[llvm/DWARF] Recursively resolve DW_AT_signature references

[labath]

findRecursively follows DW_AT_specification and DW_AT_abstract_origin references, but not DW_AT_signature. As far as I can tell, there is no fundamental difference between these attributes that would make this behavior desirable, and this just seems like a consequence of the fact that this attribute is newer. This patch aims to change that.

The motivation is some code in lldb, which assumes that it can construct a qualified name of a type by just walking the parent chain and looking at the name attribute. This works for "regular" debug info, even when some of the DIEs are just forward declarations, but it breaks in the presence of type units, because of the need to explicitly resolve the signature reference.

While LLDB does not use the llvm's DWARFDie class (yet?), this seems like a very important change in the overall API, and any divergence here would complicate eventual reunification, which is why I am making the change in the llvm API first. However, putting lldb aside, I think this change is beneficial in llvm on its own, as it allows us to remove the explicit DW_AT_signature resolution in the DWARFTypePrinter.

[llvmbot]

@llvm/pr-subscribers-debuginfo

Author: Pavel Labath (labath)

Changes

findRecursively follows DW_AT_specification and DW_AT_abstract_origin references, but not DW_AT_signature. As far as I can tell, there is no fundamental difference between these attributes that would make this behavior desirable, and this just seems like a consequence of the fact that this attribute is newer. This patch aims to change that.

The motivation is some code in lldb, which assumes that it can construct a qualified name of a type by just walking the parent chain and looking at the name attribute. This works for "regular" debug info, even when some of the DIEs are just forward declarations, but it breaks in the presence of type units, because of the need to explicitly resolve the signature reference.

While LLDB does not use the llvm's DWARFDie class (yet?), this seems like a very important change in the overall API, and any divergence here would complicate eventual reunification, which is why I am making the change in the llvm API first. However, putting lldb aside, I think this change is beneficial in llvm on its own, as it allows us to remove the explicit DW_AT_signature resolution in the DWARFTypePrinter.

---

Full diff: https://github.com/llvm/llvm-project/pull/97423.diff

3 Files Affected:

• (modified) llvm/include/llvm/DebugInfo/DWARF/DWARFDie.h (-2)
• (modified) llvm/lib/DebugInfo/DWARF/DWARFDie.cpp (+15-28)
• (modified) llvm/lib/DebugInfo/DWARF/DWARFTypePrinter.cpp (+25-27)

diff --git a/llvm/include/llvm/DebugInfo/DWARF/DWARFDie.h b/llvm/include/llvm/DebugInfo/DWARF/DWARFDie.h
index 421b84d644db6..497d3bee048ab 100644
--- a/llvm/include/llvm/DebugInfo/DWARF/DWARFDie.h
+++ b/llvm/include/llvm/DebugInfo/DWARF/DWARFDie.h
@@ -181,8 +181,6 @@ class DWARFDie {
   DWARFDie getAttributeValueAsReferencedDie(dwarf::Attribute Attr) const;
   DWARFDie getAttributeValueAsReferencedDie(const DWARFFormValue &V) const;
 
-  DWARFDie resolveTypeUnitReference() const;
-
   /// Extract the range base attribute from this DIE as absolute section offset.
   ///
   /// This is a utility function that checks for either the DW_AT_rnglists_base
diff --git a/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp b/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp
index 410842a80b015..2cc5b416fe1a2 100644
--- a/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp
@@ -103,10 +103,6 @@ static void dumpLocationExpr(raw_ostream &OS, const DWARFFormValue &FormValue,
       .print(OS, DumpOpts, U);
 }
 
