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[NewPM] LiveIntervals: Check dependencies for invalidation #123563

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Jan 24, 2025
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[NewPM] LiveIntervals: Check dependencies for invalidation #123563

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[NewPM] LiveIntervals: Check dependencies for invalidation
optimisan Jan 20, 2025
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remove parenthesis
optimisan Jan 20, 2025
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3 changes: 3 additions & 0 deletions llvm/include/llvm/CodeGen/LiveIntervals.h
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Original file line number Diff line number Diff line change
Expand Up @@ -113,6 +113,9 @@ class LiveIntervals {
LiveIntervals(LiveIntervals &&) = default;
~LiveIntervals();

bool invalidate(MachineFunction &MF, const PreservedAnalyses &PA,
MachineFunctionAnalysisManager::Invalidator &Inv);

/// Calculate the spill weight to assign to a single instruction.
/// If \p PSI is provided the calculation is altered for optsize functions.
static float getSpillWeight(bool isDef, bool isUse,
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14 changes: 14 additions & 0 deletions llvm/lib/CodeGen/LiveIntervals.cpp
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Expand Up @@ -127,6 +127,20 @@ LiveIntervalsWrapperPass::LiveIntervalsWrapperPass() : MachineFunctionPass(ID) {

LiveIntervals::~LiveIntervals() { clear(); }

bool LiveIntervals::invalidate(
MachineFunction &MF, const PreservedAnalyses &PA,
MachineFunctionAnalysisManager::Invalidator &Inv) {
auto PAC = PA.getChecker<LiveIntervalsAnalysis>();

if (!PAC.preserved() && !PAC.preservedSet<AllAnalysesOn<MachineFunction>>())
return true;

// LiveIntervals holds pointers to these results, so check for their
// invalidation.
return (Inv.invalidate<SlotIndexesAnalysis>(MF, PA) ||
Inv.invalidate<MachineDominatorTreeAnalysis>(MF, PA));
}
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IIUC when control flow reaches here, it implies live intervals or all machine function analyses are preserved. It invalidates them forcibly. Move them to SlotIndexesAnalysis?

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invalidate() actually means isThisResultInvalid(IR, PA), so if SlotIndexes reports that it has been made invalid, this analysis should also be invalid. SI and MDT are good with the default invalidate() implementation since they don't depend on other analyses.

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Then

PreservedAnalyses SomePass::run(IRUnitT &, AnalysisManager &) {
  PreservedAnalyses PA;
  PA.preserve<LiveIntervals>();
  return PA;
}

will no longer work, which is counter-intuitive... because it doesn't preserve SlotIndexes explicitly.

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Yes, it is related here #123045 (comment)

New PM can't report such cases. Could add such a functionality behind a flag but I guess it's unlikely for more cases like this to exist, since they would lead to invalid reads (as the PM deletes the results).

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Then

PreservedAnalyses SomePass::run(IRUnitT &, AnalysisManager &) {
  PreservedAnalyses PA;
  PA.preserve<LiveIntervals>();
  return PA;
}

will no longer work, which is counter-intuitive... because it doesn't preserve SlotIndexes explicitly.

if SlotIndexes is invalidated, then LiveIntervals may reference an invalid SlotIndexes, so if we kept around LiveIntervals it may be invalid to use. so invalidating it regardless makes sense.

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Yes, it is related here #123045 (comment)

New PM can't report such cases. Could add such a functionality behind a flag but I guess it's unlikely for more cases like this to exist, since they would lead to invalid reads (as the PM deletes the results).

I don't think it's easily trackable which analyses depend on other analyses without a bunch of extra infrastructure, so unless this is a problem that comes up a lot I'm inclined to say it's not worth the effort


void LiveIntervals::clear() {
// Free the live intervals themselves.
for (unsigned i = 0, e = VirtRegIntervals.size(); i != e; ++i)
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