[DebugInfo] Add DW_AT_artificial for compiler generated static member.

[CarlosAlbertoEnciso]

Consider the case when the compiler generates a static member. Any consumer of the debug info generated for that case, would benefit if that member has the DW_AT_artificial flag.

[llvmbot]

@llvm/pr-subscribers-debuginfo

Author: Carlos Alberto Enciso (CarlosAlbertoEnciso)

Changes

Consider the case when the compiler generates a static member. Any consumer of the debug info generated for that case, would benefit if that member has the DW_AT_artificial flag.

---

Full diff: https://github.com/llvm/llvm-project/pull/115851.diff

2 Files Affected:

• (modified) llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp (+4)
• (added) llvm/test/DebugInfo/Generic/artificial-static-member.ll (+41)

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index c6d9fcfb0b76c1..76f7b0920434bb 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1784,6 +1784,10 @@ DIE *DwarfUnit::getOrCreateStaticMemberDIE(const DIDerivedType *DT) {
   addFlag(StaticMemberDIE, dwarf::DW_AT_external);
   addFlag(StaticMemberDIE, dwarf::DW_AT_declaration);
 
+  // Consider the case when the static member was created by the compiler.
+  if (DT->isArtificial())
+    addFlag(StaticMemberDIE, dwarf::DW_AT_artificial);
+
   // FIXME: We could omit private if the parent is a class_type, and
   // public if the parent is something else.
   addAccess(StaticMemberDIE, DT->getFlags());
diff --git a/llvm/test/DebugInfo/Generic/artificial-static-member.ll b/llvm/test/DebugInfo/Generic/artificial-static-member.ll
new file mode 100644
index 00000000000000..3263c976e39158
--- /dev/null
+++ b/llvm/test/DebugInfo/Generic/artificial-static-member.ll
@@ -0,0 +1,41 @@
+; RUN: llc -O0 -filetype=obj < %s |   \
+; RUN: llvm-dwarfdump --debug-info - | FileCheck %s
+
+; LLVM IR generated from:
+
+; struct S {
+;   static int Member;   <-- Manually marked as artificial
+; };
+; int S::Member = 1;
+
+source_filename = "artificial-static-member.cpp"
+target triple = "x86_64-pc-linux-gnu"
+
+@_ZN1S6MemberE = dso_local global i32 1, align 4, !dbg !0
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!9, !10}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "Member", linkageName: "_ZN1S6MemberE", scope: !2, type: !5, isLocal: false, isDefinition: true, declaration: !6)
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !3, emissionKind: FullDebug, globals: !4)
+!3 = !DIFile(filename: "artificial-static-member.cpp", directory: "")
+!4 = !{!0}
+!5 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!6 = !DIDerivedType(tag: DW_TAG_variable, name: "Member", scope: !7, baseType: !5, flags: DIFlagArtificial | DIFlagStaticMember)
+!7 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S", size: 8, flags: DIFlagTypePassByValue, elements: !8, identifier: "_ZTS1S")
+!8 = !{!6}
+!9 = !{i32 7, !"Dwarf Version", i32 5}
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+
+; CHECK: {{.*}}DW_TAG_structure_type
+; CHECK-NEXT: DW_AT_calling_convention
+; CHECK-NEXT: DW_AT_name	("S")
+; CHECK-NEXT: DW_AT_byte_size
+
+; CHECK: {{.*}}DW_TAG_variable
+; CHECK-NEXT: DW_AT_name	("Member")
+; CHECK-NEXT: DW_AT_type
+; CHECK-NEXT: DW_AT_external	(true)
+; CHECK-NEXT: DW_AT_declaration	(true)
+; CHECK-NEXT: DW_AT_artificial	(true)

[dwblaikie]

Any particular compiler generated static variables you have in mind? There aren't any C++ ones that come to mind - though of course LLVM isn't just for Clang/C++, but would be nice to have some documented use case, if there is one, for this. If there isn't one we can document/mention, it's probably still OK to do though, it's simple enough/cheap enough that it's not a big deal.

[CarlosAlbertoEnciso]

For classes with virtual functions, Clang generates a class member _vptr$XXX with XXX being the class name. It points to the vtable function pointers.

.debug_info contents:
DW_TAG_structure_type
  DW_AT_name	("MyClass")
  ...
  DW_TAG_member
    DW_AT_name	("_vptr$MyClass")
    DW_AT_type	(0x000000ba "int (**)()")
    DW_AT_data_member_location	(0x00)
    DW_AT_artificial	(true)
  ...

[CarlosAlbertoEnciso]

We want to allow our (SCE) debugger to have a direct access to the whole vtable, thru a static data member _vtable$.

.debug_info contents:
...
DW_TAG_variable
  DW_AT_specification	("_vtable$")
  DW_AT_location	(DW_OP_addrx 0x0)     --> index (0) in the '.debug_addr'
  DW_AT_linkage_name	("_ZTVN7MyClassE")
...
DW_TAG_structure_type
  DW_AT_name	("MyClass")
  ...
  DW_TAG_variable
    DW_AT_name	("_vtable$")
    DW_AT_type	(0x000000d7 "void *")
    DW_AT_artificial	(true)
    ...

.debug_addr contents:
...
Addrs: [
 Address of vtable for MyClass
...
]

We need for the generated static data member to be artificial.

[dwblaikie]

SGTM

[CarlosAlbertoEnciso]

@dwblaikie Thanks for your feedback.

[llvm-ci]

LLVM Buildbot has detected a new failure on builder llvm-clang-aarch64-darwin running on doug-worker-5 while building llvm at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/9521

Here is the relevant piece of the build log for the reference

Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: DebugInfo/Generic/artificial-static-member.ll' FAILED ********************
Exit Code: 2

Command Output (stderr):
--
RUN: at line 1: /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llc -O0 -filetype=obj < /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/DebugInfo/Generic/artificial-static-member.ll |    /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llvm-dwarfdump --debug-info - | /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/DebugInfo/Generic/artificial-static-member.ll
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llc -O0 -filetype=obj
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/DebugInfo/Generic/artificial-static-member.ll
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llvm-dwarfdump --debug-info -
/Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llc: error: unable to get target for 'x86_64-pc-linux-gnu', see --version and --triple.error: -: The file was not recognized as a valid object file
FileCheck error: '<stdin>' is empty.
FileCheck command line:  /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/DebugInfo/Generic/artificial-static-member.ll

--

********************

[CarlosAlbertoEnciso]

LLVM Buildbot has detected a new failure on builder llvm-clang-aarch64-darwin running on doug-worker-5 while building llvm at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/9521

Created PR that fix the test case: #116327