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Spill mode: Hoist back-copies locally.

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The leaveIntvAfter() function normally inserts a back-copy after the
requested instruction, making the back-copy kill the live range.

In spill mode, try to insert the back-copy before the last use instead.
That means the last use becomes the kill instead of the back-copy.  This
lowers the register pressure because the last use can now redefine the
same register it was reading.

This will also improve compile time: The back-copy isn't a kill, so
hoisting it in hoistCopiesForSize() won't force a recomputation of the
source live range.  Similarly, if the back-copy isn't hoisted by the
splitter, the spiller will not attempt hoisting it locally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139883 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jakob Stoklund Olesen 2011-09-16 00:03:35 +00:00
parent 69cf1cac3f
commit ebac0c1747
1 changed files with 17 additions and 6 deletions
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23
lib/CodeGen/SplitKit.cpp
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@ -520,17 +520,28 @@ SlotIndex SplitEditor::leaveIntvAfter(SlotIndex Idx) {
DEBUG(dbgs() << " leaveIntvAfter " << Idx);
// The interval must be live beyond the instruction at Idx.
Idx = Idx.getBoundaryIndex();
VNInfo *ParentVNI = Edit->getParent().getVNInfoAt(Idx);
SlotIndex Boundary = Idx.getBoundaryIndex();
VNInfo *ParentVNI = Edit->getParent().getVNInfoAt(Boundary);
if (!ParentVNI) {
DEBUG(dbgs() << ": not live\n");
return Idx.getNextSlot();
return Boundary.getNextSlot();
}
DEBUG(dbgs() << ": valno " << ParentVNI->id << '\n');
MachineInstr *MI = LIS.getInstructionFromIndex(Idx);
MachineInstr *MI = LIS.getInstructionFromIndex(Boundary);
assert(MI && "No instruction at index");
VNInfo *VNI = defFromParent(0, ParentVNI, Idx, *MI->getParent(),
// In spill mode, make live ranges as short as possible by inserting the copy
// before MI. This is only possible if that instruction doesn't redefine the
// value. The inserted COPY is not a kill, and we don't need to recompute
// the source live range. The spiller also won't try to hoist this copy.
if (SpillMode && !SlotIndex::isSameInstr(ParentVNI->def, Idx) &&
MI->readsVirtualRegister(Edit->getReg())) {
forceRecompute(0, ParentVNI);
defFromParent(0, ParentVNI, Idx, *MI->getParent(), MI);
return Idx;
}
VNInfo *VNI = defFromParent(0, ParentVNI, Boundary, *MI->getParent(),
llvm::next(MachineBasicBlock::iterator(MI)));
return VNI->def;
}