[llvm] [RISCV] Implement tail call optimization in machine outliner (PR #115297)

[Craig Topper via llvm-commits]

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@@ -2937,19 +2938,47 @@ bool RISCVInstrInfo::shouldOutlineFromFunctionByDefault(
   return MF.getFunction().hasMinSize();
 }
 
+static bool IsCandidatePatchable(const MachineInstr &MI) {
+  const MachineBasicBlock *MBB = MI.getParent();
+  const MachineFunction *MF = MBB->getParent();
+  const Function &F = MF->getFunction();
+  return F.getFnAttribute("fentry-call").getValueAsBool() ||
+         F.hasFnAttribute("patchable-function-entry");
+}
+
+static bool CannotInsertTailCall(const MachineInstr &MI) {
+  if (MI.isTerminator())
+    return IsCandidatePatchable(MI);
+  return true;
+}
+
+static bool MIUseX5(const MachineInstr &MI, const TargetRegisterInfo *TRI) {
+  return MI.modifiesRegister(RISCV::X5, TRI) ||
+         MI.getDesc().hasImplicitDefOfPhysReg(RISCV::X5);
+}
+
 std::optional<std::unique_ptr<outliner::OutlinedFunction>>
 RISCVInstrInfo::getOutliningCandidateInfo(
     const MachineModuleInfo &MMI,
     std::vector<outliner::Candidate> &RepeatedSequenceLocs,
     unsigned MinRepeats) const {
 
-  // First we need to filter out candidates where the X5 register (IE t0) can't
-  // be used to setup the function call.
-  auto CannotInsertCall = [](outliner::Candidate &C) {
+  auto CandidateUseX5 = [](outliner::Candidate &C) {
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topperc wrote:

Use -> Uses

https://github.com/llvm/llvm-project/pull/115297