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Improving reload inheritance code generation and predictability


I have been continually vexed by the "joys" of reload inheritance. In particular its near randomness in identifying inheritance opportunities.

The fundamental problem is how we select a spill register. Right now we keep track of what hard regs are spill regs and we use them in a round-robin fashion. The idea is by using them in a round-robin fashion, a spill reg holds a useful value for a longer period of time, which in turn means we're more likely to find a reload inheritance optimization opportunity. Understandable, but it can be quite annoying in practice.

The problem with this is subtle changes in what's presented to reload changes the spills, which in turn changes which spill registers are selected for a particular insn needing spills. That in turn can mask or unmask reload inheritance opportunities, effectively at random because of the round robin selection of spill regs.

The randomness makes it extremely difficult to track down regressions in the range splitting code -- not to mention that it's totally silly that eliminating a reload somewhere in the stream suddenly causes an inheritance opportunity to be missed because last_reload_reg didn't get bumped due to the elimination of an unrelated reload.

So while I hate the inheritance code and loathe to spend time making it better, I just can't justify spending a days looking for these kinds of regresions. So attached you'll find a revamping of how we choose a register from the set of spill registers.

Basically we start searching forward in the stream from the current insn needing a reload noting uses of spill regs as we go. The further away from the insn needing reloads the spill reg is used the more desirable that spill reg becomes. If we can't find all the spill regs before reaching the end of the block, the remaining spill regs are considered the most desirable and we allocate these stragglers in the round-robin fashion.

The net result is that a spill reg holds a useful value for an even longer stretch of insns which in turn exposes more reload inheritance opportunities. Furthermore, reload inheritance becomes more stable in the opportunities it finds.

Obviously this has to be queued for stage1, but I wanted to get any thoughts/comments early...



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