[Clang] Add "extend lifetime" flags and release note (#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.

Co-authored-by: Stephen Tozer <[email protected]>

Author: wolfy1961

clang/docs/ReleaseNotes.rst

@@ -519,6 +519,20 @@ 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.
 
 - The ``-Warray-compare`` warning has been added to warn about array comparison
   on versions older than C++20.

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)
 

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,

clang/include/clang/Driver/Options.td

@@ -4349,6 +4349,26 @@ 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,

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7706,6 +7706,8 @@ 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.

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"