[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.

Author: SLTozer

clang/docs/ReleaseNotes.rst

@@ -412,6 +412,16 @@ 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
 -------------------------

clang/include/clang/Basic/CodeGenOptions.def

@@ -393,6 +393,12 @@ 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)
 

clang/include/clang/Driver/Options.td

@@ -4298,6 +4298,15 @@ 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,

clang/lib/CodeGen/CGCall.cpp

@@ -2567,6 +2567,10 @@ 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);

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7654,6 +7654,11 @@ 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.

clang/lib/Frontend/CompilerInvocation.cpp

@@ -2245,6 +2245,11 @@ 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)

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() {}