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Break false dependencies before partial register updates.

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Two new TargetInstrInfo hooks lets the target tell ExecutionDepsFix
about instructions with partial register updates causing false unwanted
dependencies.

The ExecutionDepsFix pass will break the false dependencies if the
updated register was written in the previoius N instructions.

The small loop added to sse-domains.ll runs twice as fast with
dependency-breaking instructions inserted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144602 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jakob Stoklund Olesen
2011-11-15 01:15:30 +00:00
parent 2947f730a9
commit c2ecf3efbf
5 changed files with 193 additions and 0 deletions
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27
lib/CodeGen/ExecutionDepsFix.cpp
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@ -471,11 +471,34 @@ void ExeDepsFix::processDefs(MachineInstr *MI, bool Kill) {
DEBUG(dbgs() << TRI->getName(RC->getRegister(rx)) << ":\t" << CurInstr
<< '\t' << *MI);
// How many instructions since rx was last written?
unsigned Clearance = CurInstr - LiveRegs[rx].Def;
LiveRegs[rx].Def = CurInstr;
// Kill off domains redefined by generic instructions.
if (Kill)
kill(rx);
// Verify clearance before partial register updates.
unsigned Pref = TII->getPartialRegUpdateClearance(MI, i, TRI);
if (!Pref)
continue;
DEBUG(dbgs() << "Clearance: " << Clearance << ", want " << Pref);
if (Pref > Clearance) {
DEBUG(dbgs() << ": Break dependency.\n");
TII->breakPartialRegDependency(MI, i, TRI);
continue;
}
// The current clearance seems OK, but we may be ignoring a def from a
// back-edge.
if (!SeenUnknownBackEdge || Pref <= unsigned(CurInstr)) {
DEBUG(dbgs() << ": OK.\n");
continue;
}
// A def from an unprocessed back-edge may make us break this dependency.
DEBUG(dbgs() << ": Wait for back-edge to resolve.\n");
}
++CurInstr;
@ -663,6 +686,10 @@ bool ExeDepsFix::runOnMachineFunction(MachineFunction &mf) {
for (unsigned i = 0, e = Loops.size(); i != e; ++i) {
MachineBasicBlock *MBB = Loops[i];
enterBasicBlock(MBB);
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;
++I)
if (!I->isDebugValue())
processDefs(I, false);
leaveBasicBlock(MBB);
}