[PATCH] D152017: [DebugInfo] Add flag to only emit referenced member functions

Fri Jun 2 11:22:37 PDT 2023

dblaikie created this revision.
dblaikie added a reviewer: probinson.
Herald added a project: All.
dblaikie requested review of this revision.
Herald added subscribers: cfe-commits, MaskRay.
Herald added a project: clang.

Complete C++ type information can be quite expensive - and there's
limited value in representing every member function, even those that
can't be called (we don't do similarly for every non-member function
anyway). So add a flag to opt out of this behavior for experimenting
with this more terse behavior.

I think Sony already does this by default, so perhaps with a change to
the defaults, Sony can migrate to this rather than a downstream patch.

This breaks current debuggers in some expected ways - but those
breakages are visible without this feature too. Consider member function
template instantiations - they can't be consistently enumerated in every
translation unit:

a.h:
	struct t1 {

  	  template <int i>
  	  static int f1() {
  	    return i;
  	  }

};
	namespace ns {
	template <int i>
	int f1() {

  	  return i;

}
	}  // namespace ns
a.cpp:
	#include "a.h"
	void f1() {

  	  t1::f1<0>();
  	  ns::f1<0>();

}
b.cpp:
	#include "a.h"
	void f1();
	int main() {

  	  f1();
  	  t1::f1<1>();
  	  ns::f1<1>();

}

(gdb) p ns::f1<0>()
$1 = 0
(gdb) p ns::f1<1>()
$2 = 1
(gdb) p t1::f1<0>()
Couldn't find method t1::f1<0>
(gdb) p t1::f1<1>()
$3 = 1
(gdb) s
f1 () at a.cpp:3
3         t1::f1<0>();
(gdb) p t1::f1<0>()
$4 = 0
(gdb) p t1::f1<1>()
Couldn't find method t1::f1<1>
(gdb)

(other similar non-canonical features are implicit special members
(copy/move ctor/assignment operator, default ctor) and nested types (eg:
pimpl idiom, where the nested type is declared-but-not-defined in one
TU, and defined in another TU))

lldb can't parse the template expressions above, so I'm not sure how to
test it there, but I'd guess it has similar problems. (
https://stackoverflow.com/questions/64602475/how-to-print-value-returned-by-template-member-function-in-gdb-lldb-debugging
so... I guess that's just totally not supported in lldb, how
unfortunate. And implicit special members are instantiated implicitly by
lldb, so missing those doesn't tickle the same issue)

@probinson - would this be an accurate upstreaming of your internal handling/would you use this functionality? If it wouldn't be useful to you, it's maybe not worth adding upstream yet - not sure we'll use it at Google, but if it was useful to you folks and meant other folks could test with it it seemed maybe useful.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D152017

Files:
  clang/include/clang/Basic/CodeGenOptions.def
  clang/include/clang/Driver/Options.td
  clang/lib/CodeGen/CGDebugInfo.cpp
  clang/lib/Driver/ToolChains/Clang.cpp
  clang/test/CodeGenCXX/debug-info-incomplete-types.cpp
  clang/test/Driver/debug-options.c


Index: clang/test/Driver/debug-options.c
===================================================================

