrow %3 ```

… with guaranteed ownership to to `tuple_extract`/`struct_extract`

By defining its own array structs, which are independent from objc interop. Also, the requires-line was wrong anyway. The test didn't run on macos either.

…hen changes are made When ownership is changed from owned to guaranteed, the enclosing values of a guaranteed value can change. Fixes a SIL verifier error.

…AddPriorityEscalationHandler` builtins The operand is a closure which escapes to the return value to the builtin. Therefore the operand ownership must be `BitwiseEscape`. Otherwise the closure might be destroyed too early. Fixes a mis-compile in the Concurrency library.

Fixes a compiler crash in LoadableByAddress. Unfortunately I don't have a test case for this.

…ined SIL function This error was not printed because the verifier complained before the error handling was done.

The thing with `optional<VerifierErrorEmitterGuard>` didn't work as expected. This resulted in a null-pointer de-reference when printing a verifier error.

…conversion in OSSA In ownership converting an unowned bitwise cast to a "real" ownership forwarding instruction can cause various troubles. Fixes a SIL verifier crash.

eeckstein · 2025-12-19T16:46:12Z

@swift-ci test

eeckstein · 2025-12-19T18:51:22Z

@swift-ci smoke test macos

eeckstein requested a review from jckarter as a code owner December 17, 2025 18:09

eeckstein force-pushed the simplify-load branch from 08d411a to 516c10b Compare December 17, 2025 18:11

eeckstein requested review from aidan-hall, atrick, elsakeirouz and meg-gupta and removed request for jckarter December 17, 2025 18:11

meg-gupta reviewed Dec 17, 2025

View reviewed changes

eeckstein force-pushed the simplify-load branch from 516c10b to 55963c0 Compare December 18, 2025 16:43

eeckstein requested a review from rjmccall as a code owner December 18, 2025 16:43

eeckstein force-pushed the simplify-load branch from 55963c0 to 05f115f Compare December 18, 2025 20:11

meg-gupta reviewed Dec 18, 2025

View reviewed changes

eeckstein force-pushed the simplify-load branch from 877eded to 03617ea Compare December 19, 2025 07:53

eeckstein added 10 commits December 19, 2025 17:43

Optimizer: split SimplifyBeginAndLoadBorrow.swift into SimplifyBeginB…

f67a310

…orrow.swift and SimplifyLoadBorrow.swift

SimplifyBeginBorrow: small refactoring

ec8fe83

* rename `lookThroughSingleForwardingUses` -> `lookThroughOwnedConvertibaleForwardingChain` * fix the comment of `replaceGuaranteed`

SimplifyBeginBorrow: ignore type-dependent operands when converting b…

0271a4f

…orrowed -> owned

SemanticARCOpts: remove the BorrowScope optimization because this is …

ccc19e0

…now covered by SimplifyBeginBorrow

SIL: add Type.isHeapObjectReferenceType

439bda6

OwnershipOptUtils: fix a wrong debug location when creating destroy_v…

cc22290

…alue When taking the location from the builder's insertion point, we must make sure it's not a return location. Fixes an assertion failure. I found this during my work on simplifying borrow scopes

Optimizer: rename SimplifyMisc.swift -> SimplifyTypeValue.swift

d5afd16

eeckstein added 8 commits December 19, 2025 17:44

SimplifyDestructure: convert destructure_tuple/destructure_struct…

fb50158

… with guaranteed ownership to to `tuple_extract`/`struct_extract`

tests: enable SILOptimizer/copy-to-borrow-optimization.sil for all OSes

7e61773

By defining its own array structs, which are independent from objc interop. Also, the requires-line was wrong anyway. The test didn't run on macos either.

CopyToBorrowOptimization: need to update borrowed-from instructions w…

7ace2c5

…hen changes are made When ownership is changed from owned to guaranteed, the enclosing values of a guaranteed value can change. Fixes a SIL verifier error.

LoadableByAddress: handle the unchecked_bitwise_cast instruction

daa8850

Fixes a compiler crash in LoadableByAddress. Unfortunately I don't have a test case for this.

SILVerifier: print a readable error message when referencing an undef…

ceafe65

…ined SIL function This error was not printed because the verifier complained before the error handling was done.

SILVerifier: don't crash on verifier errors in the function header

e265eb0

The thing with `optional<VerifierErrorEmitterGuard>` didn't work as expected. This resulted in a null-pointer de-reference when printing a verifier error.

SILCombine: disable unchecked_bitwise_cast -> unchekced_ref_cast …

a06733b

…conversion in OSSA In ownership converting an unowned bitwise cast to a "real" ownership forwarding instruction can cause various troubles. Fixes a SIL verifier crash.

eeckstein force-pushed the simplify-load branch from 03617ea to a06733b Compare December 19, 2025 16:44

eeckstein merged commit cd3ebfb into swiftlang:main Dec 20, 2025
4 of 5 checks passed

eeckstein deleted the simplify-load branch December 20, 2025 06:23

Several optimization improvements, mainly for begin_borrow, load and load_borrow #86113

Several optimization improvements, mainly for begin_borrow, load and load_borrow #86113

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Several optimization improvements, mainly for `begin_borrow`, `load` and `load_borrow` #86113

Several optimization improvements, mainly for `begin_borrow`, `load` and `load_borrow` #86113

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