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

[Matt Arsenault via llvm-commits]

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@@ -1099,11 +1099,45 @@ 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 TargetInstrInfo *TII) {
+  const MachineFunction *MF = MI.getMF();
+
+  // 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;
+  }
+
+  return false;
+}
+
 /// emitComments - Pretty-print comments for instructions.
 static void emitComments(const MachineInstr &MI, raw_ostream &CommentOS) {
   const MachineFunction *MF = MI.getMF();
   const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
 
+  // If this is a reload of a spilled register that only feeds into a FAKE_USE
+  // instruction, meaning the load value has no effect on the program and has
+  // only been kept alive for debugging; since it is still available on the
+  // stack, we can skip the load itself.
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arsenm wrote:

I don't understand the comment in this context. The load isn't being skipped, it's just skipping the comments about the instruction 

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