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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The analysis will be needed by both the greedy register allocator and the
X86FloatingPoint pass. It only needs to be computed once when the CFG doesn't
change.
This pass is very fast, usually showing up as 0.0% wall time.
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Edge bundles is an annotation on the CFG that turns it into a bipartite directed
graph where each basic block is connected to an outgoing and an ingoing bundle.
These bundles are useful for identifying regions of the CFG for live range
splitting.
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the loop predecessors.
The register can be live-out from a predecessor without being live-in to the
loop header if there is a critical edge from the predecessor.
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Bypass loops have the current live range live through, but contain no uses or
defs. Splitting around a bypass loop can free registers for other uses inside
the loop by spilling the split range.
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Whenever splitting wants to insert a copy, it checks if the value can be
rematerialized cheaply instead.
Missing features:
- Delete instructions when all uses have been rematerialized.
- Truncate live ranges to the remaining uses after rematerialization.
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source, and let rewrite() clean it up.
This way, kill flags on the inserted copies are fixed as well during rewrite().
We can't just assume that all the copies we insert are going to be kills since
critical edges into loop headers sometimes require both source and dest to be
live out of a block.
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in SSAUpdaterImpl.h
Verifying live intervals revealed that the old method was completely wrong, and
we need an iterative approach to calculating PHI placemant. Fortunately, we have
MachineDominators available, so we don't have to compute that over and over
like SSAUpdaterImpl.h must.
Live-out values are cached between calls to mapValue() and computed in a greedy
way, so most calls will be working with very small block sets.
Thanks to Bob for explaining how this should work.
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proper SSA updating.
This doesn't cause MachineDominators to be recomputed since we are already
requiring MachineLoopInfo which uses dominators as well.
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Critical edges going into a loop are not as bad as critical exits. We can handle
them by splitting the critical edge, or by having both inside and outside
registers live out of the predecessor.
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the remainder register.
Example:
bb0:
x = 1
bb1:
use(x)
...
x = 2
jump bb1
When x is isolated in bb1, the inner part breaks into two components, x1 and x2:
bb0:
x0 = 1
bb1:
x1 = x0
use(x1)
...
x2 = 2
x0 = x2
jump bb1
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Jakob Stoklund Olesen
Benjamin Kramer