[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[Matt Arsenault via llvm-commits]

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@@ -1099,11 +1099,46 @@ void AsmPrinter::emitFunctionEntryLabel() {
   }
 }
 
+// Recognize cases where a spilled register is reloaded solely to feed into a
+// FAKE_USE.
+static bool isLoadFeedingIntoFakeUse(const MachineInstr &MI) {
+  const MachineFunction *MF = MI.getMF();
+  const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
+
+  // If the restore size is std::nullopt then we are not dealing with a reload
+  // of a spilled register.
+  if (!MI.getRestoreSize(TII))
+    return false;
+
+  // Check if this register is the operand of a FAKE_USE and
+  // does it have the kill flag set there.
+  auto NextI = std::next(MI.getIterator());
+  if (NextI == MI.getParent()->end() || !NextI->isFakeUse())
+    return false;
+
+  unsigned Reg = MI.getOperand(0).getReg();
+  for (const MachineOperand &MO : NextI->operands()) {
+    // Return true if we came across the register from the
+    // previous spill instruction that is killed in NextI.
+    if (MO.isReg() && MO.isUse() && MO.isKill() && MO.getReg() == Reg)
+      return true;
+  }
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arsenm wrote:

The preferred method is to maintain LiveRegUnits while walking from the end of the block (which is the wrong direction for the asmprinter...)

https://github.com/llvm/llvm-project/pull/86149