Registers are not allocated strictly in spill weight order when live range
splitting and spilling has created new shorter intervals with higher spill
weights.
When one of the new heavy intervals conflicts with a single lighter interval,
simply evict the old interval instead of trying to split the heavy one.
The lighter interval is a better candidate for splitting, it has a smaller use
density.
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The last split point can be anywhere in the block, so it interferes with the
strictly monotonic requirements of advanceTo().
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This is a lot easier than trying to get kill flags right during live range
splitting and rematerialization.
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If a live range is used by a terminator instruction, and that live range needs
to leave the block on the stack or in a different register, it can be necessary
to have both sides of the split live at the terminator instruction.
Example:
%vreg2 = COPY %vreg1
JMP %vreg1
Becomes after spilling %vreg2:
SPILL %vreg1
JMP %vreg1
The spill doesn't kill the register as is normally the case.
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If interference reaches the last split point, it is effectively live out and
should be marked as 'MustSpill'.
This can make a difference when the terminator uses a register. There is no way
that register can be reused in the outgoing CFG bundle, even if it isn't live
out.
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We should not be attempting a region split if it won't lead to at least one
directly allocatable interval. That could cause infinite splitting loops.
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When the live range is live through a block that doesn't use the register, but
that has interference, region splitting wants to split at the top and bottom of
the basic block.
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These end points come from the inserted copies, and can be passed directly to
useIntv. This simplifies the coloring code.
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The greedy register allocator revealed some problems with the value mapping in
SplitKit. We would sometimes start mapping values before all defs were known,
and that could change a value from a simple 1-1 mapping to a multi-def mapping
that requires ssa update.
The new approach collects all defs and register assignments first without
filling in any live intervals. Only when finish() is called, do we compute
liveness and mapped values. At this time we know with certainty which values map
to multiple values in a split range.
This also has the advantage that we can compute live ranges based on the
remaining uses after rematerializing at split points.
The current implementation has many opportunities for compile time optimization.
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The value mapping gets confused about which original values have multiple new
definitions so they may need phi insertions.
This could probably be simplified by letting enterIntvBefore() take a live range
to be added following the instruction. As long as the range stays inside the
same basic block, value mapping shouldn't be a problem.
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interval after an instruction. The leaveIntvAfter() method only adds liveness
from the instruction's boundary index to the inserted copy.
Ideally, SplitKit should be smarter about this, perhaps by combining useIntv()
and leaveIntvAfter() into one method that guarantees continuity.
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Region splitting includes loop splitting as a subset, and it is more generic.
The splitting heuristics for variables that are live in more than one block are
now:
1. Try to create a region that covers multiple basic blocks.
2. Try to create a new live range for each block with multiple uses.
3. Spill.
Steps 2 and 3 are similar to what the standard spiller is doing.
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Analyze the live range's behavior entering and leaving basic blocks. Compute an
interference pattern for each allocation candidate, and use SpillPlacement to
find an optimal region where that register can be live.
This code is still not enabled.
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createMachineVerifierPass and MachineFunction::verify.
The banner is printed before the machine code dump, just like the printer pass.
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RegAllocBase::VerifyEnabled.
Run the machine code verifier in a few interesting places during RegAllocGreedy.
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The heuristics split around the largest loop where the current register may be
allocated without interference.
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live range splitting around loops guided by register pressure.
So far, trySplit() simply prints a lot of debug output.
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spill weight. Filter out fixed registers instead.
Add support for reassigning an interference that was assigned to an alias.
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abstract priority queue interface in subclasses that want to override the
priority calculations.
Subclasses must provide a getPriority() implementation instead.
This approach requires less code as long as priorities are expressable as simple
floats, and it avoids the dangers of defining potentially expensive priority
comparison functions.
It also should speed up priority_queue operations since they no longer have to
chase pointers when comparing registers. This is not measurable, though.
Preferably, we shouldn't use floats to guide code generation. The use of floats
here is derived from the use of floats for spill weights. Spill weights have a
dynamic range that doesn't lend itself easily to a fixpoint implementation.
When someone invents a stable spill weight representation, it can be reused for
allocation priorities.
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