[vm/ffi] FfiTransform RangeError

[dcharkes]

@dcharkes ,

I'm getting a strange compilation error when using unions on current Flutter master channel, which is:

Flutter is already up to date on channel master
Flutter 2.3.0-2.0.pre.116 • channel master •
Framework • revision b7dd9acbed (54 minutes ago) • 2021-05-11 13:49:02 -0400
Engine • revision 5f9eb39f7d
Tools • Dart 2.14.0 (build 2.14.0-74.0.dev)

Error log

Crash when compiling null,
at character offset null:
RangeError (index): Invalid value: Not in inclusive range 0..2: 3
#0      List.[] (dart:core-patch/array.dart:110:52)
#1      _FfiDefinitionTransformer._replaceFields.<anonymous closure> (package:vm/transformations/ffi_definitions.dart:546:70)
#2      MapMixin.map (dart:collection/maps.dart:170:28)
#3      _FfiDefinitionTransformer._replaceFields (package:vm/transformations/ffi_definitions.dart:546:12)
#4      _FfiDefinitionTransformer.visitClassInTopologicalOrder (package:vm/transformations/ffi_definitions.dart:198:28)
#5      _FfiDefinitionTransformer.manualVisitInTopologicalOrder.<anonymous closure> (package:vm/transformations/ffi_definitions.dart:158:9)
#6      List.forEach (dart:core-patch/growable_array.dart:403:8)
#7      _FfiDefinitionTransformer.manualVisitInTopologicalOrder (package:vm/transformations/ffi_definitions.dart:134:25)
#8      transformLibraries (package:vm/transformations/ffi_definitions.dart:89:15)
#9      VmTarget.performModularTransformationsOnLibraries (package:vm/target/vm.dart:162:34)
#10     KernelTarget.runBuildTransformations (package:front_end/src/fasta/kernel/kernel_target.dart:1236:19)
#11     KernelTarget.buildComponent.<anonymous closure> (package:front_end/src/fasta/kernel/kernel_target.dart:372:7)
<asynchronous suspension>
#12     withCrashReporting (package:front_end/src/fasta/crash.dart:121:12)
<asynchronous suspension>
#13     generateKernelInternal.<anonymous closure> (package:front_end/src/kernel_generator_impl.dart:164:19)
<asynchronous suspension>
#14     withCrashReporting (package:front_end/src/fasta/crash.dart:121:12)
<asynchronous suspension>
#15     generateKernel.<anonymous closure> (package:front_end/src/kernel_generator_impl.dart:53:12)
<asynchronous suspension>
#16     generateKernel (package:front_end/src/kernel_generator_impl.dart:52:10)
<asynchronous suspension>
#17     kernelForModule (package:front_end/src/api_prototype/kernel_generator.dart:99:11)
<asynchronous suspension>
#18     SingleShotCompilerWrapper.compileInternal (file:///b/s/w/ir/cache/builder/src/third_party/dart/pkg/vm/bin/kernel_service.dart:404:11)
<asynchronous suspension>
#19     Compiler.compile.<anonymous closure> (file:///b/s/w/ir/cache/builder/src/third_party/dart/pkg/vm/bin/kernel_service.dart:218:45)
<asynchronous suspension>
#20     _processLoadRequest (file:///b/s/w/ir/cache/builder/src/third_party/dart/pkg/vm/bin/kernel_service.dart:885:37)
<asynchronous suspension>


#0      List.[] (dart:core-patch/array.dart:110:52)
#1      _FfiDefinitionTransformer._replaceFields.<anonymous closure> (package:vm/transformations/ffi_definitions.dart:546:70)
#2      MapMixin.map (dart:collection/maps.dart:170:28)
#3      _FfiDefinitionTransformer._replaceFields (package:vm/transformations/ffi_definitions.dart:546:12)
#4      _FfiDefinitionTransformer.visitClassInTopologicalOrder (package:vm/transformations/ffi_definitions.dart:198:28)
#5      _FfiDefinitionTransformer.manualVisitInTopologicalOrder.<anonymous closure> (package:vm/transformations/ffi_definitions.dart:158:9)
#6      List.forEach (dart:core-patch/growable_array.dart:403:8)
#7      _FfiDefinitionTransformer.manualVisitInTopologicalOrder (package:vm/transformations/ffi_definitions.dart:134:25)
#8      transformLibraries (package:vm/transformations/ffi_definitions.dart:89:15)
#9      VmTarget.performModularTransformationsOnLibraries (package:vm/target/vm.dart:162:34)
#10     KernelTarget.runBuildTransformations (package:front_end/src/fasta/kernel/kernel_target.dart:1236:19)
#11     KernelTarget.buildComponent.<anonymous closure> (package:front_end/src/fasta/kernel/kernel_target.dart:372:7)
<asynchronous suspension>
#12     withCrashReporting (package:front_end/src/fasta/crash.dart:121:12)
<asynchronous suspension>
#13     generateKernelInternal.<anonymous closure> (package:front_end/src/kernel_generator_impl.dart:164:19)
<asynchronous suspension>
#14     withCrashReporting (package:front_end/src/fasta/crash.dart:121:12)
<asynchronous suspension>
#15     generateKernel.<anonymous closure> (package:front_end/src/kernel_generator_impl.dart:53:12)
<asynchronous suspension>
#16     generateKernel (package:front_end/src/kernel_generator_impl.dart:52:10)
<asynchronous suspension>
#17     kernelForModule (package:front_end/src/api_prototype/kernel_generator.dart:99:11)
<asynchronous suspension>
#18     SingleShotCompilerWrapper.compileInternal (file:///b/s/w/ir/cache/builder/src/third_party/dart/pkg/vm/bin/kernel_service.dart:404:11)
<asynchronous suspension>
#19     Compiler.compile.<anonymous closure> (file:///b/s/w/ir/cache/builder/src/third_party/dart/pkg/vm/bin/kernel_service.dart:218:45)
<asynchronous suspension>
#20     _processLoadRequest (file:///b/s/w/ir/cache/builder/src/third_party/dart/pkg/vm/bin/kernel_service.dart:885:37)
<asynchronous suspension>

This error happens whenever my union has more than three fields. E.g. :

class DartValueFields extends Union {
  Pointer p1;
  Pointer p2;
  Pointer p3;
  Pointer p4;
}

Is this a known limitation or should i open an issue in dart-lang/ffi?

Originally posted by @alexlapa in #38491 (comment)

Offending code in 2.14.0-74.0.dev:

sdk/pkg/vm/lib/transformations/ffi_definitions.dart

Lines 545 to 546 in b8f4018

	final fieldOffsets = compoundLayout
	.map((Abi abi, CompoundLayout v) => MapEntry(abi, v.offsets[i]));