[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 7e42a82d73e3be98aab8e523b16a2e43d31b0552 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/Frontend/CompilerInvocation.cpp |  8 ++++++
 clang/test/CodeGen/fake-use-opt-flags.cpp | 30 +++++++++++++++++++++++
 4 files changed, 47 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/CodeGen/fake-use-opt-flags.cpp

diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index ca8176f854729b..8f3f0de9446b41 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-lifetimes` flag is also
+    set, which tries to prevent optimizations from reducing the lifetime of
+    user variables.
 
     :option:`-O` Equivalent to :option:`-O1`.
 
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 601a233b81904f..14fec70dfb85f7 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -441,6 +441,10 @@ Modified Compiler Flags
   ``memset`` and similar functions for which it is a documented undefined
   behavior. It is implied by ``-Wnontrivial-memaccess``
 
+- The ``-Og`` optimization flag now sets ``-fextend-lifetimes``, a new compiler
+  flag, resulting in slightly reduced optimization compared to ``-O1`` in
+  exchange for improved variable visibility.
+
 Removed Compiler Flags
 -------------------------
 
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 3dd94c31b2bc7a..09322d0617aaad 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1834,6 +1834,14 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
     Opts.setInlining(CodeGenOptions::NormalInlining);
   }
 
+  // If we have specified -Og and have not set any explicit -fextend-lifetimes
+  // value, then default to -fextend-lifetimes=all.
+  if (Arg *A = Args.getLastArg(options::OPT_O_Group);
+      A && A->containsValue("g")) {
+    if (!Args.hasArg(options::OPT_fextend_lifetimes))
+      Opts.ExtendLifetimes = true;
+  }
+
   // PIC defaults to -fno-direct-access-external-data while non-PIC defaults to
   // -fdirect-access-external-data.
   Opts.DirectAccessExternalData =
diff --git a/clang/test/CodeGen/fake-use-opt-flags.cpp b/clang/test/CodeGen/fake-use-opt-flags.cpp
new file mode 100644
index 00000000000000..a99ed7a573ab22
--- /dev/null
+++ b/clang/test/CodeGen/fake-use-opt-flags.cpp
@@ -0,0 +1,30 @@
+/// Check that we generate fake uses only when -fextend-lifetimes is set and we
+/// are not setting optnone, or when we have optimizations set to -Og and we have
+/// not passed -fno-extend-lifetimes.
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -O0 -fextend-lifetimes %s -o - | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -O0 -disable-O0-optnone -fextend-lifetimes %s -o - | FileCheck %s --check-prefixes=CHECK,EXTEND
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -Og %s -o - | FileCheck %s --check-prefixes=CHECK,EXTEND
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -Og -fno-extend-lifetimes %s -o - | FileCheck %s
+
+/// Test various optimization flags with -fextend-lifetimes...
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -O1 %s -o - | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -O1 -fextend-lifetimes %s -o - | FileCheck %s --check-prefixes=CHECK,EXTEND
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -O2 %s -o - | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -O2 -fextend-lifetimes %s -o - | FileCheck %s --check-prefixes=CHECK,EXTEND
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -O3 %s -o - | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -O3 -fextend-lifetimes %s -o - | FileCheck %s --check-prefixes=CHECK,EXTEND
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -Os %s -o - | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -Os -fextend-lifetimes %s -o - | FileCheck %s --check-prefixes=CHECK,EXTEND
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -Oz %s -o - | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm -disable-llvm-passes -Oz -fextend-lifetimes %s -o - | FileCheck %s --check-prefixes=CHECK,EXTEND
+
+// CHECK-LABEL: define{{.*}} void @_Z3fooi(i32{{.*}} %a)
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    %a.addr = alloca i32
+// CHECK-NEXT:    store i32 %a, ptr %a.addr
+// EXTEND-NEXT:   %fake.use = load i32, ptr %a.addr
+// EXTEND-NEXT:   call void (...) @llvm.fake.use(i32 %fake.use)
+// CHECK-NEXT:    ret void
+// CHECK-NEXT:  }
+
+void foo(int a) {}

>From ec32824c6677486f4cc54c6827abf9e9ce709ad7 Mon Sep 17 00:00:00 2001
From: Stephen Tozer <stephen.tozer at sony.com>
Date: Fri, 29 Nov 2024 17:15:38 +0000
Subject: [PATCH 2/2] Reword release note, fix comment

---
 clang/docs/ReleaseNotes.rst               | 4 ++--
 clang/lib/Frontend/CompilerInvocation.cpp | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 14fec70dfb85f7..0b5c2e3c5f35bb 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -442,8 +442,8 @@ Modified Compiler Flags
   behavior. It is implied by ``-Wnontrivial-memaccess``
 
 - The ``-Og`` optimization flag now sets ``-fextend-lifetimes``, a new compiler
-  flag, resulting in slightly reduced optimization compared to ``-O1`` in
-  exchange for improved variable visibility.
+  flag which trades a small amount of optimization in exchange for improved
+  variable visibility.
 
 Removed Compiler Flags
 -------------------------
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 09322d0617aaad..e2e433319b7e58 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1834,8 +1834,8 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
     Opts.setInlining(CodeGenOptions::NormalInlining);
   }
 
-  // If we have specified -Og and have not set any explicit -fextend-lifetimes
-  // value, then default to -fextend-lifetimes=all.
+  // If we have specified -Og and have not explicitly set -fno-extend-lifetimes,
+  // then default to -fextend-lifetimes.
   if (Arg *A = Args.getLastArg(options::OPT_O_Group);
       A && A->containsValue("g")) {
     if (!Args.hasArg(options::OPT_fextend_lifetimes))