[clang] [llvm] [Clang][CMake] Add CSSPGO support to LLVM_BUILD_INSTRUMENTED (PR #79942)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> In general, CSSPGO is meant as a more practical alternative for IRPGO thanks to i) the use of sampled profile, ii) context-sensitivity. However, IRPGO is still expected to provide better performance in all cases where it's applicable thanks to accurate profile information.
Due to that, I don't think it makes sense to apply CSSPGO on top of IRPGO, and CMake automation would not permit mixing the two (both implemented as LLVM_BUILD_INSTRUMENTED exclusive options).

Ah. Might've been getting this confused with another technique (maybe IRPGO+instrumented context sensitive PGO?). I remember Google saying in https://discourse.llvm.org/t/practical-compiler-optimizations-for-wsc-workshop-us-llvm-dev-meeting-2023/73998 that a valid pipeline was doing two rounds of profiling with different levels of instrumentation.

Thanks for the link to the existing performance results.

> With CMake stuff implemented in this diff, it's expected that the amount of profile information collected would be miniscule (in-tree perf-training only has a single hello world source file) and inadequate for getting much perf boost with CSSPGO (it even prints the warning that it needs 6985000.0x more profile).

Right. Not sure if you've seen https://github.com/llvm/llvm-project/pull/78879, but that should allow making that pretty easy to do. There's also https://github.com/llvm/llvm-project/pull/77347 proposing to change the default, but there hasn't really been a consensus on that one.

Thank you for the additional information. Sorry for the derailment/noise.

https://github.com/llvm/llvm-project/pull/79942