[KeyInstr] Add release note & update option (#148244)

Make the option visible, improve the help text, and add a release note.

Author: OCHyams

clang/docs/ReleaseNotes.rst

@@ -376,6 +376,8 @@ New Compiler Flags
 
 - New option ``-ignore-pch`` added to disable precompiled headers. It overrides ``-emit-pch`` and ``-include-pch``. (#GH142409, `PCHDocs <https://clang.llvm.org/docs/UsersManual.html#ignoring-a-pch-file>`_).
 
+- New options ``-g[no-]key-instructions`` added, disabled by default. Reduces jumpiness of debug stepping for optimized code in some debuggers (not LLDB at this time). Not recommended for use without optimizations. DWARF only. Note both the positive and negative flags imply ``-g``.
+
 Deprecated Compiler Flags
 -------------------------
 

clang/include/clang/Driver/Options.td

@@ -4720,9 +4720,9 @@ def gno_embed_source : Flag<["-"], "gno-embed-source">, Group<g_flags_Group>,
 defm key_instructions : BoolGOption<"key-instructions",
     CodeGenOpts<"DebugKeyInstructions">, DefaultFalse,
     NegFlag<SetFalse>, PosFlag<SetTrue, [], [],
-        "Enable Key Instructions, which reduces the jumpiness of optimized code stepping (DWARF only)."
-        " Requires LLVM built with LLVM_EXPERIMENTAL_KEY_INSTRUCTIONS.">,
-    BothFlags<[HelpHidden], [ClangOption, CLOption, CC1Option]>>;
+        "Enable Key Instructions, which reduces the jumpiness of debug stepping in optimized C/C++ code"
+        " in some debuggers. DWARF only. Implies -g.">,
+    BothFlags<[], [ClangOption, CLOption, CC1Option]>>;
 def headerpad__max__install__names : Joined<["-"], "headerpad_max_install_names">;
 def help : Flag<["-", "--"], "help">,
     Visibility<[ClangOption, CC1Option, CC1AsOption,

clang/test/DebugInfo/KeyInstructions/flag.cpp

@@ -3,6 +3,10 @@
 //// Default: Off.
 // RUN: %clang -### -target x86_64 -c -gdwarf %s 2>&1 | FileCheck %s --check-prefixes=NO-KEY-INSTRUCTIONS
 
+//// Help.
+// RUN %clang --help | FileCheck %s --check-prefix=HELP
+// HELP: -gkey-instructions  Enable Key Instructions, which reduces the jumpiness of debug stepping in optimized C/C++ code in some debuggers. DWARF only. Implies -g.
+
 // KEY-INSTRUCTIONS: "-gkey-instructions"
 // NO-KEY-INSTRUCTIONS-NOT: key-instructions