Subzero: Recognize single-block loops during loop depth analysis.

src/IceLoopAnalyzer.cpp

@@ -34,6 +34,14 @@ void LoopAnalyzer::LoopNode::incrementLoopNestDepth() {
   BB->incrementLoopNestDepth();
 }
 
+bool LoopAnalyzer::LoopNode::hasSelfEdge() const {
+  for (CfgNode *Succ : BB->getOutEdges()) {
+    if (Succ == BB)
+      return true;
+  }
+  return false;
+}
+
 LoopAnalyzer::LoopAnalyzer(Cfg *Fn) : Func(Fn) {
   const NodeList &Nodes = Func->getNodes();
 
@@ -115,6 +123,8 @@ LoopAnalyzer::processNode(LoopAnalyzer::LoopNode &Node) {
   // Single node means no loop in the CFG
   if (LoopStack.back() == &Node) {
     LoopStack.back()->setOnStack(false);
+    if (Node.hasSelfEdge())
+      LoopStack.back()->incrementLoopNestDepth();
     LoopStack.back()->setDeleted();
     ++NumDeletedNodes;
     LoopStack.pop_back();

src/IceLoopAnalyzer.h

@@ -78,6 +78,7 @@ private:
     bool isDeleted() const { return Deleted; }
 
     void incrementLoopNestDepth();
+    bool hasSelfEdge() const;
 
   private:
     CfgNode *BB;

tests_lit/llvm2ice_tests/loop-nest-depth.ll

@@ -279,3 +279,28 @@ out:
 ; CHECK-NEXT: out:
 ; CHECK-NEXT: LoopNestDepth = 0
 ; CHECK-LABEL: Before RMW
+
+define internal void @test_single_block_loop(i32 %count) {
+entry:
+  br label %body
+body:
+;  %i = phi i32 [ 0, %entry ], [ %inc, %body ]
+; A normal loop would have a phi instruction like above for the induction
+; variable, but that may introduce new basic blocks due to phi edge splitting,
+; so we use an alternative definition for %i to make the test more clear.
+  %i = add i32 %count, 1
+  %inc = add i32 %i, 1
+  %cmp = icmp slt i32 %inc, %count
+  br i1 %cmp, label %body, label %exit
+exit:
+  ret void
+}
+
+; CHECK-LABEL: After loop nest depth analysis
+; CHECK-NEXT: entry:
+; CHECK-NEXT: LoopNestDepth = 0
+; CHECK-NEXT: body:
+; CHECK-NEXT: LoopNestDepth = 1
+; CHECK-NEXT: exit:
+; CHECK-NEXT: LoopNestDepth = 0
+; CHECK-LABEL: Before RMW