-static DWARFDie resolveReferencedType(DWARFDie D, DWARFFormValue F) {
-  return D.getAttributeValueAsReferencedDie(F).resolveTypeUnitReference();
-}
-
 static void dumpAttribute(raw_ostream &OS, const DWARFDie &Die,
                           const DWARFAttribute &AttrValue, unsigned Indent,
                           DIDumpOptions DumpOpts) {
@@ -198,8 +194,8 @@ static void dumpAttribute(raw_ostream &OS, const DWARFDie &Die,
                 DINameKind::LinkageName))
       OS << Space << "\"" << Name << '\"';
   } else if (Attr == DW_AT_type || Attr == DW_AT_containing_type) {
-    DWARFDie D = resolveReferencedType(Die, FormValue);
-    if (D && !D.isNULL()) {
+    if (DWARFDie D = Die.getAttributeValueAsReferencedDie(FormValue);
+        D && !D.isNULL()) {
       OS << Space << "\"";
       dumpTypeQualifiedName(D, OS);
       OS << '"';
@@ -291,13 +287,12 @@ DWARFDie::findRecursively(ArrayRef<dwarf::Attribute> Attrs) const {
     if (auto Value = Die.find(Attrs))
       return Value;
 
-    if (auto D = Die.getAttributeValueAsReferencedDie(DW_AT_abstract_origin))
-      if (Seen.insert(D).second)
-        Worklist.push_back(D);
-
-    if (auto D = Die.getAttributeValueAsReferencedDie(DW_AT_specification))
-      if (Seen.insert(D).second)
-        Worklist.push_back(D);
+    for (dwarf::Attribute Attr :
+         {DW_AT_abstract_origin, DW_AT_specification, DW_AT_signature}) {
+      if (auto D = Die.getAttributeValueAsReferencedDie(Attr))
+        if (Seen.insert(D).second)
+          Worklist.push_back(D);
+    }
   }
 
   return std::nullopt;
@@ -312,27 +307,19 @@ DWARFDie::getAttributeValueAsReferencedDie(dwarf::Attribute Attr) const {
 
 DWARFDie
 DWARFDie::getAttributeValueAsReferencedDie(const DWARFFormValue &V) const {
-  DWARFDie Result;
   if (auto SpecRef = V.getAsRelativeReference()) {
     if (SpecRef->Unit)
-      Result = SpecRef->Unit->getDIEForOffset(SpecRef->Unit->getOffset() +
-                                              SpecRef->Offset);
-    else if (auto SpecUnit =
-                 U->getUnitVector().getUnitForOffset(SpecRef->Offset))
-      Result = SpecUnit->getDIEForOffset(SpecRef->Offset);
-  }
-  return Result;
-}
-
-DWARFDie DWARFDie::resolveTypeUnitReference() const {
-  if (auto Attr = find(DW_AT_signature)) {
-    if (std::optional<uint64_t> Sig = Attr->getAsReferenceUVal()) {
+      return SpecRef->Unit->getDIEForOffset(SpecRef->Unit->getOffset() +
+                                            SpecRef->Offset);
+    if (V.getForm() == dwarf::DW_FORM_ref_sig8) {
       if (DWARFTypeUnit *TU = U->getContext().getTypeUnitForHash(
-              U->getVersion(), *Sig, U->isDWOUnit()))
+              U->getVersion(), SpecRef->Offset, U->isDWOUnit()))
         return TU->getDIEForOffset(TU->getTypeOffset() + TU->getOffset());
     }
+    if (auto *SpecUnit = U->getUnitVector().getUnitForOffset(SpecRef->Offset))
+      return SpecUnit->getDIEForOffset(SpecRef->Offset);
   }
-  return *this;
+  return {};
 }
 
 std::optional<uint64_t> DWARFDie::getRangesBaseAttribute() const {
diff --git a/llvm/lib/DebugInfo/DWARF/DWARFTypePrinter.cpp b/llvm/lib/DebugInfo/DWARF/DWARFTypePrinter.cpp
index a26431e8313f6..fc1aae77a9293 100644
--- a/llvm/lib/DebugInfo/DWARF/DWARFTypePrinter.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFTypePrinter.cpp
@@ -62,17 +62,10 @@ void DWARFTypePrinter::appendArrayType(const DWARFDie &D) {
   EndedWithTemplate = false;
 }
 
