Add a SWIFT_STDLIB_OS_VERSIONING flag to avoid querying OS version at runtime, use it in the freestanding build (swiftlang#33791)

Author: kubamracek

stdlib/CMakeLists.txt

@@ -25,6 +25,10 @@ option(SWIFT_STDLIB_SINGLE_THREADED_RUNTIME
     "Build the standard libraries assuming that they will be used in an environment with only a single thread."
     FALSE)
 
+option(SWIFT_STDLIB_OS_VERSIONING
+    "Build stdlib with availability based on OS versions (Darwin only)."
+    TRUE)
+
 #
 # End of user-configurable options.
 #

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -313,6 +313,10 @@ function(_add_target_variant_c_compile_flags)
     list(APPEND result "-DSWIFT_STDLIB_SINGLE_THREADED_RUNTIME")
   endif()
 
+  if(SWIFT_STDLIB_OS_VERSIONING)
+    list(APPEND result "-DSWIFT_RUNTIME_OS_VERSIONING")
+  endif()
+
   set("${CFLAGS_RESULT_VAR_NAME}" "${result}" PARENT_SCOPE)
 endfunction()
 

stdlib/cmake/modules/SwiftSource.cmake

@@ -264,6 +264,10 @@ function(_add_target_variant_swift_compile_flags
     list(APPEND result "-D" "SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING")
   endif()
 
+  if(SWIFT_STDLIB_OS_VERSIONING)
+    list(APPEND result "-D" "SWIFT_RUNTIME_OS_VERSIONING")
+  endif()
+
   set("${result_var_name}" "${result}" PARENT_SCOPE)
 endfunction()
 

stdlib/public/core/Availability.swift

@@ -24,7 +24,7 @@ public func _stdlib_isOSVersionAtLeast(
   _ minor: Builtin.Word,
   _ patch: Builtin.Word
 ) -> Builtin.Int1 {
-#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
+#if (os(macOS) || os(iOS) || os(tvOS) || os(watchOS)) && SWIFT_RUNTIME_OS_VERSIONING
   if Int(major) == 9999 {
     return true._value
   }
@@ -43,7 +43,7 @@ public func _stdlib_isOSVersionAtLeast(
 #endif
 }
 
-#if os(macOS)
+#if os(macOS) && SWIFT_RUNTIME_OS_VERSIONING
 // This is a magic entry point known to the compiler. It is called in
 // generated code for API availability checking.
 @_semantics("availability.osversion")

stdlib/public/runtime/BackDeployment.cpp

@@ -15,7 +15,7 @@
 #include "../SwiftShims/FoundationShims.h"
 #include <stdlib.h>
 
-#if defined(__APPLE__) && defined(__MACH__)
+#if defined(__APPLE__) && defined(__MACH__) && defined(SWIFT_RUNTIME_OS_VERSIONING)
 
 #if SWIFT_CLASS_IS_SWIFT_MASK_GLOBAL_VARIABLE
 static unsigned long long computeIsSwiftMask() {

stdlib/public/stubs/Availability.mm

@@ -16,7 +16,7 @@
 
 #include "swift/Runtime/Config.h"
 
-#if SWIFT_OBJC_INTEROP
+#if SWIFT_OBJC_INTEROP && defined(SWIFT_RUNTIME_OS_VERSIONING)
 #include "swift/Basic/Lazy.h"
 #include "swift/Runtime/Debug.h"
 #include <TargetConditionals.h>

utils/build-presets.ini

@@ -2455,6 +2455,7 @@ swift-objc-interop=0
 swift-enable-compatibility-overrides=0
 swift-runtime-macho-no-dyld=1
 swift-stdlib-single-threaded-runtime=1
+swift-stdlib-os-versioning=0
 
 [preset: stdlib_S_standalone_minimal_macho_x86_64,build]
 mixin-preset=

utils/build-script-impl

@@ -196,6 +196,7 @@ KNOWN_SETTINGS=(
     swift-enable-compatibility-overrides          "1"               "whether to support back-deploying compatibility fixes for newer apps running on older runtimes"
     swift-runtime-macho-no-dyld                   "0"               "whether to build stdlib assuming the runtime environment does not support dynamic modules"
     swift-stdlib-single-threaded-runtime          "0"               "whether to build stdlib as a single-threaded runtime only"
+    swift-stdlib-os-versioning                    "1"               "whether to build stdlib with availability based on OS versions (Darwin only)"
 
     ## FREESTANDING Stdlib Options
     swift-freestanding-sdk                      ""                "which SDK to use when building the FREESTANDING stdlib"
@@ -1776,6 +1777,7 @@ for host in "${ALL_HOSTS[@]}"; do
                     -DSWIFT_STDLIB_SINGLE_THREADED_RUNTIME:BOOL=$(true_false "${SWIFT_STDLIB_SINGLE_THREADED_RUNTIME}")
                     -DSWIFT_ENABLE_RUNTIME_FUNCTION_COUNTERS:BOOL=$(true_false "${SWIFT_ENABLE_RUNTIME_FUNCTION_COUNTERS}")
                     -DSWIFT_RUNTIME_MACHO_NO_DYLD:BOOL=$(true_false "${SWIFT_RUNTIME_MACHO_NO_DYLD}")
+                    -DSWIFT_STDLIB_OS_VERSIONING:BOOL=$(true_false "${SWIFT_STDLIB_OS_VERSIONING}")
                     -DSWIFT_NATIVE_LLVM_TOOLS_PATH:STRING="${native_llvm_tools_path}"
                     -DSWIFT_NATIVE_CLANG_TOOLS_PATH:STRING="${native_clang_tools_path}"
                     -DSWIFT_NATIVE_SWIFT_TOOLS_PATH:STRING="${native_swift_tools_path}"