LoopRotate: don't rotate loops where the header block branches to a dead-end block.

Incomplete liveranges in the dead-end exit block can cause a missing adjacent phi-argument for a re-borrow if there is a borrow-scope is in the loop.
But even when we have complete lifetimes, it's probably not worth rotating a loop where the header block branches to a dead-end block.

Fixes a SIL verifier crash

Author: eeckstein

lib/SILOptimizer/LoopTransforms/LoopRotate.cpp

@@ -18,6 +18,7 @@
 #include "swift/SIL/SILBuilder.h"
 #include "swift/SIL/SILInstruction.h"
 #include "swift/SILOptimizer/Analysis/Analysis.h"
+#include "swift/SILOptimizer/Analysis/DeadEndBlocksAnalysis.h"
 #include "swift/SILOptimizer/Analysis/DominanceAnalysis.h"
 #include "swift/SILOptimizer/Analysis/LoopAnalysis.h"
 #include "swift/SILOptimizer/PassManager/Passes.h"
@@ -374,6 +375,15 @@ static bool rotateLoop(SILLoop *loop, DominanceInfo *domInfo,
     return false;
   }
 
+  // Incomplete liveranges in the dead-end exit block can cause a missing adjacent
+  // phi-argument for a re-borrow if there is a borrow-scope is in the loop.
+  // But even when we have complete lifetimes, it's probably not worth rotating
+  // a loop where the header block branches to a dead-end block.
+  auto *deBlocks = pm->getAnalysis<DeadEndBlocksAnalysis>()->get(exit->getParent());
+  if (deBlocks->isDeadEnd(exit)) {
+    return false;
+  }
+
   // We don't want to rotate such that we merge two headers of separate loops
   // into one. This can be turned into an assert again once we have guaranteed
   // preheader insertions.

test/SILOptimizer/looprotate_ossa.sil

@@ -18,6 +18,7 @@ class Bar {
   @objc func foo_objc()
 }
 
+sil [ossa] @getBar : $@convention(thin) () -> @owned Bar
 sil [ossa] @_TFC4main3Bar3foofS0_FT_T_ : $@convention(method) (@guaranteed Bar) -> ()
 sil [ossa] @_TFC4main3Bar3boofS0_FT_T_ : $@convention(method) (@guaranteed Bar) -> Int64
 sil [ossa] @_TFC4main3Bar3foo_objcfS0_FT_T_ : $@convention(objc_method) (Bar) -> ()
@@ -713,3 +714,39 @@ bb14:
   return %27
 }
 
+// Incomplete liveranges in dead-end exit blocks can cause a missing phi in the rotated loop.
+// CHECK-LABEL: sil [ossa] @incomplete_liverange_in_dead_end_exit :
+// CHECK:       bb1:
+// CHECK:         apply
+// CHECK:       bb2:
+// CHECK-LABEL: } // end sil function 'incomplete_liverange_in_dead_end_exit'
+sil [ossa] @incomplete_liverange_in_dead_end_exit : $@convention(thin) () -> () {
+bb0:
+  br bb1
+
+bb1:
+  %1 = function_ref @getBar : $@convention(thin) () -> @owned Bar
+  %2 = apply %1() : $@convention(thin) () -> @owned Bar
+  %3 = begin_borrow %2
+  cond_br undef, bb3, bb2
+
+bb2:
+  br bb4
+
+bb3:
+  end_borrow %3
+  unreachable
+
+bb4:
+  end_borrow %3
+  destroy_value %2
+  cond_br undef, bb6, bb5
+
+bb5:
+  br bb1
+
+bb6:
+  %12 = tuple ()
+  return %12
+}
+