-static DWARFDie resolveReferencedType(DWARFDie D,
-                                      dwarf::Attribute Attr = DW_AT_type) {
-  return D.getAttributeValueAsReferencedDie(Attr).resolveTypeUnitReference();
-}
-static DWARFDie resolveReferencedType(DWARFDie D, DWARFFormValue F) {
-  return D.getAttributeValueAsReferencedDie(F).resolveTypeUnitReference();
-}
 DWARFDie DWARFTypePrinter::skipQualifiers(DWARFDie D) {
   while (D && (D.getTag() == DW_TAG_const_type ||
                D.getTag() == DW_TAG_volatile_type))
-    D = resolveReferencedType(D);
+    D = D.getAttributeValueAsReferencedDie(DW_AT_type);
   return D;
 }
 
@@ -103,7 +96,9 @@ DWARFTypePrinter::appendUnqualifiedNameBefore(DWARFDie D,
     return DWARFDie();
   }
   DWARFDie InnerDIE;
-  auto Inner = [&] { return InnerDIE = resolveReferencedType(D); };
+  auto Inner = [&] {
+    return InnerDIE = D.getAttributeValueAsReferencedDie(DW_AT_type);
+  };
   const dwarf::Tag T = D.getTag();
   switch (T) {
   case DW_TAG_pointer_type: {
@@ -134,7 +129,8 @@ DWARFTypePrinter::appendUnqualifiedNameBefore(DWARFDie D,
       OS << '(';
     else if (Word)
       OS << ' ';
-    if (DWARFDie Cont = resolveReferencedType(D, DW_AT_containing_type)) {
+    if (DWARFDie Cont =
+            D.getAttributeValueAsReferencedDie(DW_AT_containing_type)) {
       appendQualifiedName(Cont);
       EndedWithTemplate = false;
       OS << "::";
@@ -173,7 +169,8 @@ DWARFTypePrinter::appendUnqualifiedNameBefore(DWARFDie D,
   case DW_TAG_base_type:
   */
   default: {
-    const char *NamePtr = dwarf::toString(D.find(DW_AT_name), nullptr);
+    const char *NamePtr =
+        dwarf::toString(D.findRecursively(DW_AT_name), nullptr);
     if (!NamePtr) {
       appendTypeTagName(D.getTag());
       return DWARFDie();
@@ -235,9 +232,9 @@ void DWARFTypePrinter::appendUnqualifiedNameAfter(
   case DW_TAG_pointer_type: {
     if (needsParens(Inner))
       OS << ')';
-    appendUnqualifiedNameAfter(Inner, resolveReferencedType(Inner),
-                               /*SkipFirstParamIfArtificial=*/D.getTag() ==
-                                   DW_TAG_ptr_to_member_type);
+    appendUnqualifiedNameAfter(
+        Inner, Inner.getAttributeValueAsReferencedDie(DW_AT_type),
+        /*SkipFirstParamIfArtificial=*/D.getTag() == DW_TAG_ptr_to_member_type);
     break;
   }
   case DW_TAG_LLVM_ptrauth_type: {
@@ -341,7 +338,7 @@ bool DWARFTypePrinter::appendTemplateParameters(DWARFDie D,
       appendTemplateParameters(C, FirstParameter);
     }
     if (C.getTag() == dwarf::DW_TAG_template_value_parameter) {
-      DWARFDie T = resolveReferencedType(C);
+      DWARFDie T = C.getAttributeValueAsReferencedDie(DW_AT_type);
       Sep();
       if (T.getTag() == DW_TAG_enumeration_type) {
         OS << '(';
@@ -461,7 +458,7 @@ bool DWARFTypePrinter::appendTemplateParameters(DWARFDie D,
       continue;
     auto TypeAttr = C.find(DW_AT_type);
     Sep();
-    appendQualifiedName(TypeAttr ? resolveReferencedType(C, *TypeAttr)
+    appendQualifiedName(TypeAttr ? C.getAttributeValueAsReferencedDie(*TypeAttr)
