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Teach MachineSinking to handle easy critical edges.

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Sometimes it is desirable to sink instructions along a critical edge:

x = ...
if (a && b) ...
else use(x);

The 'a && b' condition creates a critical edge to the else block, but we still
want to sink the computation of x into the block. The else block is dominated by
the parent block, so we are not pushing instructions into new code paths.

llvm-svn: 101165
This commit is contained in:
Jakob Stoklund Olesen
2010-04-13 19:06:14 +00:00
parent dd9a259975
commit 20b71e28cc
1 changed files with 17 additions and 2 deletions
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19
llvm/lib/CodeGen/MachineSink.cpp
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@@ -276,8 +276,23 @@ bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
// but for now we just punt.
// FIXME: Split critical edges if not backedges.
if (SuccToSinkTo->pred_size() > 1) {
DEBUG(dbgs() << " *** PUNTING: Critical edge found\n");
return false;
// We cannot sink a load across a critical edge - there may be stores in
// other code paths.
bool store = true;
if (!MI->isSafeToMove(TII, AA, store)) {
DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n");
return false;
}
// We don't want to sink across a critical edge if we don't dominate the
// successor. We could be introducing calculations to new code paths.
if (!DT->dominates(ParentBlock, SuccToSinkTo)) {
DEBUG(dbgs() << " *** PUNTING: Critical edge found\n");
return false;
}
// Otherwise we are OK with sinking along a critical edge.
DEBUG(dbgs() << "Sinking along critical edge.\n");
}
// Determine where to insert into. Skip phi nodes.