https://github.com/SLTozer created
https://github.com/llvm/llvm-project/pull/110000
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.
>From a33566974fbf260181d20d3f1b67081d21493506 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] [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 | 10 ++++++++++
clang/include/clang/Basic/CodeGenOptions.def | 6 ++++++
clang/include/clang/Driver/Options.td | 9 +++++++++
clang/lib/CodeGen/CGCall.cpp | 4 ++++
clang/lib/Driver/ToolChains/Clang.cpp | 5 +++++
clang/lib/Frontend/CompilerInvocation.cpp | 5 +++++
clang/test/CodeGen/extend-lifetimes-hasfakeuses.c | 12 ++++++++++++
7 files changed, 51 insertions(+)
create mode 100644 clang/test/CodeGen/extend-lifetimes-hasfakeuses.c
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 14907e7db18de3..249f2171babda1 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -211,6 +211,16 @@ 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-lifetimes`` and ``-fextend-this-ptr`` flags have been added to
+ allow for improved debugging of optimized code. Using ``-fextend-lifetimes``
+ will cause Clang to generate code that tries to preserve the lifetimes of
+ source variables, meaning that variables will typically be visible in a
+ debugger more often. The ``-fextend-this-ptr`` flag has the same behaviour,
+ but applies only to the ``this`` variable in C++ class member functions. Note
+ that this flag modifies the optimizations that Clang performs, which will
+ result in reduced performance in generated code; however, this feature will
+ not extend the lifetime of some variables in cases where doing so would have
+ too severe an 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 2893377e5a38be..18f5225813133f 100644
--- a/clang/include/clang/Basic/CodeGenOptions.def
+++ b/clang/include/clang/Basic/CodeGenOptions.def
@@ -388,6 +388,12 @@ CODEGENOPT(EnableTLSDESC, 1, 0)
/// Bit size of immediate TLS offsets (0 == use the default).
VALUE_CODEGENOPT(TLSSize, 8, 0)
+/// Whether to extend the live range of the `this` pointer.
+CODEGENOPT(ExtendThisPtr, 1, 0)
+
+/// Whether to extend the live ranges of all local variables.
+CODEGENOPT(ExtendLifetimes, 1, 0)
+
/// The default stack protector guard offset to use.
VALUE_CODEGENOPT(StackProtectorGuardOffset, 32, INT_MAX)
diff --git a/clang/include/clang/Driver/Options.td
b/clang/include/clang/Driver/Options.td
index 23bd686a85f526..a82dbbc113d105 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -4263,6 +4263,15 @@ def stack_usage_file : Separate<["-"],
"stack-usage-file">,
Visibility<[CC1Option]>,
HelpText<"Filename (or -) to write stack usage output to">,
MarshallingInfoString<CodeGenOpts<"StackUsageOutput">>;
+def fextend_this_ptr : Flag <["-"], "fextend-this-ptr">, Group<f_Group>,
+ MarshallingInfoFlag<CodeGenOpts<"ExtendThisPtr">>,
+ HelpText<"Extend the lifetime of the 'this' pointer to improve visibility "
+ "in optimized debugging">, Visibility<[ClangOption, CC1Option]>;
+def fextend_lifetimes : Flag <["-"], "fextend-lifetimes">, Group<f_Group>,
+ MarshallingInfoFlag<CodeGenOpts<"ExtendLifetimes">>,
+ HelpText<"Extend the lifetimes of local variables and parameters to improve "
+ "visibility in optimized debugging">,
+ Visibility<[ClangOption, CC1Option]>;
defm unique_basic_block_section_names :
BoolFOption<"unique-basic-block-section-names",
CodeGenOpts<"UniqueBasicBlockSectionNames">, DefaultFalse,
diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 4ae981e4013e9c..2453c580f7db0f 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -2543,6 +2543,10 @@ void CodeGenModule::ConstructAttributeList(StringRef
Name,
if (shouldDisableTailCalls())
FuncAttrs.addAttribute("disable-tail-calls", "true");
+ // Mark the function as having fake uses when -fextend-lifetimes is on.
+ if (CodeGenOpts.ExtendLifetimes || CodeGenOpts.ExtendThisPtr)
+ FuncAttrs.addAttribute(llvm::Attribute::HasFakeUses);
+
// CPU/feature overrides. addDefaultFunctionDefinitionAttributes
// handles these separately to set them based on the global defaults.
GetCPUAndFeaturesAttributes(CalleeInfo.getCalleeDecl(), FuncAttrs);
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp
b/clang/lib/Driver/ToolChains/Clang.cpp
index 0bab48caf1a5e2..3fe7817acf61c2 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -7583,6 +7583,11 @@ 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");
+ if (Args.hasArg(options::OPT_fextend_this_ptr))
+ CmdArgs.push_back("-fextend-this-ptr");
+ if (Args.hasArg(options::OPT_fextend_lifetimes))
+ CmdArgs.push_back("-fextend-lifetimes");
+
// Forward -fcomment-block-commands to -cc1.
Args.AddAllArgs(CmdArgs, options::OPT_fcomment_block_commands);
// Forward -fparse-all-comments to -cc1.
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp
b/clang/lib/Frontend/CompilerInvocation.cpp
index efd852593468aa..93972c37ac6821 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -2217,6 +2217,11 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions
&Opts, ArgList &Args,
Args.getAllArgValues(OPT_fsanitize_trap_EQ), Diags,
Opts.SanitizeTrap);
+ Opts.ExtendThisPtr =
+ Opts.OptimizationLevel > 0 && Args.hasArg(OPT_fextend_this_ptr);
+ Opts.ExtendLifetimes =
+ Opts.OptimizationLevel > 0 && Args.hasArg(OPT_fextend_lifetimes);
+
Opts.EmitVersionIdentMetadata = Args.hasFlag(OPT_Qy, OPT_Qn, true);
if (!LangOpts->CUDAIsDevice)
diff --git a/clang/test/CodeGen/extend-lifetimes-hasfakeuses.c
b/clang/test/CodeGen/extend-lifetimes-hasfakeuses.c
new file mode 100644
index 00000000000000..081bace3dc7cce
--- /dev/null
+++ b/clang/test/CodeGen/extend-lifetimes-hasfakeuses.c
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 %s -emit-llvm -O2 -fextend-lifetimes -o - | FileCheck
--check-prefixes=CHECK-ALL,CHECK-O2 %s
+// RUN: %clang_cc1 %s -emit-llvm -O0 -fextend-lifetimes -o - | FileCheck
--check-prefixes=CHECK-ALL,CHECK-O0 %s
+
+// Checks that we emit the function attribute has_fake_uses when
+// -fextend-lifetimes is on and optimizations are enabled, and that it does not
+// when optimizations are disabled.
+
+// CHECK-ALL: define {{.*}}void @foo
+// CHECK-O2: attributes #0 = {{{.*}}has_fake_uses
+// CHECK-O0-NOT: attributes #0 = {{{.*}}has_fake_uses
+
+void foo() {}
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