Merge pull request swiftlang#77523 from eeckstein/fix-simplification

Simplification: don't delete trivially dead borrowed-from instruction

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Utilities/OptUtils.swift

@@ -297,6 +297,9 @@ extension Instruction {
     switch self {
     case is TermInst, is MarkUninitializedInst, is DebugValueInst:
       return false
+    case is BorrowedFromInst:
+      // A dead borrowed-from can only be removed if the argument (= operand) is also removed.
+      return false
     case let bi as BuiltinInst:
       if bi.id == .OnFastPath {
         return false

test/SILOptimizer/sil_combine_inst_passes.sil

@@ -148,3 +148,21 @@ bb0:
   return %16 : $Int
 }
 
+// CHECK-LABEL: sil [ossa] @test_dead_borrowed_from
+// CHECK:         borrowed {{.*}} from
+// CHECK:       } // end sil function 'test_dead_borrowed_from'
+sil [ossa] @test_dead_borrowed_from : $@convention(thin) (@guaranteed Buffer, @guaranteed Buffer) -> () {
+bb0(%0 : @guaranteed $Buffer, %1 : @guaranteed $Buffer):
+  cond_br undef, bb1, bb2
+
+bb1:
+  br bb3(%0 : $Buffer)
+
+bb2:
+  br bb3(%1 : $Buffer)
+
+bb3(%9 : @guaranteed $Buffer):
+  %10 = borrowed %9 : $Buffer from (%1 : $Buffer)
+  %r = tuple ()
+  return %r : $()
+}