[SILOptimizer]: slow OSSA lifetime canonicalization mitigation

OSSA lifetime canonicalization can take a very long time in certain
cases in which there are large basic blocks. to mitigate this, add logic
to skip walking the liveness boundary for extending liveness to dead
ends when there aren't any dead ends in the function.

Updates `DeadEndBlocks` with a new `isEmpty` method and cache to
determine if there are any dead-end blocks in a given function.

Author: jamieQ

include/swift/SIL/BasicBlockUtils.h

@@ -68,6 +68,10 @@ class DeadEndBlocks {
   const SILFunction *f;
   bool didComputeValue = false;
 
+  /// When non-null, indicates whether dead-end blocks are present
+  /// in the current function.
+  std::optional<bool> hasAnyDeadEnds = std::nullopt;
+
   void compute();
 
 public:
@@ -85,6 +89,17 @@ class DeadEndBlocks {
     return reachableBlocks.count(block) == 0;
   }
 
+  /// Returns true iff none of the function's blocks is a dead-end.
+  /// Note: The underlying value is lazily computed & cached.
+  bool isEmpty() {
+    if (!hasAnyDeadEnds.has_value()) {
+      hasAnyDeadEnds = llvm::any_of(
+          *f, [this](const SILBasicBlock &BB) { return isDeadEnd(&BB); });
+    }
+
+    return !hasAnyDeadEnds.value();
+  }
+
   /// Return true if this dead end blocks has computed its internal cache yet.
   ///
   /// Used to determine if we need to verify a DeadEndBlocks.

include/swift/SILOptimizer/Utils/CanonicalizeOSSALifetime.h

@@ -475,6 +475,10 @@ class CanonicalizeOSSALifetime final {
     return !endingLifetimeAtExplicitEnds();
   }
 
+  bool hasAnyDeadEnds() const {
+    return !deadEndBlocksAnalysis->get(function)->isEmpty();
+  }
+
   bool respectsDeinitBarriers() const {
     if (!currentDef->isLexical())
       return false;

lib/SIL/Utils/BasicBlockUtils.cpp

@@ -446,6 +446,16 @@ static FunctionTest DeadEndBlocksTest("dead_end_blocks", [](auto &function,
   }
 #endif
 });
+
+// Arguments:
+// - none
+// Dumps:
+// - message
+static FunctionTest HasAnyDeadEndBlocksTest(
+    "has_any_dead_ends", [](auto &function, auto &arguments, auto &test) {
+      auto deb = test.getDeadEndBlocks();
+      llvm::outs() << (deb->isEmpty() ? "no dead ends\n" : "has dead ends\n");
+    });
 } // end namespace swift::test
 
 //===----------------------------------------------------------------------===//

lib/SILOptimizer/Utils/CanonicalizeOSSALifetime.cpp

@@ -1406,7 +1406,7 @@ bool CanonicalizeOSSALifetime::computeLiveness() {
     clear();
     return false;
   }
-  if (respectsDeadEnds()) {
+  if (respectsDeadEnds() && hasAnyDeadEnds()) {
     if (respectsDeinitBarriers()) {
       extendLexicalLivenessToDeadEnds();
     }

test/SILOptimizer/dead_end_blocks.sil

@@ -47,4 +47,64 @@ exit:
   return %retval : $()
 }
 
+// no dead ends - simple return
+// CHECK-LABEL: begin running test {{.*}} on simple_function: has_any_dead_ends
+// CHECK:         no dead ends
+// CHECK-LABEL: end running test {{.*}} on simple_function: has_any_dead_ends
+sil @simple_function : $@convention(thin) () -> () {
+entry:
+  specify_test "has_any_dead_ends"
+  %retval = tuple ()
+  return %retval : $()
+}
+
+// dead ends - unreachable blocks
+// CHECK-LABEL: begin running test {{.*}} on function_with_dead_ends: has_any_dead_ends
+// CHECK:         has dead ends
+// CHECK-LABEL: end running test {{.*}} on function_with_dead_ends: has_any_dead_ends
+sil @function_with_dead_ends : $@convention(thin) () -> () {
+entry:
+  specify_test "has_any_dead_ends"
+  cond_br undef, die, exit
+
+die:
+  unreachable
+
+exit:
+  %retval = tuple ()
+  return %retval : $()
+}
 
+// dead ends – infinite loop
+// CHECK-LABEL: begin running test {{.*}} on function_with_loop: has_any_dead_ends
+// CHECK:         has dead ends
+// CHECK-LABEL: end running test {{.*}} on function_with_loop: has_any_dead_ends
+sil @function_with_loop : $@convention(thin) () -> () {
+entry:
+  specify_test "has_any_dead_ends"
+  cond_br undef, exit, loop
+
+loop:
+  br loop
+
+exit:
+  %retval = tuple ()
+  return %retval : $()
+}
+
+// no dead ends – conditional branches but all paths return
+// CHECK-LABEL: begin running test {{.*}} on branching_no_dead_ends: has_any_dead_ends
+// CHECK:         no dead ends
+// CHECK-LABEL: end running test {{.*}} on branching_no_dead_ends: has_any_dead_ends
+sil @branching_no_dead_ends : $@convention(thin) () -> () {
+entry:
+  specify_test "has_any_dead_ends"
+  cond_br undef, then, else
+
+then:
+  br else
+
+else:
+  %retval2 = tuple ()
+  return %retval2 : $()
+}