All dexpreopted Java code falls into three categories:
Dexpreopt implementation for bootclasspath libraries (boot images) is located in soong/java (see e.g. soong/java/dexpreopt_bootjars.go), and install rules are in make/core/dex_preopt.mk.
Dexpreopt implementation for system server, libraries and apps is located in soong/dexpreopt. For the rest of this section we focus primarily on it (and not boot images).
Dexpeopt implementation is split across the Soong part and the Make part. The core logic is in Soong, and Make only generates configs and scripts to pass information to Soong.
The build system generates a global JSON dexpreopt config that is populated from product variables. This is static configuration that is passed to both Soong and Make. The $OUT/soong/dexpreopt.config file is generated in make/core/dex_preopt_config.mk. Soong reads it in soong/dexpreopt/config.go and makes a device-specific copy (this is needed to ensure incremental build correctness). The global config contains lists of bootclasspath jars, system server jars, dex2oat options, global switches that enable and disable parts of dexpreopt and so on.
The build system also generates a module config for each dexpreopted package. It contains package-specific configuration that is derived from the global configuration and Android.bp or Android.mk module for the package.
Module configs for Make packages are generated in make/core/dex_preopt_odex_install.mk; they are materialized as per-package JSON dexpreopt.config files.
Module configs in Soong are not materialized as dexpreopt.config files and exist as Go structures in memory, unless it is necessary to materialize them as a file for dependent Make packages or for post-dexpreopting. Module configs are defined in soong/dexpreopt/config.go.
The Soong implementation of dexpreopt consists roughly of the following steps:
Read global dexpreopt config passed from Make (soong/dexpreopt/config.go).
Construct a static boot image config (soong/java/dexpreopt_config.go).
During dependency mutator pass, for each suitable module:
During rule generation pass, for each suitable module:
compute transitive uses-library dependency closure (soong/java/java.go:addCLCFromDep)
construct CLC from the dependency closure (soong/dexpreopt/class_loader_context.go)
construct module config with CLC, boot image locations, etc. (soong/java/dexpreopt.go)
generate build rules to verify build-time CLC against the manifest (e.g. for apps: soong/java/app.go:verifyUsesLibraries)
generate dexpreopt build rule (soong/dexpreopt/dexpreopt.go)
At the end of rule generation pass:
In order to reuse the same dexpreopt implementation for both Soong and Make packages, part of Soong is compiled into a standalone binary dexpreopt_gen. It runs during the Ninja stage of the build and generates shell scripts with dexpreopt build rules for Make packages, and then executes them.
This setup causes many inconveniences. To name a few:
Errors in the build rules are only revealed at the late stage of the build.
These rules are not tested by the presubmit builds that run m nothing on many build targets/products.
It is impossible to find dexpreopt build rules in the generated Ninja files.
However all these issues are a lesser evil compared to having a duplicate dexpreopt implementation in Make. Also note that it would be problematic to reimplement the logic in Make anyway, because Android.mk modules are not processed in the order of uses-library dependencies and propagating dependency information from one module to another would require a similar workaround with a script.
Dexpreopt for Make packages involves a few steps:
At Soong phase (during m nothing), see dexpreopt_gen:
At Make/Kati phase (during m nothing), see make/core/dex_preopt_odex_install.mk:
generate build rules for module dexpreopt.config
generate build rules for merging dependency dexpreopt.config files (see make/core/dex_preopt_config_merger.py)
generate build rules for dexpreopt_gen invocation
generate build rules for executing dexpreopt.sh scripts
At Ninja phase (during m):
generate dexpreopt.config files
execute dexpreopt_gen rules (generate dexpreopt.sh scripts)
execute dexpreopt.sh scripts (this runs the actual dexpreopt)
The Make/Kati phase adds all the necessary dependencies that trigger dexpreopt_gen and dexpreopt.sh rules. The real dexpreopt command (dex2oat invocation that will be executed to AOT-compile a package) is in the dexpreopt.sh script, which is generated close to the end of the build.
