One of the main missing features in Clang/LLVM profile runtime is the lack of support for online/in-process profile merging support. Profile data collected for different workloads for the same executable binary need to be collected and merged later by the offline post-processing tool. This limitation makes it hard to handle cases where the instrumented binary needs to be run with large number of small workloads, possibly in parallel. For instance, to do PGO for clang, we may choose to build a large project with the instrumented Clang binary. This is because
- to avoid profile from different runs from overriding others, %p substitution needs to be specified in either the command line or an environment variable so that different process can dump profile data into its own file named using pid. This will create huge requirement on the disk storage. For instance, clang’s raw profile size is typically 80M – if the instrumented clang is used to build a medium to large size project (such as clang itself), profile data can easily use up hundreds of Gig bytes of local storage.
- pid can also be recycled. This means that some of the profile data may be overridden without being noticed.
The way to solve this problem is to allow profile data to be merged in process. I have a prototype implementation and plan to send it out for review soon after some clean ups. By default, the profiling merging is off and it can be turned on with an user option or via an environment variable. The following summarizes the issues involved in adding this feature:
- the target platform needs to have file locking support
- there needs an efficient way to identify the profile data and associate it with the binary using binary/profdata signature;
- Currently without merging, profile data from shared libraries (including dlopen/dlcose ones) are concatenated into the primary profile file. This can complicate matters, as the merger also needs to find the matching shared libs, and the merger also needs to avoid unnecessary data movement/copy;
- value profile data is variable in length even for the same binary.
All the above issues are resolved and clang self build with instrumented binary passes (with both j1 and high parallelism).
If you have any concerns, please let me know.