This project demonstrates how to leverage PMU feedback with SEP on Windows or perf on Linux.
For portability, this project is described as a CMake project.
Included is an attempt at an HWPGO.cmake module which can help to add HWPGO feedback to an existing project.
The top-level CMakeLists.txt contains comments describing how to use HWPGO.cmake, which should be suitable for use in other CMake projects, although it has not been widely tested.
Building without HWPGO:
cmake -G Ninja -B nopgo -S . -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=icx
cmake --build nopgo --parallel
Building with HWPGO:
cmake -G Ninja -B hwpgo -S . -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=icx -DHWPGO=On
cmake --build hwpgo --parallel
In the case of the unpredictable executable the "if" condition in the hot loop is difficult to predict:
hwpgo-mispredict-example/unpredictable.c
Lines 24 to 31 in 493f07b
Profitability heuristics based on static analysis are unlikely to eliminate control flow and implement the selection of p with cmov because it would require unconditionally computing z in case its value is needed.
HWPGO allows the compiler to understand that more aggressive speculation is worthwhile, and so it implements the loop body with conditional moves rather than a branch.
$ time ./nopgo/unpredictable
./nopgo/unpredictable 0.99s user 0.00s system 99% cpu 0.992 total
$ time ./hwpgo/unpredictable
./hwpgo/3 0.19s user 0.00s system 99% cpu 0.197 total
The result is approximately a 5x speedup.
The predictable example shows the case where a branch is predictable and cmov conversion would be harmful.
Although the branch remains taken about 50% of the time, the taken/not-taken pattern is predictable to hardware.
HWPGO generates an empty mispredict profile in this case, indicating that there is no mispredict problem.
As a result, cmov is not used and performance is preserved.
hwpgo-mispredict-example/predictable.c
Lines 22 to 29 in 493f07b