                                  : DWARFDie());
   }
   if (IsTemplate && *FirstParameter && FirstParameter == &FirstParameterValue) {
@@ -473,15 +470,15 @@ bool DWARFTypePrinter::appendTemplateParameters(DWARFDie D,
 void DWARFTypePrinter::decomposeConstVolatile(DWARFDie &N, DWARFDie &T,
                                               DWARFDie &C, DWARFDie &V) {
   (N.getTag() == DW_TAG_const_type ? C : V) = N;
-  T = resolveReferencedType(N);
+  T = N.getAttributeValueAsReferencedDie(DW_AT_type);
   if (T) {
     auto Tag = T.getTag();
     if (Tag == DW_TAG_const_type) {
       C = T;
-      T = resolveReferencedType(T);
+      T = T.getAttributeValueAsReferencedDie(DW_AT_type);
     } else if (Tag == DW_TAG_volatile_type) {
       V = T;
-      T = resolveReferencedType(T);
+      T = T.getAttributeValueAsReferencedDie(DW_AT_type);
     }
   }
 }
@@ -491,10 +488,11 @@ void DWARFTypePrinter::appendConstVolatileQualifierAfter(DWARFDie N) {
   DWARFDie T;
   decomposeConstVolatile(N, T, C, V);
   if (T && T.getTag() == DW_TAG_subroutine_type)
-    appendSubroutineNameAfter(T, resolveReferencedType(T), false, C.isValid(),
-                              V.isValid());
+    appendSubroutineNameAfter(T, T.getAttributeValueAsReferencedDie(DW_AT_type),
+                              false, C.isValid(), V.isValid());
   else
-    appendUnqualifiedNameAfter(T, resolveReferencedType(T));
+    appendUnqualifiedNameAfter(T,
+                               T.getAttributeValueAsReferencedDie(DW_AT_type));
 }
 void DWARFTypePrinter::appendConstVolatileQualifierBefore(DWARFDie N) {
   DWARFDie C;
@@ -504,7 +502,7 @@ void DWARFTypePrinter::appendConstVolatileQualifierBefore(DWARFDie N) {
   bool Subroutine = T && T.getTag() == DW_TAG_subroutine_type;
   DWARFDie A = T;
   while (A && A.getTag() == DW_TAG_array_type)
-    A = resolveReferencedType(A);
+    A = A.getAttributeValueAsReferencedDie(DW_AT_type);
   bool Leading =
       (!A || (A.getTag() != DW_TAG_pointer_type &&
               A.getTag() != llvm::dwarf::DW_TAG_ptr_to_member_type)) &&
@@ -546,7 +544,7 @@ void DWARFTypePrinter::appendSubroutineNameAfter(
     if (P.getTag() != DW_TAG_formal_parameter &&
         P.getTag() != DW_TAG_unspecified_parameters)
       return;
-    DWARFDie T = resolveReferencedType(P);
+    DWARFDie T = P.getAttributeValueAsReferencedDie(DW_AT_type);
     if (SkipFirstParamIfArtificial && RealFirst && P.find(DW_AT_artificial)) {
       FirstParamIfArtificial = T;
       RealFirst = false;
@@ -567,7 +565,7 @@ void DWARFTypePrinter::appendSubroutineNameAfter(
     if (DWARFDie P = FirstParamIfArtificial) {
       if (P.getTag() == DW_TAG_pointer_type) {
         auto CVStep = [&](DWARFDie CV) {
-          if (DWARFDie U = resolveReferencedType(CV)) {
+          if (DWARFDie U = CV.getAttributeValueAsReferencedDie(DW_AT_type)) {
             Const |= U.getTag() == DW_TAG_const_type;
             Volatile |= U.getTag() == DW_TAG_volatile_type;
             return U;
@@ -653,7 +651,8 @@ void DWARFTypePrinter::appendSubroutineNameAfter(
   if (D.find(DW_AT_rvalue_reference))
     OS << " &&";
 
