Programs compiled by R8 are not structurally the same as the input. For example, names, invokes and line numbers can all change when compiling with R8. The correspondence between the input and the output is recorded in a mapping file. The mapping file can be used with retrace to recover the original stack trace from the stack traces produced by running the R8 compiled program. More information about R8 and mapping files can be found here.
The format for additional information is encoded as comments with json formatted data. The data has an ID to disambiguate it from other information.
The version information states what version the content of the mapping file is using.
The format of the version information is:
# {"id":"com.android.tools.r8.mapping","version":"1.0"}
Here id must be com.android.tools.r8.mapping and version is the version of the mapping file.
The version information applies to content of the file following the position of the version entry until either the end of the file or another version entry is present.
If no version information is present, the content is assumed to have version zero and no additional mapping information except Source File.
When interpreting the mapping file, any additional mapping information pertaining to a later version should be ignored. In other words, treated as a normal comment.
Retracing tools supporting this versioning scheme should issue a warning when given mapping files with versions higher than the version supported by the tool.
The source file information states what source file a class originated from.
The source file information must be placed directly below the class mapping it pertains to. The format of the source file information is:
some.package.Class -> foo.a:
# {"id":"sourceFile","fileName":"R8Test.kt"}
Here id must be the string sourceFile and fileName is the source file name as a string value (in this example R8Test.kt).
Note that the id of the source file information is unqualified. It is the only allowed unqualified identifier as it was introduced prior to the versioning scheme.
The synthesized information states what parts of the compiled output program are synthesized by the compiler. A retracing tool should use the synthesized information to strip out synthesized method frames from retraced stacks.
The synthesized information must be placed directly below the class, field or method mapping it pertains to. The format of the synthesized information is:
# {'id':'com.android.tools.r8.synthesized'}
Here id must be com.android.tools.r8.synthesized. There is no other content.
A class mapping would be:
some.package.SomeSynthesizedClass -> x.y.z:
# {'id':'com.android.tools.r8.synthesized'}
This specifies that the class x.y.z has been synthesized by the compiler and thus it did not exist in the original input program.
Notice that the left-hand side and right-hand side, here some.package.SomeSynthesizedClass and x.y.z respectively, could be any class name. It is likely that the mapping for synthetics is the identity, but a useful name can be placed here if possible which legacy retrace tools would use when retracing.
A field mapping would be:
some.package.Class -> x.y.z:
int someField -> a
# {'id':'com.android.tools.r8.synthesized'}
This specifies that the field x.y.z.a has been synthesized by the compiler. As for classes, since the field is not part of the original input program, the left- and right-hand side names could be anything. Note that a field can be synthesized without the class being synthesized.
A method mapping would be:
some.package.SomeSynthesizedClass -> x.y.z:
void someMethod() -> a
# {'id':'com.android.tools.r8.synthesized'}
This specifies that the method x.y.z.a() has been synthesized by the compiler. As for classes, since the method is not part of the original input program, the left- and right-hand side names could be anything. Note that a method can be synthesized without the class being synthesized.
For inline frames a mapping would be:
some.package.Class -> foo.a:
4:4:void example.Foo.lambda$main$0():225 -> a
4:4:void run(example.Foo):2 -> a
# {'id':'com.android.tools.r8.synthesized'}
5:5:void example.Foo.lambda$main$1():228 -> a
5:5:void run(example.Foo):4 -> a
# {'id':'com.android.tools.r8.synthesized'} <-- redundant
This specifies that line 4 in the method foo.a.a is in a method that has been synthesized by the compiler. Since the method is either synthesized or not any extra synthesized comments will have no effect.
Synthesized information should never be placed on inlined frames:
some.package.Class -> foo.a:
4:4:void example.Foo.syntheticThatIsInlined():225 -> a
# {'id':'com.android.tools.r8.synthesized'}
4:4:void run(example.Foo):2 -> a
In the above, the mapping information suggests that the inline frame example.Foo.syntheticThatIsInlined should be marked as synthesized. However, since that method was not part of the input program it should not be in the output mapping information at all.