LiveIntervalMap maps values from a parent LiveInterval to a child interval that
is a strict subset. It will create phi-def values as needed to preserve the
VNInfo SSA form in the child interval.
This leads to an algorithm very similar to the one in SSAUpdaterImpl.h, but with
enough differences that the code can't be reused:
- We don't need to manipulate PHI instructions.
- LiveIntervals have kills.
- We have MachineDominatorTree.
- We can use df_iterator.
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The earliestStart argument is entirely specific to linear scan allocation, and
can be easily calculated by RegAllocLinearScan.
Replace std::vector with SmallVector.
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When a live range is contained a single block, we can split it around
instruction clusters. The current approach is very primitive, splitting before
and after the largest gap between uses.
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split intervals. THis means the analysis can be used for multiple splits as long
as curli doesn't shrink.
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Before spilling a live range, we split it into a separate range for each basic
block where it is used. That way we only get one reload per basic block if the
new smaller ranges can allocate to a register.
This type of splitting is already present in the standard spiller.
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necessary.
Sometimes, live range splitting doesn't shrink the current interval, but simply
changes some instructions to use a new interval. That makes the original more
suitable for spilling. In this case, we don't need to duplicate the original.
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We are now at a point where we can split around simple single-entry, single-exit
loops, although still with some bugs.
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The spillers can pluck the analyses they need from the pass reference.
Switch some never-null pointers to references.
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Determine which loop exit blocks need a 'pre-exit' block inserted.
Recognize when this would be impossible.
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This is a work in progress. So far we have some basic loop analysis to help
determine where it is useful to split a live range around a loop.
The actual loop splitting code from Splitter.cpp is also going to move in here.
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