Merge pull request #82313 from jamieQ/copy-prop-ossa-dead-ends-build-time-fix

[SILOptimizer]: slow OSSA lifetime canonicalization mitigation

Author: nate-chandler

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 : $()
+}