[Profiler] Avoid introducing empty unreachable regions

lib/SIL/IR/SILProfiler.cpp

@@ -341,31 +341,65 @@ class CounterExpr {
   /// Returns true if this is a Zero node.
   bool isZero() const { return K == Kind::Zero; }
 
+  /// Returns true if the counter is semantically a Zero node. This considers
+  /// the simplified version of the counter that has eliminated redundant
+  /// operations.
+  bool isSemanticallyZero() const {
+    // Run the counter through the counter builder to simplify it, using a dummy
+    // mapping of unique counter indices for each node reference. The value of
+    // the indices doesn't matter, but we need to ensure that e.g subtraction
+    // of a node from itself cancels out.
+    llvm::coverage::CounterExpressionBuilder Builder;
+    llvm::DenseMap<ASTNode, unsigned> DummyIndices;
+    unsigned LastIdx = 0;
+    auto Counter = expand(Builder, [&](auto Node) {
+      if (!DummyIndices.count(Node)) {
+        DummyIndices[Node] = LastIdx;
+        LastIdx += 1;
+      }
+      return DummyIndices[Node];
+    });
+    return Counter.isZero();
+  }
+
   /// Expand this node into an llvm::coverage::Counter.
   ///
   /// Updates \c Builder with any expressions that are needed to represent this
   /// counter.
   llvm::coverage::Counter
   expand(llvm::coverage::CounterExpressionBuilder &Builder,
-         llvm::DenseMap<ASTNode, unsigned> &Counters) const {
+         llvm::function_ref<unsigned(ASTNode)> GetCounterIdx) const {
     switch (K) {
     case Kind::Zero:
       return llvm::coverage::Counter::getZero();
     case Kind::Node:
-      return llvm::coverage::Counter::getCounter(Counters[Node]);
+      return llvm::coverage::Counter::getCounter(GetCounterIdx(Node));
     case Kind::Add:
-      return Builder.add(LHS->expand(Builder, Counters),
-                         RHS->expand(Builder, Counters));
+      return Builder.add(LHS->expand(Builder, GetCounterIdx),
+                         RHS->expand(Builder, GetCounterIdx));
     case Kind::Sub:
-      return Builder.subtract(LHS->expand(Builder, Counters),
-                              RHS->expand(Builder, Counters));
+      return Builder.subtract(LHS->expand(Builder, GetCounterIdx),
+                              RHS->expand(Builder, GetCounterIdx));
     case Kind::Ref:
-      return LHS->expand(Builder, Counters);
+      return LHS->expand(Builder, GetCounterIdx);
     }
 
     llvm_unreachable("Unhandled Kind in switch.");
   }
 
+  /// Expand this node into an llvm::coverage::Counter.
+  ///
+  /// Updates \c Builder with any expressions that are needed to represent this
+  /// counter.
+  llvm::coverage::Counter
+  expand(llvm::coverage::CounterExpressionBuilder &Builder,
+         const llvm::DenseMap<ASTNode, unsigned> &Counters) const {
+    return expand(Builder, [&](auto Node) {
+      // FIXME: We ought to assert that the node is present.
+      return Counters.lookup(Node);
+    });
+  }
+
   void print(raw_ostream &OS) const {
     switch (K) {
     case Kind::Zero:
@@ -780,7 +814,17 @@ struct CoverageMapping : public ASTWalker {
     if (ControlFlowAdjust)
       Count = &createCounter(CounterExpr::Sub(*Count, *ControlFlowAdjust));
 
-    RegionStack.emplace_back(ASTNode(), *Count, getEndLoc(Scope), None);
+    if (Count->isSemanticallyZero()) {
+      // If the counter is semantically zero, form an 'incomplete' region with
+      // no starting location. This prevents forming unreachable regions unless
+      // there is a following statement or expression to extend the region.
+      RegionStack.emplace_back(ASTNode(), *Count, None, None);
+    } else {
+      // Otherwise, we have a non-zero counter, so form a new region starting
+      // at the end of the previous scope. This ensures the user covers both
+      // branches of a condition.
+      RegionStack.emplace_back(ASTNode(), *Count, getEndLoc(Scope), None);
+    }
   }
 
