[MachinePipeliner] Detect a cycle in PHI dependencies early on

Abort pipelining in the below case

%1 = phi i32 [ %a, %entry ], [ %3, %loop ]
%2 = phi i32 [ %a, %entry ], [ %1, %loop ]
%3 = phi i32 [ %b, %entry ], [ %2, %loop ]

Author: quic-asaravan

llvm/lib/CodeGen/MachinePipeliner.cpp

@@ -485,6 +485,56 @@ void MachinePipeliner::setPragmaPipelineOptions(MachineLoop &L) {
   }
 }
 
+bool hasPHICycle(const MachineBasicBlock *LoopHeader,
+                 const MachineRegisterInfo &MRI) {
+  DenseMap<unsigned, SmallVector<unsigned, 2>> PhiDeps;
+  SmallSet<unsigned, 8> PhiRegs;
+
+  // Collect PHI nodes and their dependencies
+  for (const MachineInstr &MI : *LoopHeader) {
+    if (!MI.isPHI())
+      continue;
+
+    unsigned DefReg = MI.getOperand(0).getReg();
+    PhiRegs.insert(DefReg);
+
+    for (unsigned i = 1; i < MI.getNumOperands(); i += 2) {
+      unsigned SrcReg = MI.getOperand(i).getReg();
+      PhiDeps[DefReg].push_back(SrcReg);
+    }
+  }
+
+  // DFS to detect cycles
+  SmallSet<unsigned, 8> Visited, RecStack;
+
+  std::function<bool(unsigned)> DFS = [&](unsigned Reg) -> bool {
+    if (!PhiRegs.count(Reg))
+      return false;
+    if (RecStack.count(Reg))
+      return true;
+    if (Visited.count(Reg))
+      return false;
+
+    Visited.insert(Reg);
+    RecStack.insert(Reg);
+
+    for (unsigned Dep : PhiDeps[Reg]) {
+      if (DFS(Dep))
+        return true;
+    }
+
+    RecStack.erase(Reg);
+    return false;
+  };
+
+  for (unsigned Reg : PhiRegs) {
+    if (DFS(Reg))
+      return true;
+  }
+
+  return false;
+}
+
 /// Return true if the loop can be software pipelined.  The algorithm is
 /// restricted to loops with a single basic block.  Make sure that the
 /// branch in the loop can be analyzed.
@@ -499,6 +549,11 @@ bool MachinePipeliner::canPipelineLoop(MachineLoop &L) {
     return false;
   }
 
+  if (hasPHICycle(L.getHeader(), MF->getRegInfo())) {
+    LLVM_DEBUG(dbgs() << "Cannot pipeline loop due to PHI cycle\n");
+    return false;
+  }
+
   if (disabledByPragma) {
     ORE->emit([&]() {
       return MachineOptimizationRemarkAnalysis(DEBUG_TYPE, "canPipelineLoop",

llvm/test/CodeGen/Hexagon/swp-phi-cycle.ll

@@ -0,0 +1,22 @@
+; RUN: llc -mtriple=hexagon-unknown-linux-gnu -enable-pipeliner -debug-only=pipeliner < %s 2>&1 | FileCheck %s
+
+; CHECK: Cannot pipeline loop due to PHI cycle
+
+define void @phi_cycle_loop(i32 %a, i32 %b) {
+entry:
+  br label %loop
+
+loop:
+  %1 = phi i32 [ %a, %entry ], [ %3, %loop ]
+  %2 = phi i32 [ %a, %entry ], [ %1, %loop ]
+  %3 = phi i32 [ %b, %entry ], [ %2, %loop ]
+
+  ; Prevent PHI elimination by using all values
+  %add1 = add i32 %1, %2
+  %add2 = add i32 %add1, %3
+  %cmp = icmp slt i32 %add2, 100
+  br i1 %cmp, label %loop, label %exit
+
+exit:
+  ret void
+}