Simplification: remove begin_borrow if the borrowed value is a thin_to_thick_function

`thin_to_thick_function` has "none" ownership and is compatible with guaranteed values.
Therefore the `begin_borrow` is not needed.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyBeginAndLoadBorrow.swift

@@ -21,6 +21,8 @@ extension BeginBorrowInst : OnoneSimplifyable {
        !findPointerEscapingUse(of: borrowedValue)
     {
       tryReplaceBorrowWithOwnedOperand(beginBorrow: self, context)
+    } else {
+      removeBorrowOfThinFunction(beginBorrow: self, context)
     }
   }
 }
@@ -77,6 +79,19 @@ private func tryReplaceBorrowWithOwnedOperand(beginBorrow: BeginBorrowInst, _ co
   }
 }
 
+private func removeBorrowOfThinFunction(beginBorrow: BeginBorrowInst, _ context: SimplifyContext) {
+  guard let thin2thickFn = beginBorrow.borrowedValue as? ThinToThickFunctionInst,
+        // For simplicity don't go into the trouble of removing reborrow phi arguments.
+        beginBorrow.uses.filterUsers(ofType: BranchInst.self).isEmpty else
+  {
+    return
+  }
+  // `thin_to_thick_function` has "none" ownership and is compatible with guaranteed values.
+  // Therefore the `begin_borrow` is not needed.
+  beginBorrow.uses.ignoreUsers(ofType: EndBorrowInst.self).replaceAll(with: thin2thickFn, context)
+  context.erase(instructionIncludingAllUsers: beginBorrow)
+}
+
 /// Replace
 /// ```
 ///   %1 = begin_borrow %0

test/SILOptimizer/simplify_begin_borrow.sil

@@ -31,6 +31,8 @@ sil @takeC : $@convention(thin) (@guaranteed C) -> ()
 sil @takeOwnedC : $@convention(thin) (@owned C) -> ()
 sil @takeS1 : $@convention(thin) (@guaranteed S1) -> ()
 sil @takeS2 : $@convention(thin) (@guaranteed S2) -> ()
+sil @simple_func : $@convention(thin) () -> ()
+sil @use_closure : $@convention(thin) (@guaranteed @callee_guaranteed () -> ()) -> ()
 
 // CHECK-LABEL: sil [ossa] @replace_simple :
 // CHECK-NOT:     begin_borrow
@@ -438,3 +440,43 @@ bb3:
   %retval = tuple ()
   return %retval : $()
 }
+
+// CHECK-LABEL: sil [ossa] @remove_borrow_of_thin_function :
+// CHECK:         %1 = thin_to_thick_function
+// CHECK-NOT:     begin_borrow
+// CHECK:         %3 = apply %2(%1)
+// CHECK:         %4 = apply %1()
+// CHECK-LABEL: } // end sil function 'remove_borrow_of_thin_function'
+sil [ossa] @remove_borrow_of_thin_function : $@convention(thin) () -> () {
+bb0:
+  %0 = function_ref @simple_func : $@convention(thin) () -> ()
+  %1 = thin_to_thick_function %0 to $@callee_guaranteed () -> ()
+  %2 = begin_borrow %1
+  %3 = function_ref @use_closure : $@convention(thin) (@guaranteed @callee_guaranteed () -> ()) -> ()
+  %4 = apply %3(%2) : $@convention(thin) (@guaranteed @callee_guaranteed () -> ()) -> ()
+  %5 = apply %2(): $@callee_guaranteed () -> ()
+  end_borrow %2
+  %r = tuple ()
+  return %r
+}
+
+// CHECK-LABEL: sil [ossa] @dont_remove_borrow_of_thin_function :
+// CHECK:         thin_to_thick_function
+// CHECK-NEXT:    begin_borrow
+// CHECK-LABEL: } // end sil function 'dont_remove_borrow_of_thin_function'
+sil [ossa] @dont_remove_borrow_of_thin_function : $@convention(thin) () -> () {
+bb0:
+  %0 = function_ref @simple_func : $@convention(thin) () -> ()
+  %1 = thin_to_thick_function %0 to $@callee_guaranteed () -> ()
+  %2 = begin_borrow %1
+  %3 = function_ref @use_closure : $@convention(thin) (@guaranteed @callee_guaranteed () -> ()) -> ()
+  %4 = apply %3(%2) : $@convention(thin) (@guaranteed @callee_guaranteed () -> ()) -> ()
+  %5 = apply %2(): $@callee_guaranteed () -> ()
+  br bb1(%2)
+bb1(%7 : @reborrow $@callee_guaranteed () -> ()):
+  %8 = borrowed %7 from ()
+  end_borrow %8
+  %r = tuple ()
+  return %r
+}
+