[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 351971bbff4e77b0f36cd92cd1a881584d17a9e7 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 1/2] 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 42aac7b25d93c..68e9e07ed0005 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/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 :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 4f640697e1817..23a3d481e66c2 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -219,6 +219,10 @@ Modified Compiler Flags
 
 - `-Wpadded` option implemented for the `x86_64-windows-msvc` target. Fixes #61702
 
+- 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 8506a5c00e7bc..b2dd4b3b54869 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/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);
diff --git a/clang/test/Driver/extend-variable-liveness.c b/clang/test/Driver/extend-variable-liveness.c
index bbfb2ece6f297..99a5409ceccea 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-variable-liveness=all -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,ALL

>From e5037472fd98a7f4a2d9f8298799f46a8b647f87 Mon Sep 17 00:00:00 2001
From: Stephen Tozer <stephen.tozer at sony.com>
Date: Thu, 17 Apr 2025 15:14:55 +0100
Subject: [PATCH 2/2] Update docs to fit current compiler/doc state

---
 clang/docs/CommandGuide/clang.rst | 2 +-
 clang/docs/ReleaseNotes.rst       | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index 68e9e07ed0005..a1e738f1afec8 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -445,7 +445,7 @@ Code Generation Options
 
     :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
+    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.
 
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 23a3d481e66c2..9a533410f1f3a 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -219,9 +219,9 @@ Modified Compiler Flags
 
 - `-Wpadded` option implemented for the `x86_64-windows-msvc` target. Fixes #61702
 
-- 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.
+- The ``-Og`` optimization flag now sets ``-fextend-variable-liveness``,
+  reducing performance slightly while reducing the number of optimized-out
+  variables.
 
 Removed Compiler Flags
 -------------------------