ivopts: Refine register pressure estimate for loop invariants
Register pressure estimation in loop invariant optimization is adjusted for more accurate spilling costs.
The ivopts pass has adjusted its register pressure estimation to more accurately reflect the cost of spilling loop invariants versus induction variables. Previously, it assumed spilling loop invariants was half the cost of spilling induction variables. This change corrects the calculation to account for the fact that invariants only need to be loaded once, while induction variables must be loaded and stored per loop iteration.
In Details
The ivopts_estimate_reg_pressure function in tree-ssa-loop-ivopts.cc previously set the register pressure cost for spilling invariants to exactly half that of spilling IV candidates. This commit corrects this by recognizing that invariants are loaded once, whereas IVs are loaded and stored each iteration. The estimate is adjusted to reflect this difference, aiming for more accurate cost-based decisions in the loop invariant optimization pass, particularly regarding register allocation trade-offs.
- ivopts
- The GCC optimization pass focused on loop-invariant code motion and induction variable analysis and optimization.
- loop invariant
- An expression within a loop whose value does not change from one iteration to the next. Compilers can often hoist these expressions out of the loop to improve performance.
- induction variable
- A variable within a loop whose value is modified by a constant or other simple arithmetic progression in each iteration, typically used for loop control or array indexing.
- register pressure
- A measure of how many registers are simultaneously needed by the program. High register pressure can lead to more spills (storing register values to memory), impacting performance.
- spilling
- The process by which a compiler moves values from registers to memory when there are not enough registers available to hold all the live values.