SimplifyBranch: don't leave an empty block after merging two basic blocks

Although empty blocks are cleaned up at the end of every Simplification pass, it's not legal SIL to have empty blocks and subsequent simplification passes may crash because of this.

rdar://115169880

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyBranch.swift

@@ -70,8 +70,10 @@ private extension BranchInst {
       }
       parentBB.moveAllInstructions(toBeginOf: targetBB, context)
       parentBB.moveAllArguments(to: targetBB, context)
+      context.erase(block: parentBB)
     } else {
       targetBB.moveAllInstructions(toEndOf: parentBB, context)
+      context.erase(block: targetBB)
     }
   }
 }

SwiftCompilerSources/Sources/Optimizer/PassManager/Context.swift

@@ -71,6 +71,10 @@ extension MutatingContext {
     erase(instruction: inst)
   }
 
+  func erase(block: BasicBlock) {
+    _bridged.eraseBlock(block.bridged)
+  }
+
   func tryOptimizeApplyOfPartialApply(closure: PartialApplyInst) -> Bool {
     if _bridged.tryOptimizeApplyOfPartialApply(closure.bridged) {
       notifyInstructionsChanged()
@@ -227,10 +231,6 @@ struct FunctionPassContext : MutatingContext {
     }
   }
 
-  func erase(block: BasicBlock) {
-    _bridged.eraseBlock(block.bridged)
-  }
-
   func modifyEffects(in function: Function, _ body: (inout FunctionEffects) -> ()) {
     notifyEffectsChanged()
     function._modifyEffects(body)

test/SILOptimizer/simplify_branch_crash.sil

@@ -0,0 +1,28 @@
+// RUN: %target-sil-opt -enable-sil-verify-all %s -sil-opt-pass-count=1.1 -simplification -simplify-instruction=br | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+
+import Swift
+import Builtin
+
+// Check that branch simplification doesn't leave empty blocks.
+// -sil-opt-pass-count=1.1  prevents dead block elimination which would hide the problem.
+
+
+
+// CHECK-LABEL: sil @dont_leave_empty_blocks
+// CHECK:       bb0(%0 : $Builtin.Int64):
+// CHECK-NEXT:    br bb1
+// CHECK:       bb1:
+// CHECK-NEXT:    return %0
+// CHECK:       } // end sil function 'dont_leave_empty_blocks'
+sil @dont_leave_empty_blocks : $@convention(thin) (Builtin.Int64) -> Builtin.Int64 {
+bb0(%0 : $Builtin.Int64):
+  br bb1
+
+bb1:
+  br bb2
+
+bb2:
+  return %0 : $Builtin.Int64
+}