-  appendUnqualifiedNameAfter(Inner, resolveReferencedType(Inner));
+  appendUnqualifiedNameAfter(
+      Inner, Inner.getAttributeValueAsReferencedDie(DW_AT_type));
 }
 void DWARFTypePrinter::appendScopes(DWARFDie D) {
   if (D.getTag() == DW_TAG_compile_unit)
@@ -666,7 +665,6 @@ void DWARFTypePrinter::appendScopes(DWARFDie D) {
     return;
   if (D.getTag() == DW_TAG_lexical_block)
     return;
-  D = D.resolveTypeUnitReference();
   if (DWARFDie P = D.getParent())
     appendScopes(P);
   appendUnqualifiedName(D);

[labath]

Ping? What do you think about this approach? Or do you know of someone who can review this?

[MaskRay]

Thanks. This seems a nice refactoring as well as a simplification.

Only few relevant tests are llvm/test/tools/llvm-dwarfdump/X86/prettyprint_type*.s

I followed the control flow in a debugger and this PR seems reasonable.

DW_TAG_GNU_template_parameter_pack
  DW_TAG_template_type_parameter
    DW_AT_type [DW_FORM_ref4]   (cu + 0x0058 => {0x00000058} "t1")

=>

DW_TAG_compile_unit
  DW_TAG_structure_type
    DW_AT_signature [DW_FORM_ref_sig8]      (0xc6694e51369161f2)

=>
Type Unit: type_signature = 0xc6694e51369161f2
DW_TAG_type_unit 
  DW_TAG_structure_type
    DW_AT_name

Best to wait for @dwblaikie who has authored 58b1b64 and
3cbc4c4

[dwblaikie]

Love the change - thanks for this! (& putting it into LLVM with the intent to match future changes to LLDB is excellent - will help with the type printer generalization work I've got going that's trying to port the type printer to work with LLDB too, which I'd just commented out/punted on the type unit support for now - and with this patch it should come out in the wash (after resolving these changes against my own)

[dwblaikie]

Seems a bit questionable that getAsRelativeReference returns a valid value for DW_FORM_ref_sig8 - it's not a CU-relative reference form... (eg: if one had called getAttributeValueAsReferencedDie on a ref_sig8 before this patch, it would have walked off to some garbage offset, right? Interpreting the sig8 as an offset)

So might be worth changing it to not do that and having an explicit/separate case here?

(aside: this case is probably untested? How'd you come across changing this function? Clang/LLVM doesn't produce direct references via ref_sig8, only via the skeleton type DIEs... ah, I see, the findRecursively change would cause us to call getAttributeValueAsReferencedDie(DW_AT_signature)? OK.

Incidentally this has probably added support for some DWARF that GCC produces - where it references the type unit directly from DW_AT_type, maybe? Currently llvm-dwarfdump doesn't resolve such references ( https://godbolt.org/z/E5boxcj1r - note the DW_TAG_variable's DW_AT_type (0xc949e2ea8b91cfb0) only has the signature, and I guess with this patch maybe we now get DW_AT_type (0xc949e2ea8b91cfb0 "t1")?)

[labath]

So might be worth changing it to not do that and having an explicit/separate case here?

Yeah, I was wondering about that. It did seem somewhat suboptimal, but I wasn't particularly keen on taking it on.

Now I am, and here's my attempt to do something about this: #98905

Incidentally this has probably added support for some DWARF that GCC produces - where it references the type unit directly from DW_AT_type, maybe? Currently llvm-dwarfdump doesn't resolve such references ( https://godbolt.org/z/E5boxcj1r - note the DW_TAG_variable's DW_AT_type (0xc949e2ea8b91cfb0) only has the signature, and I guess with this patch maybe we now get DW_AT_type (0xc949e2ea8b91cfb0 "t1")?)