--- clang/test/Driver/debug-options.c
+++ clang/test/Driver/debug-options.c
@@ -259,6 +259,9 @@
 // RUN: %clang -### -c %s 2>&1 | FileCheck -check-prefix=NORNGBSE %s
 // RUN: %clang -### -c -fdebug-ranges-base-address -fno-debug-ranges-base-address %s 2>&1 | FileCheck -check-prefix=NORNGBSE %s
 //
+// RUN: %clang -### -c -gincomplete-types %s 2>&1 | FileCheck -check-prefix=INCTYPES %s
+// RUN: %clang -### -c %s 2>&1 | FileCheck -check-prefix=NOINCTYPES %s
+//
 // RUN: %clang -### -c -glldb %s 2>&1 | FileCheck -check-prefix=NOPUB %s
 // RUN: %clang -### -c -glldb -gno-pubnames %s 2>&1 | FileCheck -check-prefix=NOPUB %s
 //
@@ -398,6 +401,9 @@
 // RNGBSE: -fdebug-ranges-base-address
 // NORNGBSE-NOT: -fdebug-ranges-base-address
 //
+// INCTYPES: -gincomplete-types
+// NOINCTYPES-NOT: -gincomplete-types
+//
 // GARANGE-DAG: -generate-arange-section
 //
 // FDTS: "-mllvm" "-generate-type-units"
Index: clang/test/CodeGenCXX/debug-info-incomplete-types.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/debug-info-incomplete-types.cpp
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -debug-info-kind=limited -gincomplete-types %s -emit-llvm -o - | FileCheck %s
+
+struct t1 {
+  void f1();
+  void f2();
+};
+
+void t1::f1() { }
+
+// CHECK: distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t1"
+// CHECK-SAME: elements: [[ELEMENTS:![0-9]+]]
+// CHECK: [[ELEMENTS]] = !{}
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -4308,6 +4308,12 @@
                          DebuggerTuning != llvm::DebuggerKind::DBX)))
     CmdArgs.push_back("-gno-column-info");
 
+  if (const Arg *A = Args.getLastArg(options::OPT_gincomplete_types))
+    (void)checkDebugInfoOption(A, Args, D, TC);
+  if (Args.hasFlag(options::OPT_gincomplete_types,
+                   options::OPT_gno_incomplete_types, false))
+    CmdArgs.push_back("-gincomplete-types");
+
   // FIXME: Move backend command line options to the module.
   if (Args.hasFlag(options::OPT_gmodules, options::OPT_gno_modules, false)) {
     // If -gline-tables-only or -gline-directives-only is the last option it
Index: clang/lib/CodeGen/CGDebugInfo.cpp
===================================================================
--- clang/lib/CodeGen/CGDebugInfo.cpp
+++ clang/lib/CodeGen/CGDebugInfo.cpp
@@ -2740,7 +2740,7 @@
 
   // Collect data fields (including static variables and any initializers).
   CollectRecordFields(RD, DefUnit, EltTys, FwdDecl);
-  if (CXXDecl)
+  if (CXXDecl && !CGM.getCodeGenOpts().DebugIncompleteTypes)
     CollectCXXMemberFunctions(CXXDecl, DefUnit, EltTys, FwdDecl);
 
   LexicalBlockStack.pop_back();
Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -3346,6 +3346,10 @@
   CodeGenOpts<"DebugStrictDwarf">, DefaultFalse,
   PosFlag<SetTrue, [CC1Option]>, NegFlag<SetFalse>, BothFlags<[CoreOption]>>,
   Group<g_flags_Group>;
+defm incomplete_types : BoolOption<"g", "incomplete-types",
+  CodeGenOpts<"DebugIncompleteTypes">, DefaultFalse,
+  PosFlag<SetTrue, [CC1Option]>, NegFlag<SetFalse>, BothFlags<[CoreOption]>>,
+  Group<g_flags_Group>;
 defm column_info : BoolOption<"g", "column-info",
   CodeGenOpts<"DebugColumnInfo">, DefaultTrue,
   NegFlag<SetFalse, [CC1Option]>, PosFlag<SetTrue>, BothFlags<[CoreOption]>>,
Index: clang/include/clang/Basic/CodeGenOptions.def
===================================================================
--- clang/include/clang/Basic/CodeGenOptions.def
+++ clang/include/clang/Basic/CodeGenOptions.def
@@ -333,6 +333,8 @@
 VALUE_CODEGENOPT(WarnStackSize     , 32, UINT_MAX) ///< Set via -fwarn-stack-size.
 CODEGENOPT(NoStackArgProbe, 1, 0) ///< Set when -mno-stack-arg-probe is used
 CODEGENOPT(DebugStrictDwarf, 1, 1) ///< Whether or not to use strict DWARF info.
+/// Whether or not to include all members in a type in debug info.
+CODEGENOPT(DebugIncompleteTypes, 1, 0)
 
 /// Control the Assignment Tracking debug info feature.
 ENUM_CODEGENOPT(AssignmentTrackingMode, AssignmentTrackingOpts, 2, AssignmentTrackingOpts::Disabled)


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