   /// Push a region covering \c Node onto the stack.

test/Profiler/coverage_if.swift

@@ -17,3 +17,74 @@ func foo(x : Bool) { // CHECK: [[@LINE]]:20 -> {{[0-9]+}}:2 : 0
 
 foo(x: true);
 foo(x: false);
+
+// rdar://29390569 – Make sure we don't add a spurious unreachable empty region.
+
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo1
+func foo1() -> Int {    // CHECK: [[@LINE]]:20 -> {{[0-9]+}}:2 : 0
+  if .random() {        // CHECK: [[@LINE]]:6 -> [[@LINE]]:15 : 0
+    return 0            // CHECK: [[@LINE-1]]:16 -> [[@LINE+2]]:4 : 1
+                        // CHECK: [[@LINE+1]]:4 -> {{[0-9]+}}:2 : (0 - 1)
+  } else if .random() { // CHECK: [[@LINE]]:13 -> [[@LINE]]:22 : (0 - 1)
+    return 1            // CHECK: [[@LINE-1]]:23 -> [[@LINE+1]]:4 : 2
+  } else {              // CHECK: [[@LINE]]:4 -> {{[0-9]+}}:2 : ((0 - 1) - 2)
+    return 2            // CHECK: [[@LINE-1]]:10 -> [[@LINE+1]]:4 : ((0 - 1) - 2)
+  }                     // CHECK-NOT: zero
+}
+
+// ...but we will add an unreachable region if you write code there.
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo2
+func foo2() -> Int {
+  if .random() {
+    return 0
+  } else if .random() {
+    return 1
+  } else {
+    return 2
+  }
+  _ = foo1() // CHECK: [[@LINE]]:3 -> [[@LINE+1]]:2 : zero
+}
+
+// Make sure we don't add unreachable regions for a labeled jump either.
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo3
+func foo3() -> Int { // CHECK: [[@LINE]]:20 -> {{[0-9]+}}:2 : 0
+  x: do {            // CHECK: [[@LINE]]:9 -> [[@LINE+6]]:4 : 0
+    if .random() {   // CHECK: [[@LINE]]:8 -> [[@LINE]]:17 : 0
+      return 0       // CHECK: [[@LINE-1]]:18 -> [[@LINE+1]]:6 : 1
+    } else {         // CHECK: [[@LINE]]:6 -> [[@LINE+3]]:4 : (0 - 1)
+      break x        // CHECK: [[@LINE-1]]:12 -> [[@LINE+1]]:6 : (0 - 1)
+    }                // CHECK-NOT: zero
+  }
+  return 2
+}
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo4
+func foo4() -> Int {
+  x: do {
+    if .random() {
+      return 0
+    } else {
+      break x
+    }
+    let y = 0 // CHECK: [[@LINE]]:13 -> [[@LINE+1]]:4 : zero
+  }
+  return 2
+}
+
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo5
+func foo5() throws -> Int { // CHECK: [[@LINE]]:27 -> {{[0-9]+}}:2 : 0
+  if .random() {            // CHECK: [[@LINE]]:6 -> [[@LINE]]:15 : 0
+    return 0                // CHECK: [[@LINE-1]]:16 -> [[@LINE+1]]:4 : 1
+  } else {                  // CHECK: [[@LINE]]:4 -> {{[0-9]+}}:2 : (0 - 1)
+    struct S: Error {}      // CHECK: [[@LINE-1]]:10 -> [[@LINE+2]]:4 : (0 - 1)
+    throw S()
+  }                         // CHECK-NOT: zero
+}
+
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo6
+func foo6(_ x: Int?) -> Int? { // CHECK: [[@LINE]]:30 -> {{[0-9]+}}:2 : 0
+  if let x = x {
+    return x                   // CHECK: [[@LINE-1]]:16 -> [[@LINE+1]]:4 : 1
+  } else {                     // CHECK: [[@LINE]]:4 -> {{[0-9]+}}:2 : (0 - 1)
+    return nil                 // CHECK: [[@LINE-1]]:10 -> [[@LINE+1]]:4 : (0 - 1)
+  }                            // CHECK-NOT: zero
+}