Interesting. I'll look into this tomorrow.

[labath]

Incidentally this has probably added support for some DWARF that GCC produces - where it references the type unit directly from DW_AT_type, maybe? Currently llvm-dwarfdump doesn't resolve such references ( https://godbolt.org/z/E5boxcj1r - note the DW_TAG_variable's DW_AT_type (0xc949e2ea8b91cfb0) only has the signature, and I guess with this patch maybe we now get DW_AT_type (0xc949e2ea8b91cfb0 "t1")?)

Interesting. I'll look into this tomorrow.

Yes, it really does fix it. I added a test (modifying an existing one) for that. PTAL.

[labath]

Sorry about the messed up merge. TIL the order of parents is important.

[dwblaikie]

Awesome!

[llvm-ci]

LLVM Buildbot has detected a new failure on builder llvm-clang-x86_64-gcc-ubuntu running on sie-linux-worker3 while building llvm at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/174/builds/1857

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

Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'cross-project-tests :: debuginfo-tests/clang_llvm_roundtrip/simplified_template_names.cpp' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
Verifying -:	file format elf64-x86-64
Verifying .debug_abbrev...
Verifying .debug_info Unit Header Chain...
Verifying .debug_types Unit Header Chain...
Verifying non-dwo Units...
Verifying unit: 1 / 1, "/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/cross-project-tests/debuginfo-tests/clang_llvm_roundtrip/simplified_template_names.cpp"
Verifying dwo Units...
Verifying .debug_line...
Verifying .debug_str_offsets...
No errors.
Verifying -:	file format elf64-x86-64
Verifying .debug_abbrev...
Verifying .debug_info Unit Header Chain...
Verifying .debug_types Unit Header Chain...
Verifying non-dwo Units...
Verifying unit: 1 / 1, "/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/cross-project-tests/debuginfo-tests/clang_llvm_roundtrip/simplified_template_names.cpp"
Verifying dwo Units...
Verifying .debug_line...
Verifying .debug_str_offsets...
No errors.
Verifying -:	file format elf64-x86-64
Verifying .debug_abbrev...
Verifying .debug_info Unit Header Chain...
Verifying .debug_types Unit Header Chain...
Verifying non-dwo Units...
Verifying unit: 1 / 83, "/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/cross-project-tests/debuginfo-tests/clang_llvm_roundtrip/simplified_template_names.cpp"
error: Simplified template DW_AT_name could not be reconstituted:
         original: t3<int, false>
    reconstituted: t3

0x000000d6: DW_TAG_structure_type [20]   (0x0000000b)
              DW_AT_declaration [DW_FORM_flag_present]	(true)
              DW_AT_signature [DW_FORM_ref_sig8]	(0x384ddc13a7a92863)


0x0000000b: DW_TAG_compile_unit [52] *
              DW_AT_producer [DW_FORM_strp]	( .debug_str[0x00000000] = "clang version 19.0.0git (https://github.com/llvm/llvm-project.git e93df78bd46b585c0bdabdbdc95410e4c08b9d38)")
              DW_AT_language [DW_FORM_data2]	(DW_LANG_C_plus_plus_14)
              DW_AT_name [DW_FORM_strp]	( .debug_str[0x0000006c] = "/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/cross-project-tests/debuginfo-tests/clang_llvm_roundtrip/simplified_template_names.cpp")
              DW_AT_stmt_list [DW_FORM_sec_offset]	(0x00000000)
              DW_AT_comp_dir [DW_FORM_strp]	( .debug_str[0x0000010a] = "/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/projects/cross-project-tests/debuginfo-tests/clang_llvm_roundtrip")
              DW_AT_low_pc [DW_FORM_addr]	(0x0000000000000000)
              DW_AT_ranges [DW_FORM_sec_offset]	(0x00000000
                 [0x0000000000000000, 0x000000000000038d) ".text"
                 [0x0000000000000390, 0x00000000000003a6) ".text"
...