The process described above for Make packages involves "indirect build rules", i.e. build rules that are generated not at the time when the build system is created (which is a small step at the very beginning of the build triggered with m nothing), but at the time when the actual build is done (m phase).
Some build systems, such as Make, allow modifications of the build graph during the build. Other build systems, such as Soong, have a clear separation into the first "generation phase" (this is when build rules are created) and the second "build phase" (this is when the build rules are executed), and they do not allow modifications of the dependency graph during the second phase. The Soong approach is better from performance standpoint, because with the Make approach there are no guarantees regarding the time of the build --- recursive build graph modfications continue until fixpoint. However the Soong approach is also more restictive, as it can only generate build rules from the information that is passed to the build system via global configuration, Android.bp files or encoded in the Go code. Any other information (such as the contents of the Java manifest files) are not accessible and cannot be used to generate build rules.
Hence the need for the "indirect build rules": during the generation phase only stubs of the build rules are generated, and the real rules are generated by the stub rules during the build phase (and executed immediately). Note that the build system still has to add all the necessary dependencies during the generation phase, because it will not be possible to change build order during the build phase.
Indirect buils rules are used in a couple of places in dexpreopt:
soong/scripts/manifest_check.py: first to extract targetSdkVersion from the manifest, and later to extract <uses-library/> tags from the manifest and compare them to the uses-library list known to the build system
soong/scripts/construct_context.py: to trim compatibility libraries in CLC
make/core/dex_preopt_config_merger.py: to merge information from dexpreopt.config files for uses-library dependencies into the dependent's dexpreopt.config file (mostly the CLC)
autogenerated dexpreopt.sh scripts: to call dexpreopt_gen
Because the information from the manifests has to be duplicated in the Android.bp/Android.mk files, there is a danger that it may get out of sync. To guard against that, the build system generates a rule that verifies uses-libraries: checks the metadata in the build files against the contents of a manifest. The manifest can be available as a source file, or as part of a prebuilt APK.
The check is implemented in soong/scripts/manifest_check.py.
It is possible to turn off the check globally for a product by setting PRODUCT_BROKEN_VERIFY_USES_LIBRARIES := true in a product makefile, or for a particular build by setting RELAX_USES_LIBRARY_CHECK=true.
Compatibility libraries are libraries that didn’t exist prior to a certain SDK version (say, N), but classes in them were in the bootclasspath jars, etc., and in version N they have been separated into a standalone uses-library. Compatibility libraries should only be in the CLC of an app if its targetSdkVersion in the manifest is less than N.
Currently compatibility libraries only affect apps (but not other libraries).
The build system cannot see targetSdkVersion of an app at the time it generates dexpreopt build rules, so it doesn't know whether to add compatibility libaries to CLC or not. As a workaround, the build system includes all compatibility libraries regardless of the app version, and appends some extra logic to the dexpreopt rule that will extract targetSdkVersion from the manifest and filter CLC based on that version during Ninja stage of the build, immediately before executing the dexpreopt command (see the soong/scripts/construct_context.py script).
As of the time of writing (January 2022), there are the following compatibility libraries:
Sometimes uses-library tags are missing from the source manifest of a library/app. This may happen for example if one of the transitive dependencies of the library/app starts using another uses-library, and the library/app's manifest isn't updated to include it.
Soong can compute some of the missing uses-library tags for a given library/app automatically as SDK libraries in the transitive dependency closure of the library/app. The closure is needed because a library/app may depend on a static library that may in turn depend on an SDK library (possibly transitively via another library).
Not all uses-library tags can be computed in this way, because some of the uses-library dependencies are not SDK libraries, or they are not reachable via transitive dependency closure. But when possible, allowing Soong to calculate the manifest entries is less prone to errors and simplifies maintenance. For example, consider a situation when many apps use some static library that adds a new uses-library dependency -- all the apps will have to be updated. That is difficult to maintain.
There is also a manifest merger, because sometimes the final manifest of an app is merged from a few dependency manifests, so the final manifest installed on devices contains a superset of uses-library tags of the source manifest of the app.