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

[Stephen Tozer via cfe-commits]

https://github.com/SLTozer updated https://github.com/llvm/llvm-project/pull/118026

>From beb5e1955f9b0aa106512bb10d4543232e4d4cfa Mon Sep 17 00:00:00 2001
From: Stephen Tozer <stephen.tozer at sony.com>
Date: Thu, 28 Nov 2024 13:53:20 +0000
Subject: [PATCH] Enable -fextend-lifetimes at -Og

---
 clang/docs/CommandGuide/clang.rst            | 7 +++++--
 clang/docs/ReleaseNotes.rst                  | 4 ++++
 clang/lib/Driver/ToolChains/Clang.cpp        | 8 +++++++-
 clang/test/Driver/extend-variable-liveness.c | 3 ++-
 4 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index ca8176f854729b..f8294a01048fec 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -442,8 +442,11 @@ 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 :option:`-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`.
 
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index c5a5da5daf8f5e..7b8670c9f48140 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -580,6 +580,10 @@ Modified Compiler Flags
   ``-fexperimental-modules-reduced-bmi`` flag. The ``-fmodules-reduced-bmi`` flag
   is intended to be enabled by default in the future.
 
+- The ``-Og`` optimization flag now sets ``-fextend-variable-liveness``, a new
+  compiler flag which trades a small amount of optimization in exchange for
+  improved variable visibility.
+
 Removed Compiler Flags
 -------------------------
 
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 589de953be5be1..0ce32d7d0f1b89 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -7706,7 +7706,13 @@ 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);
diff --git a/clang/test/Driver/extend-variable-liveness.c b/clang/test/Driver/extend-variable-liveness.c
index bdd89d6f7721ce..95c9f6bc6904c1 100644
--- a/clang/test/Driver/extend-variable-liveness.c
+++ b/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-this-ptr-liveness -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,THIS