Swift Optimizer: add simplification of debug_step

When compiling with optimizations, unconditionally remove `debug_step`

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/CMakeLists.txt

@@ -13,6 +13,7 @@ swift_compiler_sources(Optimizer
   SimplifyBuiltin.swift
   SimplifyCondBranch.swift
   SimplifyCondFail.swift
+  SimplifyDebugStep.swift
   SimplifyGlobalValue.swift
   SimplifyStrongRetainRelease.swift
   SimplifyStructExtract.swift

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyDebugStep.swift

@@ -0,0 +1,22 @@
+//===--- SimplifyDebugStep.swift ------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SIL
+
+extension DebugStepInst : Simplifyable {
+  func simplify(_ context: SimplifyContext) {
+    // When compiling with optimizations (note: it's not a OnoneSimplifyable transformation),
+    // unconditionally remove debug_step instructions.
+    context.erase(instruction: self)
+  }
+}
+

test/SILOptimizer/simplify_debug_step.sil

@@ -0,0 +1,19 @@
+// RUN: %target-sil-opt -enable-sil-verify-all %s -onone-simplification -simplify-instruction=debug_step | %FileCheck %s --check-prefix=CHECK-ONONE --check-prefix=CHECK
+// RUN: %target-sil-opt -enable-sil-verify-all %s -simplification -simplify-instruction=debug_step | %FileCheck %s --check-prefix=CHECK-O --check-prefix=CHECK
+
+// REQUIRES: swift_in_compiler
+
+import Swift
+import Builtin
+
+// CHECK-LABEL: sil @testit
+// CHECK-O-NOT:   debug_step
+// CHECK-ONONE:   debug_step
+// CHECK:       } // end sil function 'testit'
+sil @testit : $@convention(thin) () -> () {
+bb0:
+  debug_step
+  %r = tuple ()
+  return %r : $()
+}
+