https://github.com/SLTozer updated
https://github.com/llvm/llvm-project/pull/118026
>From efd7f58e421b0afdc886688128f72170e0c4a970 Mon Sep 17 00:00:00 2001
From: Stephen Tozer <stephen.to...@sony.com>
Date: Wed, 25 Sep 2024 15:08:39 +0100
Subject: [PATCH 1/2] [Clang] Add "extend lifetime" flags and release note
Following the commit that added the fake use intrinsic to LLVM, this patch
adds a pair of flags for the clang frontend that emit fake use intrinsics,
for the purpose of extending the lifetime of variables (either all source
variables, or just the `this` pointer). This patch does not implement the
fake use intrinsic emission of the flags themselves, it simply adds the flags,
the corresponding release note, and the attachment of the `has_fake_uses`
attribute to affected functions; the remaining functionality appears in the
next patch.
---
clang/docs/ReleaseNotes.rst | 14 ++++++++++++++
clang/include/clang/Basic/CodeGenOptions.def | 3 +++
clang/include/clang/Basic/CodeGenOptions.h | 6 ++++++
clang/include/clang/Driver/Options.td | 20 ++++++++++++++++++++
clang/lib/Driver/ToolChains/Clang.cpp | 2 ++
clang/test/Driver/extend-variable-liveness.c | 15 +++++++++++++++
6 files changed, 60 insertions(+)
create mode 100644 clang/test/Driver/extend-variable-liveness.c
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 601a233b81904f..19a40d4666c727 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -412,6 +412,20 @@ New Compiler Flags
only for thread-local variables, and none (which corresponds to the
existing ``-fno-c++-static-destructors`` flag) skips all static
destructors registration.
+- The ``-fextend-variable-liveness`` flag has been added to allow for improved
+ debugging of optimized code. Using ``-fextend-variable-liveness`` will cause
+ Clang to generate code that tries to preserve the liveness of source
variables
+ through optimizations, meaning that variables will typically be visible in a
+ debugger more often. The flag has two levels: ``-fextend-variable-liveness``,
+ or ``-fextend-variable-liveness=all``, extendes the liveness of all user
+ variables and the ``this`` pointer. Alternatively ``-fextend-this-ptr``, or
+ ``-fextend-variable-liveness=this``, has the same behaviour but applies only
+ to the ``this`` variable in C++ class member functions, meaning its effect is
+ a strict subset of ``-fextend-variable-liveness``. Note that this flag
+ modifies the results of optimizations that Clang performs, which will result
+ in reduced performance in generated code; however, this feature will not
+ extend the liveness of some variables in cases where doing so would likely
+ have a severe impact on generated code performance.
Deprecated Compiler Flags
-------------------------
diff --git a/clang/include/clang/Basic/CodeGenOptions.def
b/clang/include/clang/Basic/CodeGenOptions.def
index 4cf22c4ee08ce0..8d181c96bc9641 100644
--- a/clang/include/clang/Basic/CodeGenOptions.def
+++ b/clang/include/clang/Basic/CodeGenOptions.def
@@ -393,6 +393,9 @@ CODEGENOPT(EnableTLSDESC, 1, 0)
/// Bit size of immediate TLS offsets (0 == use the default).
VALUE_CODEGENOPT(TLSSize, 8, 0)
+/// The types of variables that we will extend the live ranges of.
+ENUM_CODEGENOPT(ExtendVariableLiveness, ExtendVariableLivenessKind, 2,
ExtendVariableLivenessKind::None)
+
/// The default stack protector guard offset to use.
VALUE_CODEGENOPT(StackProtectorGuardOffset, 32, INT_MAX)
diff --git a/clang/include/clang/Basic/CodeGenOptions.h
b/clang/include/clang/Basic/CodeGenOptions.h
index 2dcf98b465661e..f55d9513443e8c 100644
--- a/clang/include/clang/Basic/CodeGenOptions.h
+++ b/clang/include/clang/Basic/CodeGenOptions.h
@@ -95,6 +95,12 @@ class CodeGenOptions : public CodeGenOptionsBase {
Embed_Marker // Embed a marker as a placeholder for bitcode.
