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LiveRangeShrink: Early exit when encountering a code motion barrier. #136806

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Merged
merged 4 commits into from
Apr 24, 2025
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LiveRangeShrink: Early exit when encountering a code motion barrier. #136806

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[𝘀𝗽𝗿] changes to main this commit is based on
pcc Apr 23, 2025
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[𝘀𝗽𝗿] initial version
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[𝘀𝗽𝗿] changes introduced through rebase
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pcc Apr 23, 2025
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18 changes: 13 additions & 5 deletions llvm/lib/CodeGen/LiveRangeShrink.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -95,14 +95,25 @@ static MachineInstr *FindDominatedInstruction(MachineInstr &New,
return Old;
}

/// Returns whether this instruction is considered a code motion barrier by this
/// pass. We can be less conservative than hasUnmodeledSideEffects() when
/// deciding whether an instruction is a barrier because it is known that pseudo
/// probes are safe to move in this pass specifically (see commit 1cb47a063e2b).
static bool isCodeMotionBarrier(MachineInstr &MI) {
return MI.hasUnmodeledSideEffects() && !MI.isPseudoProbe();
}
Comment on lines +102 to +104
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@arsenm arsenm Apr 23, 2025

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Document this? What is the exact problematic condition? We have several different barrier concepts already spread in MachineInstr and TargetInstrInfo, can we use one of those

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@pcc pcc Apr 23, 2025

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Added a comment that describes the situation to the best of my understanding. I don't think the existing ones can be used because the condition is specific to what this pass is doing.


/// Builds Instruction to its dominating order number map \p M by traversing
/// from instruction \p Start.
static void BuildInstOrderMap(MachineBasicBlock::iterator Start,
InstOrderMap &M) {
M.clear();
unsigned i = 0;
for (MachineInstr &I : make_range(Start, Start->getParent()->end()))
for (MachineInstr &I : make_range(Start, Start->getParent()->end())) {
if (isCodeMotionBarrier(I))
break;
M[&I] = i++;
}
}

bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
Expand Down Expand Up @@ -142,8 +153,6 @@ bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
while (Next != MBB.end()) {
MachineInstr &MI = *Next;
Next = MBB.SkipPHIsLabelsAndDebug(++Next);
if (MI.mayStore())
SawStore = true;

unsigned CurrentOrder = IOM[&MI];
unsigned Barrier = 0;
Expand All @@ -166,8 +175,7 @@ bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
// If MI has side effects, it should become a barrier for code motion.
// IOM is rebuild from the next instruction to prevent later
// instructions from being moved before this MI.
if (MI.hasUnmodeledSideEffects() && !MI.isPseudoProbe() &&
Next != MBB.end()) {
if (isCodeMotionBarrier(MI) && Next != MBB.end()) {
BuildInstOrderMap(Next, IOM);
SawStore = false;
}
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