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[vm] Review parameter unboxing. #64348
Description
See cl/552801 for context and discussions.
Parameter unboxing has been buggy across graph-intrinsics which was uncovered by moving SIMD value type bootstrap implementations from C++ to Dart.
Martin pointed out we have code paths for unboxed parameters on graph-intrinsics. It looks to me like they are not actively exercised because other conditions prevent that from happening.
We also have weird code like:
sdk/sdk/lib/_internal/vm/lib/integers.dart
Lines 9 to 12 in ca915eb
Which doesn't make sense on first sight. We should unbox parameters here and inline this, why aren't we?
I think we should eventually review those parts and see if it works properly or whether pragma combinations like those two above are just a workaround for a bug.
Activity
alexmarkov commentedon Sep 23, 2026
In general, graph intrinsics should not prevent unboxing of the parameters and/or result. Smi intrinsics might be a bit special because they have specialized inline cache stubs which do not expect to see unboxed arguments.
There are 2 cases:
when parameter unboxing is prevented by other means (external implementation, pragma), graph intrinsics can assert that they always take boxed parameter and not handle unboxed parameter (because handling an unboxed parameter would be a dead code). This is done via
VerifyParameterIsBoxed.if parameter could be both boxed and unboxed, graph intrinsic can convert it to boxed representation via
CreateBoxedParameterIfNeededor explicitly unbox it as needed. Also, if result can be both boxed and unboxed, graph intrinsic should useCreateBoxedResultIfNeededandCreateUnboxedResultIfNeededto convert the result.Rewriting fallbacks of graph intrinsics from external to Dart moves intrinsic from case 1 to case 2, so we should be careful to update graph intrinsics as needed.
However, even better way would be to replace all graph intrinsics with
FlowGraphBuilder::BuildGraphOfRecognizedMethodimplementation which handles boxing/unboxing in a general way.