About
The ZiCondops extension provides a conditional zero primitive upon which subsets of conditional move and conditional arithmetic/logical operations can be implemented. Transforming control flow into conditional operations can improve code performance by eliminating branch mispredict costs as well as reducing the load on the branch predictors. The earlier in the optimizer pipeline these transformations are performed the more likely they are to expose secondary optimization opportunities as well since the transformations result in larger basic blocks (a fundamental unit of code most compiler optimizations work on).
This work consists of 3 subprojects:
- Basic ZICondops patterns in the RISC-V target description
- Improvements to generate those patterns early in the RTL optimizer pipeline
- Improvements to the if-converter to consistently utilize the conditional zero primitive provided by ZICondops
Evaluation of the initial implementation has indicated that idioms to implement 32 bit operations on rv64 are consistently not if-converted. Improvements to capture those cases are in progress.
This work was initially implemented by a contractor working for Ventana with significant improvements from Ventana's engineers over time.
Dependencies
Binutils needs to have ZICondops approved before compilers can utilize it.
Status
Updates
- Project reported as priority for 2H23
- Work posted for upstream review. Adjustments/Updates in progress.
- ZICondops binutils patch (re)approved