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Deal with irreducible control flow when building traces.

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We filter out MachineLoop back-edges during the trace-building PO
traversals, but it is possible to have CFG cycles that aren't natural
loops, and MachineLoopInfo doesn't include such cycles.

Use a standard visited set to detect such CFG cycles, and completely
ignore them when picking traces.

llvm-svn: 161532
This commit is contained in:
Jakob Stoklund Olesen 2012-08-08 22:12:01 +00:00
parent 674a10c78a
commit 98908a7b0c
1 changed files with 22 additions and 17 deletions
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39
lib/CodeGen/MachineTraceMetrics.cpp
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@ -233,7 +233,9 @@ MinInstrCountEnsemble::pickTracePred(const MachineBasicBlock *MBB) {
const MachineBasicBlock *Pred = *I;
const MachineTraceMetrics::TraceBlockInfo *PredTBI =
getDepthResources(Pred);
assert(PredTBI && "Predecessor must be visited first");
// Ignore cycles that aren't natural loops.
if (!PredTBI)
continue;
// Pick the predecessor that would give this block the smallest InstrDepth.
unsigned Depth = PredTBI->InstrDepth + CurCount;
if (!Best || Depth < BestDepth)
@ -261,7 +263,9 @@ MinInstrCountEnsemble::pickTraceSucc(const MachineBasicBlock *MBB) {
continue;
const MachineTraceMetrics::TraceBlockInfo *SuccTBI =
getHeightResources(Succ);
assert(SuccTBI && "Successor must be visited first");
// Ignore cycles that aren't natural loops.
if (!SuccTBI)
continue;
// Pick the successor that would give this block the smallest InstrHeight.
unsigned Height = SuccTBI->InstrHeight;
if (!Best || Height < BestHeight)
@ -315,6 +319,7 @@ void MachineTraceMetrics::verifyAnalysis() const {
namespace {
struct LoopBounds {
MutableArrayRef<MachineTraceMetrics::TraceBlockInfo> Blocks;
SmallPtrSet<const MachineBasicBlock*, 8> Visited;
const MachineLoopInfo *Loops;
bool Downward;
LoopBounds(MutableArrayRef<MachineTraceMetrics::TraceBlockInfo> blocks,
@ -339,21 +344,19 @@ public:
if (LB.Downward ? TBI.hasValidHeight() : TBI.hasValidDepth())
return false;
// From is null once when To is the trace center block.
if (!From)
return true;
const MachineLoop *FromLoop = LB.Loops->getLoopFor(From);
if (!FromLoop)
return true;
// Don't follow backedges, don't leave FromLoop when going upwards.
if ((LB.Downward ? To : From) == FromLoop->getHeader())
return false;
// Don't leave FromLoop.
if (isExitingLoop(FromLoop, LB.Loops->getLoopFor(To)))
return false;
// This is a new block. The PO traversal will compute height/depth
// resources, causing us to reject new edges to To. This only works because
// we reject back-edges, so the CFG is cycle-free.
return true;
if (From) {
if (const MachineLoop *FromLoop = LB.Loops->getLoopFor(From)) {
// Don't follow backedges, don't leave FromLoop when going upwards.
if ((LB.Downward ? To : From) == FromLoop->getHeader())
return false;
// Don't leave FromLoop.
if (isExitingLoop(FromLoop, LB.Loops->getLoopFor(To)))
return false;
}
}
// To is a new block. Mark the block as visited in case the CFG has cycles
// that MachineLoopInfo didn't recognize as a natural loop.
return LB.Visited.insert(To);
}
};
}
@ -367,6 +370,7 @@ void MachineTraceMetrics::Ensemble::computeTrace(const MachineBasicBlock *MBB) {
// Run an upwards post-order search for the trace start.
Bounds.Downward = false;
Bounds.Visited.clear();
typedef ipo_ext_iterator<const MachineBasicBlock*, LoopBounds> UpwardPO;
for (UpwardPO I = ipo_ext_begin(MBB, Bounds), E = ipo_ext_end(MBB, Bounds);
I != E; ++I) {
@ -386,6 +390,7 @@ void MachineTraceMetrics::Ensemble::computeTrace(const MachineBasicBlock *MBB) {
// Run a downwards post-order search for the trace end.
Bounds.Downward = true;
Bounds.Visited.clear();
typedef po_ext_iterator<const MachineBasicBlock*, LoopBounds> DownwardPO;
for (DownwardPO I = po_ext_begin(MBB, Bounds), E = po_ext_end(MBB, Bounds);
I != E; ++I) {