[Clang] Enable -fextend-lifetimes at -Og (#118026)

Recently, a new flag -fextend-variable-liveness was added that prevents
optimizations from removing the values of source variables in some cases,
improving the quality of debugging for optimized builds where it is
enabled.

Following the inclusion of the flag, this patch enables it by default
when `-Og` is set. Currently, `-Og` is equivalent to `-O1` - it is
effectively just an alias. By enabling `-fextend-lifetimes`, this patch
changes the code generated by Clang with `-Og` to have reduced
optimization and greater debuggability than `-O1`, differentiating the
two according to their respective purposes. This idea was discussed
previously on Discourse where there was general agreement with the
principle of this change:

https://discourse.llvm.org/t/rfc-redefine-og-o1-and-add-a-new-level-of-og

Author: SLTozer

clang/docs/CommandGuide/clang.rst

@@ -443,8 +443,11 @@ Code Generation Options
     :option:`-Oz` Like :option:`-Os` (and thus :option:`-O2`), but reduces code
     size further.
 
-    :option:`-Og` Like :option:`-O1`. In future versions, this option might
-    disable different optimizations in order to improve debuggability.
+    :option:`-Og` Similar to :option:`-O1`, but with slightly reduced
+    optimization and better variable visibility. The same optimizations are run
+    as at :option:`-O1`, but the ``-fextend-variable-liveness`` flag is
+    also set, which tries to prevent optimizations from reducing the liveness of
+    user variables, improving their availability when debugging.
 
     :option:`-O` Equivalent to :option:`-O1`.
 

clang/docs/ReleaseNotes.rst

@@ -221,6 +221,10 @@ Modified Compiler Flags
 
 - The ``-mexecute-only`` and ``-mpure-code`` flags are now accepted for AArch64 targets. (#GH125688)
 
+- The ``-Og`` optimization flag now sets ``-fextend-variable-liveness``,
+  reducing performance slightly while reducing the number of optimized-out
+  variables.
+
 Removed Compiler Flags
 -------------------------
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7681,7 +7681,13 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (Args.hasArg(options::OPT_fretain_comments_from_system_headers))
     CmdArgs.push_back("-fretain-comments-from-system-headers");
 
-  Args.AddLastArg(CmdArgs, options::OPT_fextend_variable_liveness_EQ);
+  if (Arg *A = Args.getLastArg(options::OPT_fextend_variable_liveness_EQ)) {
+    A->render(Args, CmdArgs);
+  } else if (Arg *A = Args.getLastArg(options::OPT_O_Group);
+             A && A->containsValue("g")) {
+    // Set -fextend-variable-liveness=all by default at -Og.
+    CmdArgs.push_back("-fextend-variable-liveness=all");
+  }
 
   // Forward -fcomment-block-commands to -cc1.
   Args.AddAllArgs(CmdArgs, options::OPT_fcomment_block_commands);

clang/test/Driver/extend-variable-liveness.c

@@ -1,7 +1,8 @@
 // Tests that -fextend-variable-liveness and its aliases are correctly passed
-// by the driver.
+// by the driver, and are set by default at -Og.
 
 // RUN: %clang -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,DEFAULT
+// RUN: %clang -### -Og -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,ALL
 // RUN: %clang -fextend-variable-liveness=none -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,NONE
 // RUN: %clang -fextend-variable-liveness=this -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,THIS
 // RUN: %clang -fextend-variable-liveness=all -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,ALL