};
+ enum class ExtendVariableLivenessKind {
+ None,
+ This,
+ All,
+ };
+
enum InlineAsmDialectKind {
IAD_ATT,
IAD_Intel,
diff --git a/clang/include/clang/Driver/Options.td
b/clang/include/clang/Driver/Options.td
index 9c356c9d2ea4ef..bd9e5407d3843b 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -4298,6 +4298,26 @@ def stack_usage_file : Separate<["-"],
"stack-usage-file">,
Visibility<[CC1Option]>,
HelpText<"Filename (or -) to write stack usage output to">,
MarshallingInfoString<CodeGenOpts<"StackUsageOutput">>;
+def fextend_variable_liveness_EQ : Joined<["-"], "fextend-variable-liveness=">,
+ Group<f_Group>, Visibility<[ClangOption, CC1Option]>,
+ HelpText<"Extend the liveness of user variables through optimizations to "
+ "prevent stale or optimized-out variable values when debugging. Can
"
+ "be applied to all user variables, or just to the C++ 'this' ptr. "
+ "May choose not to extend the liveness of some variables, such as "
+ "non-scalars larger than 4 unsigned ints, or variables in any "
+ "inlined functions.">,
+ Values<"all,this,none">,
+ NormalizedValues<["All", "This", "None"]>,
+ NormalizedValuesScope<"CodeGenOptions::ExtendVariableLivenessKind">,
+ MarshallingInfoEnum<CodeGenOpts<"ExtendVariableLiveness">, "None">;
+def fextend_this_ptr_liveness : Flag<["-"], "fextend-this-ptr-liveness">,
+ Visibility<[ClangOption, CC1Option]>,
+ Alias<fextend_variable_liveness_EQ>, AliasArgs<["this"]>,
+ HelpText<"Alias for -fextend-variable-liveness=this.">;
+def fextend_variable_liveness : Flag<["-"], "fextend-variable-liveness">,
+ Visibility<[ClangOption, CC1Option]>,
+ Alias<fextend_variable_liveness_EQ>, AliasArgs<["all"]>,
+ HelpText<"Alias for -fextend-variable-liveness=all.">;
defm unique_basic_block_section_names :
BoolFOption<"unique-basic-block-section-names",
CodeGenOpts<"UniqueBasicBlockSectionNames">, DefaultFalse,
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp
b/clang/lib/Driver/ToolChains/Clang.cpp
index 217c1a845f0a47..ece14d1e3cbf7d 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -7654,6 +7654,8 @@ 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);
+
// Forward -fcomment-block-commands to -cc1.
Args.AddAllArgs(CmdArgs, options::OPT_fcomment_block_commands);
// Forward -fparse-all-comments to -cc1.
diff --git a/clang/test/Driver/extend-variable-liveness.c
b/clang/test/Driver/extend-variable-liveness.c
new file mode 100644
index 00000000000000..bdd89d6f7721ce
--- /dev/null
+++ b/clang/test/Driver/extend-variable-liveness.c
@@ -0,0 +1,15 @@
+// Tests that -fextend-variable-liveness and its aliases are correctly passed
+// by the driver.
+
+// RUN: %clang -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,DEFAULT
+// 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
+// RUN: %clang -fextend-variable-liveness=all -### -c %s 2>&1 | FileCheck %s
--check-prefixes=CHECK,ALL
+// RUN: %clang -fextend-variable-liveness -### -c %s 2>&1 | FileCheck %s
--check-prefixes=CHECK,ALL
+
+// CHECK: "-cc1"
+// DEFAULT-NOT: -fextend-variable-liveness
+// NONE-SAME: "-fextend-variable-liveness=none"
+// THIS-SAME: "-fextend-variable-liveness=this"
+// ALL-SAME: "-fextend-variable-liveness=all"
>From 2b59ce78f1b84081966db17f48f86776236066ad Mon Sep 17 00:00:00 2001
From: Stephen Tozer <stephen.to...@sony.com>
Date: Thu, 28 Nov 2024 13:53:20 +0000
Subject: [PATCH 2/2] 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 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 19a40d4666c727..2771e45aabeb43 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -455,6 +455,10 @@ 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-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 ece14d1e3cbf7d..629f7dd21ce38d 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -7654,7 +7654,13 @@ 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
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